From: Thomas Gleixner <tglx@linutronix.de>
From: Thomas Gleixner <tglx@linutronix.de>
struct timekeeper is ordered suboptimal vs. cachelines. The layout,
including the preceding seqcount (see struct tk_core in timekeeper.c) is:
cacheline 0: seqcount, tkr_mono
cacheline 1: tkr_raw, xtime_sec
cacheline 2: ktime_sec ... tai_offset, internal variables
cacheline 3: next_leap_ktime, raw_sec, internal variables
cacheline 4: internal variables
So any access to via ktime_get*() except for access to CLOCK_MONOTONIC_RAW
will use either cachelines 0 + 1 or cachelines 0 + 2. Access to
CLOCK_MONOTONIC_RAW uses cachelines 0 + 1 + 3.
Reorder the members so that the result is more efficient:
cacheline 0: seqcount, tkr_mono
cacheline 1: xtime_sec, ktime_sec ... tai_offset
cacheline 2: tkr_raw, raw_sec
cacheline 3: internal variables
cacheline 4: internal variables
That means ktime_get*() will access cacheline 0 + 1 and CLOCK_MONOTONIC_RAW
access will use cachelines 0 + 2.
Update kernel-doc and fix formatting issues while at it. Also fix a typo
in struct tk_read_base kernel-doc.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
---
include/linux/timekeeper_internal.h | 102 +++++++++++++++++++++---------------
1 file changed, 61 insertions(+), 41 deletions(-)
diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
index 902c20ef495a..430e40549136 100644
--- a/include/linux/timekeeper_internal.h
+++ b/include/linux/timekeeper_internal.h
@@ -26,7 +26,7 @@
* occupies a single 64byte cache line.
*
* The struct is separate from struct timekeeper as it is also used
- * for a fast NMI safe accessors.
+ * for the fast NMI safe accessors.
*
* @base_real is for the fast NMI safe accessor to allow reading clock
* realtime from any context.
@@ -44,33 +44,41 @@ struct tk_read_base {
/**
* struct timekeeper - Structure holding internal timekeeping values.
- * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
- * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
- * @xtime_sec: Current CLOCK_REALTIME time in seconds
- * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
- * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
- * @offs_real: Offset clock monotonic -> clock realtime
- * @offs_boot: Offset clock monotonic -> clock boottime
- * @offs_tai: Offset clock monotonic -> clock tai
- * @tai_offset: The current UTC to TAI offset in seconds
- * @clock_was_set_seq: The sequence number of clock was set events
- * @cs_was_changed_seq: The sequence number of clocksource change events
- * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
- * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
- * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
- * @cycle_interval: Number of clock cycles in one NTP interval
- * @xtime_interval: Number of clock shifted nano seconds in one NTP
- * interval.
- * @xtime_remainder: Shifted nano seconds left over when rounding
- * @cycle_interval
- * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
- * @ntp_error: Difference between accumulated time and NTP time in ntp
- * shifted nano seconds.
- * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
- * ntp shifted nano seconds.
- * @last_warning: Warning ratelimiter (DEBUG_TIMEKEEPING)
- * @underflow_seen: Underflow warning flag (DEBUG_TIMEKEEPING)
- * @overflow_seen: Overflow warning flag (DEBUG_TIMEKEEPING)
+ * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
+ * @xtime_sec: Current CLOCK_REALTIME time in seconds
+ * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
+ * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
+ * @offs_real: Offset clock monotonic -> clock realtime
+ * @offs_boot: Offset clock monotonic -> clock boottime
+ * @offs_tai: Offset clock monotonic -> clock tai
+ * @tai_offset: The current UTC to TAI offset in seconds
+ * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
+ * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
+ * @clock_was_set_seq: The sequence number of clock was set events
+ * @cs_was_changed_seq: The sequence number of clocksource change events
+ * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
+ * @cycle_interval: Number of clock cycles in one NTP interval
+ * @xtime_interval: Number of clock shifted nano seconds in one NTP
+ * interval.
+ * @xtime_remainder: Shifted nano seconds left over when rounding
+ * @cycle_interval
+ * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
+ * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
+ * @ntp_tick: The ntp_tick_length() value currently being
+ * used. This cached copy ensures we consistently
+ * apply the tick length for an entire tick, as
+ * ntp_tick_length may change mid-tick, and we don't
+ * want to apply that new value to the tick in
+ * progress.
+ * @ntp_error: Difference between accumulated time and NTP time in ntp
+ * shifted nano seconds.
+ * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
+ * ntp shifted nano seconds.
+ * @ntp_err_mult: Multiplication factor for scaled math conversion
+ * @skip_second_overflow: Flag used to avoid updating NTP twice with same second
+ * @last_warning: Warning ratelimiter (DEBUG_TIMEKEEPING)
+ * @underflow_seen: Underflow warning flag (DEBUG_TIMEKEEPING)
+ * @overflow_seen: Overflow warning flag (DEBUG_TIMEKEEPING)
*
* Note: For timespec(64) based interfaces wall_to_monotonic is what
* we need to add to xtime (or xtime corrected for sub jiffy times)
@@ -88,10 +96,25 @@ struct tk_read_base {
*
* @monotonic_to_boottime is a timespec64 representation of @offs_boot to
* accelerate the VDSO update for CLOCK_BOOTTIME.
+ *
+ * The cacheline ordering of the structure is optimized for in kernel usage
+ * of the ktime_get() and ktime_get_ts64() family of time accessors. Struct
+ * timekeeper is prepended in the core timekeeeping code with a sequence
+ * count, which results in the following cacheline layout:
+ *
+ * 0: seqcount, tkr_mono
+ * 1: xtime_sec ... tai_offset
+ * 2: tkr_raw, raw_sec
+ * 3,4: Internal variables
+ *
+ * Cacheline 0,1 contain the data which is used for accessing
+ * CLOCK_MONOTONIC/REALTIME/BOOTTIME/TAI, while cacheline 2 contains the
+ * data for accessing CLOCK_MONOTONIC_RAW. Cacheline 3,4 are internal
+ * variables which are only accessed during timekeeper updates once per
+ * tick.
*/
struct timekeeper {
struct tk_read_base tkr_mono;
- struct tk_read_base tkr_raw;
u64 xtime_sec;
unsigned long ktime_sec;
struct timespec64 wall_to_monotonic;
@@ -99,31 +122,28 @@ struct timekeeper {
ktime_t offs_boot;
ktime_t offs_tai;
s32 tai_offset;
+
+ struct tk_read_base tkr_raw;
+ u64 raw_sec;
+
+ /* The following members are for timekeeping internal use */
unsigned int clock_was_set_seq;
u8 cs_was_changed_seq;
- ktime_t next_leap_ktime;
- u64 raw_sec;
+
struct timespec64 monotonic_to_boot;
- /* The following members are for timekeeping internal use */
u64 cycle_interval;
u64 xtime_interval;
s64 xtime_remainder;
u64 raw_interval;
- /* The ntp_tick_length() value currently being used.
- * This cached copy ensures we consistently apply the tick
- * length for an entire tick, as ntp_tick_length may change
- * mid-tick, and we don't want to apply that new value to
- * the tick in progress.
- */
+
+ ktime_t next_leap_ktime;
u64 ntp_tick;
- /* Difference between accumulated time and NTP time in ntp
- * shifted nano seconds. */
s64 ntp_error;
u32 ntp_error_shift;
u32 ntp_err_mult;
- /* Flag used to avoid updating NTP twice with same second */
u32 skip_second_overflow;
+
#ifdef CONFIG_DEBUG_TIMEKEEPING
long last_warning;
/*
--
2.39.5On Wed, Oct 9, 2024 at 1:29 AM Anna-Maria Behnsen
<anna-maria@linutronix.de> wrote:
>
> From: Thomas Gleixner <tglx@linutronix.de>
>
> From: Thomas Gleixner <tglx@linutronix.de>
>
> struct timekeeper is ordered suboptimal vs. cachelines. The layout,
> including the preceding seqcount (see struct tk_core in timekeeper.c) is:
>
> cacheline 0: seqcount, tkr_mono
> cacheline 1: tkr_raw, xtime_sec
> cacheline 2: ktime_sec ... tai_offset, internal variables
> cacheline 3: next_leap_ktime, raw_sec, internal variables
> cacheline 4: internal variables
>
> So any access to via ktime_get*() except for access to CLOCK_MONOTONIC_RAW
> will use either cachelines 0 + 1 or cachelines 0 + 2. Access to
> CLOCK_MONOTONIC_RAW uses cachelines 0 + 1 + 3.
>
> Reorder the members so that the result is more efficient:
>
> cacheline 0: seqcount, tkr_mono
> cacheline 1: xtime_sec, ktime_sec ... tai_offset
> cacheline 2: tkr_raw, raw_sec
> cacheline 3: internal variables
> cacheline 4: internal variables
>
> That means ktime_get*() will access cacheline 0 + 1 and CLOCK_MONOTONIC_RAW
> access will use cachelines 0 + 2.
>
> Update kernel-doc and fix formatting issues while at it. Also fix a typo
> in struct tk_read_base kernel-doc.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
> ---
> include/linux/timekeeper_internal.h | 102 +++++++++++++++++++++---------------
> 1 file changed, 61 insertions(+), 41 deletions(-)
>
> diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
> index 902c20ef495a..430e40549136 100644
> --- a/include/linux/timekeeper_internal.h
> +++ b/include/linux/timekeeper_internal.h
> @@ -26,7 +26,7 @@
> * occupies a single 64byte cache line.
> *
> * The struct is separate from struct timekeeper as it is also used
> - * for a fast NMI safe accessors.
> + * for the fast NMI safe accessors.
> *
> * @base_real is for the fast NMI safe accessor to allow reading clock
> * realtime from any context.
> @@ -44,33 +44,41 @@ struct tk_read_base {
>
> /**
> * struct timekeeper - Structure holding internal timekeeping values.
> - * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
> - * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
> - * @xtime_sec: Current CLOCK_REALTIME time in seconds
> - * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
> - * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
> - * @offs_real: Offset clock monotonic -> clock realtime
> - * @offs_boot: Offset clock monotonic -> clock boottime
> - * @offs_tai: Offset clock monotonic -> clock tai
> - * @tai_offset: The current UTC to TAI offset in seconds
> - * @clock_was_set_seq: The sequence number of clock was set events
> - * @cs_was_changed_seq: The sequence number of clocksource change events
> - * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
> - * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
> - * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
> - * @cycle_interval: Number of clock cycles in one NTP interval
> - * @xtime_interval: Number of clock shifted nano seconds in one NTP
> - * interval.
> - * @xtime_remainder: Shifted nano seconds left over when rounding
> - * @cycle_interval
> - * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
> - * @ntp_error: Difference between accumulated time and NTP time in ntp
> - * shifted nano seconds.
> - * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
> - * ntp shifted nano seconds.
> - * @last_warning: Warning ratelimiter (DEBUG_TIMEKEEPING)
> - * @underflow_seen: Underflow warning flag (DEBUG_TIMEKEEPING)
> - * @overflow_seen: Overflow warning flag (DEBUG_TIMEKEEPING)
> + * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
> + * @xtime_sec: Current CLOCK_REALTIME time in seconds
> + * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
> + * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
> + * @offs_real: Offset clock monotonic -> clock realtime
> + * @offs_boot: Offset clock monotonic -> clock boottime
> + * @offs_tai: Offset clock monotonic -> clock tai
> + * @tai_offset: The current UTC to TAI offset in seconds
> + * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
> + * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
> + * @clock_was_set_seq: The sequence number of clock was set events
> + * @cs_was_changed_seq: The sequence number of clocksource change events
> + * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
> + * @cycle_interval: Number of clock cycles in one NTP interval
> + * @xtime_interval: Number of clock shifted nano seconds in one NTP
> + * interval.
> + * @xtime_remainder: Shifted nano seconds left over when rounding
> + * @cycle_interval
> + * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
> + * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
> + * @ntp_tick: The ntp_tick_length() value currently being
> + * used. This cached copy ensures we consistently
> + * apply the tick length for an entire tick, as
> + * ntp_tick_length may change mid-tick, and we don't
> + * want to apply that new value to the tick in
> + * progress.
> + * @ntp_error: Difference between accumulated time and NTP time in ntp
> + * shifted nano seconds.
> + * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
> + * ntp shifted nano seconds.
> + * @ntp_err_mult: Multiplication factor for scaled math conversion
> + * @skip_second_overflow: Flag used to avoid updating NTP twice with same second
> + * @last_warning: Warning ratelimiter (DEBUG_TIMEKEEPING)
> + * @underflow_seen: Underflow warning flag (DEBUG_TIMEKEEPING)
> + * @overflow_seen: Overflow warning flag (DEBUG_TIMEKEEPING)
> *
> * Note: For timespec(64) based interfaces wall_to_monotonic is what
> * we need to add to xtime (or xtime corrected for sub jiffy times)
> @@ -88,10 +96,25 @@ struct tk_read_base {
> *
> * @monotonic_to_boottime is a timespec64 representation of @offs_boot to
> * accelerate the VDSO update for CLOCK_BOOTTIME.
> + *
> + * The cacheline ordering of the structure is optimized for in kernel usage
> + * of the ktime_get() and ktime_get_ts64() family of time accessors. Struct
> + * timekeeper is prepended in the core timekeeeping code with a sequence
> + * count, which results in the following cacheline layout:
> + *
> + * 0: seqcount, tkr_mono
> + * 1: xtime_sec ... tai_offset
> + * 2: tkr_raw, raw_sec
> + * 3,4: Internal variables
> + *
> + * Cacheline 0,1 contain the data which is used for accessing
> + * CLOCK_MONOTONIC/REALTIME/BOOTTIME/TAI, while cacheline 2 contains the
> + * data for accessing CLOCK_MONOTONIC_RAW. Cacheline 3,4 are internal
> + * variables which are only accessed during timekeeper updates once per
> + * tick.
Would it make sense to add divider comments or something in the struct
to make this more visible? I fret in the context of a patch, a + line
adding a new structure element that breaks the ordered alignment might
not be obvious.
thanks
-john
John Stultz <jstultz@google.com> writes:
> On Wed, Oct 9, 2024 at 1:29 AM Anna-Maria Behnsen
> <anna-maria@linutronix.de> wrote:
>>
>> From: Thomas Gleixner <tglx@linutronix.de>
>>
>> From: Thomas Gleixner <tglx@linutronix.de>
>>
>> struct timekeeper is ordered suboptimal vs. cachelines. The layout,
>> including the preceding seqcount (see struct tk_core in timekeeper.c) is:
>>
>> cacheline 0: seqcount, tkr_mono
>> cacheline 1: tkr_raw, xtime_sec
>> cacheline 2: ktime_sec ... tai_offset, internal variables
>> cacheline 3: next_leap_ktime, raw_sec, internal variables
>> cacheline 4: internal variables
>>
>> So any access to via ktime_get*() except for access to CLOCK_MONOTONIC_RAW
>> will use either cachelines 0 + 1 or cachelines 0 + 2. Access to
>> CLOCK_MONOTONIC_RAW uses cachelines 0 + 1 + 3.
>>
>> Reorder the members so that the result is more efficient:
>>
>> cacheline 0: seqcount, tkr_mono
>> cacheline 1: xtime_sec, ktime_sec ... tai_offset
>> cacheline 2: tkr_raw, raw_sec
>> cacheline 3: internal variables
>> cacheline 4: internal variables
>>
>> That means ktime_get*() will access cacheline 0 + 1 and CLOCK_MONOTONIC_RAW
>> access will use cachelines 0 + 2.
>>
>> Update kernel-doc and fix formatting issues while at it. Also fix a typo
>> in struct tk_read_base kernel-doc.
>>
>> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
>> Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
>
> Acked-by: John Stultz <jstultz@google.com>
>
>> ---
>> include/linux/timekeeper_internal.h | 102 +++++++++++++++++++++---------------
>> 1 file changed, 61 insertions(+), 41 deletions(-)
>>
>> diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
>> index 902c20ef495a..430e40549136 100644
>> --- a/include/linux/timekeeper_internal.h
>> +++ b/include/linux/timekeeper_internal.h
>> @@ -26,7 +26,7 @@
>> * occupies a single 64byte cache line.
>> *
>> * The struct is separate from struct timekeeper as it is also used
>> - * for a fast NMI safe accessors.
>> + * for the fast NMI safe accessors.
>> *
>> * @base_real is for the fast NMI safe accessor to allow reading clock
>> * realtime from any context.
>> @@ -44,33 +44,41 @@ struct tk_read_base {
>>
>> /**
>> * struct timekeeper - Structure holding internal timekeeping values.
>> - * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
>> - * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
>> - * @xtime_sec: Current CLOCK_REALTIME time in seconds
>> - * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
>> - * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
>> - * @offs_real: Offset clock monotonic -> clock realtime
>> - * @offs_boot: Offset clock monotonic -> clock boottime
>> - * @offs_tai: Offset clock monotonic -> clock tai
>> - * @tai_offset: The current UTC to TAI offset in seconds
>> - * @clock_was_set_seq: The sequence number of clock was set events
>> - * @cs_was_changed_seq: The sequence number of clocksource change events
>> - * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
>> - * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
>> - * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
>> - * @cycle_interval: Number of clock cycles in one NTP interval
>> - * @xtime_interval: Number of clock shifted nano seconds in one NTP
>> - * interval.
>> - * @xtime_remainder: Shifted nano seconds left over when rounding
>> - * @cycle_interval
>> - * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
>> - * @ntp_error: Difference between accumulated time and NTP time in ntp
>> - * shifted nano seconds.
>> - * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
>> - * ntp shifted nano seconds.
>> - * @last_warning: Warning ratelimiter (DEBUG_TIMEKEEPING)
>> - * @underflow_seen: Underflow warning flag (DEBUG_TIMEKEEPING)
>> - * @overflow_seen: Overflow warning flag (DEBUG_TIMEKEEPING)
>> + * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
>> + * @xtime_sec: Current CLOCK_REALTIME time in seconds
>> + * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
>> + * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
>> + * @offs_real: Offset clock monotonic -> clock realtime
>> + * @offs_boot: Offset clock monotonic -> clock boottime
>> + * @offs_tai: Offset clock monotonic -> clock tai
>> + * @tai_offset: The current UTC to TAI offset in seconds
>> + * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
>> + * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
>> + * @clock_was_set_seq: The sequence number of clock was set events
>> + * @cs_was_changed_seq: The sequence number of clocksource change events
>> + * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
>> + * @cycle_interval: Number of clock cycles in one NTP interval
>> + * @xtime_interval: Number of clock shifted nano seconds in one NTP
>> + * interval.
>> + * @xtime_remainder: Shifted nano seconds left over when rounding
>> + * @cycle_interval
>> + * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
>> + * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
>> + * @ntp_tick: The ntp_tick_length() value currently being
>> + * used. This cached copy ensures we consistently
>> + * apply the tick length for an entire tick, as
>> + * ntp_tick_length may change mid-tick, and we don't
>> + * want to apply that new value to the tick in
>> + * progress.
>> + * @ntp_error: Difference between accumulated time and NTP time in ntp
>> + * shifted nano seconds.
>> + * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
>> + * ntp shifted nano seconds.
>> + * @ntp_err_mult: Multiplication factor for scaled math conversion
>> + * @skip_second_overflow: Flag used to avoid updating NTP twice with same second
>> + * @last_warning: Warning ratelimiter (DEBUG_TIMEKEEPING)
>> + * @underflow_seen: Underflow warning flag (DEBUG_TIMEKEEPING)
>> + * @overflow_seen: Overflow warning flag (DEBUG_TIMEKEEPING)
>> *
>> * Note: For timespec(64) based interfaces wall_to_monotonic is what
>> * we need to add to xtime (or xtime corrected for sub jiffy times)
>> @@ -88,10 +96,25 @@ struct tk_read_base {
>> *
>> * @monotonic_to_boottime is a timespec64 representation of @offs_boot to
>> * accelerate the VDSO update for CLOCK_BOOTTIME.
>> + *
>> + * The cacheline ordering of the structure is optimized for in kernel usage
>> + * of the ktime_get() and ktime_get_ts64() family of time accessors. Struct
>> + * timekeeper is prepended in the core timekeeeping code with a sequence
>> + * count, which results in the following cacheline layout:
>> + *
>> + * 0: seqcount, tkr_mono
>> + * 1: xtime_sec ... tai_offset
>> + * 2: tkr_raw, raw_sec
>> + * 3,4: Internal variables
>> + *
>> + * Cacheline 0,1 contain the data which is used for accessing
>> + * CLOCK_MONOTONIC/REALTIME/BOOTTIME/TAI, while cacheline 2 contains the
>> + * data for accessing CLOCK_MONOTONIC_RAW. Cacheline 3,4 are internal
>> + * variables which are only accessed during timekeeper updates once per
>> + * tick.
>
> Would it make sense to add divider comments or something in the struct
> to make this more visible? I fret in the context of a patch, a + line
> adding a new structure element that breaks the ordered alignment might
> not be obvious.
This is an argument! I'll add simple comments with /* Cachline X: */
Thanks,
Anna-Maria
From: Thomas Gleixner <tglx@linutronix.de>
struct timekeeper is ordered suboptimal vs. cachelines. The layout,
including the preceding seqcount (see struct tk_core in timekeeper.c) is:
cacheline 0: seqcount, tkr_mono
cacheline 1: tkr_raw, xtime_sec
cacheline 2: ktime_sec ... tai_offset, internal variables
cacheline 3: next_leap_ktime, raw_sec, internal variables
cacheline 4: internal variables
So any access to via ktime_get*() except for access to CLOCK_MONOTONIC_RAW
will use either cachelines 0 + 1 or cachelines 0 + 2. Access to
CLOCK_MONOTONIC_RAW uses cachelines 0 + 1 + 3.
Reorder the members so that the result is more efficient:
cacheline 0: seqcount, tkr_mono
cacheline 1: xtime_sec, ktime_sec ... tai_offset
cacheline 2: tkr_raw, raw_sec
cacheline 3: internal variables
cacheline 4: internal variables
That means ktime_get*() will access cacheline 0 + 1 and CLOCK_MONOTONIC_RAW
access will use cachelines 0 + 2.
Update kernel-doc and fix formatting issues while at it. Also fix a typo
in struct tk_read_base kernel-doc.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
---
include/linux/timekeeper_internal.h | 106 +++++++++++++++++-----------
1 file changed, 65 insertions(+), 41 deletions(-)
diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
index 902c20ef495a..a3b6380a7777 100644
--- a/include/linux/timekeeper_internal.h
+++ b/include/linux/timekeeper_internal.h
@@ -26,7 +26,7 @@
* occupies a single 64byte cache line.
*
* The struct is separate from struct timekeeper as it is also used
- * for a fast NMI safe accessors.
+ * for the fast NMI safe accessors.
*
* @base_real is for the fast NMI safe accessor to allow reading clock
* realtime from any context.
@@ -44,33 +44,41 @@ struct tk_read_base {
/**
* struct timekeeper - Structure holding internal timekeeping values.
- * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
- * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
- * @xtime_sec: Current CLOCK_REALTIME time in seconds
- * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
- * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
- * @offs_real: Offset clock monotonic -> clock realtime
- * @offs_boot: Offset clock monotonic -> clock boottime
- * @offs_tai: Offset clock monotonic -> clock tai
- * @tai_offset: The current UTC to TAI offset in seconds
- * @clock_was_set_seq: The sequence number of clock was set events
- * @cs_was_changed_seq: The sequence number of clocksource change events
- * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
- * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
- * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
- * @cycle_interval: Number of clock cycles in one NTP interval
- * @xtime_interval: Number of clock shifted nano seconds in one NTP
- * interval.
- * @xtime_remainder: Shifted nano seconds left over when rounding
- * @cycle_interval
- * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
- * @ntp_error: Difference between accumulated time and NTP time in ntp
- * shifted nano seconds.
- * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
- * ntp shifted nano seconds.
- * @last_warning: Warning ratelimiter (DEBUG_TIMEKEEPING)
- * @underflow_seen: Underflow warning flag (DEBUG_TIMEKEEPING)
- * @overflow_seen: Overflow warning flag (DEBUG_TIMEKEEPING)
+ * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
+ * @xtime_sec: Current CLOCK_REALTIME time in seconds
+ * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
+ * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
+ * @offs_real: Offset clock monotonic -> clock realtime
+ * @offs_boot: Offset clock monotonic -> clock boottime
+ * @offs_tai: Offset clock monotonic -> clock tai
+ * @tai_offset: The current UTC to TAI offset in seconds
+ * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
+ * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
+ * @clock_was_set_seq: The sequence number of clock was set events
+ * @cs_was_changed_seq: The sequence number of clocksource change events
+ * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
+ * @cycle_interval: Number of clock cycles in one NTP interval
+ * @xtime_interval: Number of clock shifted nano seconds in one NTP
+ * interval.
+ * @xtime_remainder: Shifted nano seconds left over when rounding
+ * @cycle_interval
+ * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
+ * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
+ * @ntp_tick: The ntp_tick_length() value currently being
+ * used. This cached copy ensures we consistently
+ * apply the tick length for an entire tick, as
+ * ntp_tick_length may change mid-tick, and we don't
+ * want to apply that new value to the tick in
+ * progress.
+ * @ntp_error: Difference between accumulated time and NTP time in ntp
+ * shifted nano seconds.
+ * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
+ * ntp shifted nano seconds.
+ * @ntp_err_mult: Multiplication factor for scaled math conversion
+ * @skip_second_overflow: Flag used to avoid updating NTP twice with same second
+ * @last_warning: Warning ratelimiter (DEBUG_TIMEKEEPING)
+ * @underflow_seen: Underflow warning flag (DEBUG_TIMEKEEPING)
+ * @overflow_seen: Overflow warning flag (DEBUG_TIMEKEEPING)
*
* Note: For timespec(64) based interfaces wall_to_monotonic is what
* we need to add to xtime (or xtime corrected for sub jiffy times)
@@ -88,10 +96,28 @@ struct tk_read_base {
*
* @monotonic_to_boottime is a timespec64 representation of @offs_boot to
* accelerate the VDSO update for CLOCK_BOOTTIME.
+ *
+ * The cacheline ordering of the structure is optimized for in kernel usage of
+ * the ktime_get() and ktime_get_ts64() family of time accessors. Struct
+ * timekeeper is prepended in the core timekeeping code with a sequence count,
+ * which results in the following cacheline layout:
+ *
+ * 0: seqcount, tkr_mono
+ * 1: xtime_sec ... tai_offset
+ * 2: tkr_raw, raw_sec
+ * 3,4: Internal variables
+ *
+ * Cacheline 0,1 contain the data which is used for accessing
+ * CLOCK_MONOTONIC/REALTIME/BOOTTIME/TAI, while cacheline 2 contains the
+ * data for accessing CLOCK_MONOTONIC_RAW. Cacheline 3,4 are internal
+ * variables which are only accessed during timekeeper updates once per
+ * tick.
*/
struct timekeeper {
+ /* Cacheline 0 (together with prepended seqcount of timekeeper core): */
struct tk_read_base tkr_mono;
- struct tk_read_base tkr_raw;
+
+ /* Cacheline 1: */
u64 xtime_sec;
unsigned long ktime_sec;
struct timespec64 wall_to_monotonic;
@@ -99,31 +125,29 @@ struct timekeeper {
ktime_t offs_boot;
ktime_t offs_tai;
s32 tai_offset;
+
+ /* Cacheline 2: */
+ struct tk_read_base tkr_raw;
+ u64 raw_sec;
+
+ /* Cachline 3 and 4 (timekeeping internal variables): */
unsigned int clock_was_set_seq;
u8 cs_was_changed_seq;
- ktime_t next_leap_ktime;
- u64 raw_sec;
+
struct timespec64 monotonic_to_boot;
- /* The following members are for timekeeping internal use */
u64 cycle_interval;
u64 xtime_interval;
s64 xtime_remainder;
u64 raw_interval;
- /* The ntp_tick_length() value currently being used.
- * This cached copy ensures we consistently apply the tick
- * length for an entire tick, as ntp_tick_length may change
- * mid-tick, and we don't want to apply that new value to
- * the tick in progress.
- */
+
+ ktime_t next_leap_ktime;
u64 ntp_tick;
- /* Difference between accumulated time and NTP time in ntp
- * shifted nano seconds. */
s64 ntp_error;
u32 ntp_error_shift;
u32 ntp_err_mult;
- /* Flag used to avoid updating NTP twice with same second */
u32 skip_second_overflow;
+
#ifdef CONFIG_DEBUG_TIMEKEEPING
long last_warning;
/*
--
2.39.5The following commit has been merged into the timers/core branch of tip:
Commit-ID: 6860d28ccb2390b4eeda32ab2ce7eb10f71921e1
Gitweb: https://git.kernel.org/tip/6860d28ccb2390b4eeda32ab2ce7eb10f71921e1
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Tue, 15 Oct 2024 12:08:39 +02:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Fri, 25 Oct 2024 19:49:13 +02:00
timekeeping: Reorder struct timekeeper
struct timekeeper is ordered suboptimal vs. cachelines. The layout,
including the preceding seqcount (see struct tk_core in timekeeper.c) is:
cacheline 0: seqcount, tkr_mono
cacheline 1: tkr_raw, xtime_sec
cacheline 2: ktime_sec ... tai_offset, internal variables
cacheline 3: next_leap_ktime, raw_sec, internal variables
cacheline 4: internal variables
So any access to via ktime_get*() except for access to CLOCK_MONOTONIC_RAW
will use either cachelines 0 + 1 or cachelines 0 + 2. Access to
CLOCK_MONOTONIC_RAW uses cachelines 0 + 1 + 3.
Reorder the members so that the result is more efficient:
cacheline 0: seqcount, tkr_mono
cacheline 1: xtime_sec, ktime_sec ... tai_offset
cacheline 2: tkr_raw, raw_sec
cacheline 3: internal variables
cacheline 4: internal variables
That means ktime_get*() will access cacheline 0 + 1 and CLOCK_MONOTONIC_RAW
access will use cachelines 0 + 2.
Update kernel-doc and fix formatting issues while at it. Also fix a typo
in struct tk_read_base kernel-doc.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/all/20241015100839.12702-1-anna-maria@linutronix.de
---
include/linux/timekeeper_internal.h | 106 ++++++++++++++++-----------
1 file changed, 65 insertions(+), 41 deletions(-)
diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
index 902c20e..a3b6380 100644
--- a/include/linux/timekeeper_internal.h
+++ b/include/linux/timekeeper_internal.h
@@ -26,7 +26,7 @@
* occupies a single 64byte cache line.
*
* The struct is separate from struct timekeeper as it is also used
- * for a fast NMI safe accessors.
+ * for the fast NMI safe accessors.
*
* @base_real is for the fast NMI safe accessor to allow reading clock
* realtime from any context.
@@ -44,33 +44,41 @@ struct tk_read_base {
/**
* struct timekeeper - Structure holding internal timekeeping values.
- * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
- * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
- * @xtime_sec: Current CLOCK_REALTIME time in seconds
- * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
- * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
- * @offs_real: Offset clock monotonic -> clock realtime
- * @offs_boot: Offset clock monotonic -> clock boottime
- * @offs_tai: Offset clock monotonic -> clock tai
- * @tai_offset: The current UTC to TAI offset in seconds
- * @clock_was_set_seq: The sequence number of clock was set events
- * @cs_was_changed_seq: The sequence number of clocksource change events
- * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
- * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
- * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
- * @cycle_interval: Number of clock cycles in one NTP interval
- * @xtime_interval: Number of clock shifted nano seconds in one NTP
- * interval.
- * @xtime_remainder: Shifted nano seconds left over when rounding
- * @cycle_interval
- * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
- * @ntp_error: Difference between accumulated time and NTP time in ntp
- * shifted nano seconds.
- * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
- * ntp shifted nano seconds.
- * @last_warning: Warning ratelimiter (DEBUG_TIMEKEEPING)
- * @underflow_seen: Underflow warning flag (DEBUG_TIMEKEEPING)
- * @overflow_seen: Overflow warning flag (DEBUG_TIMEKEEPING)
+ * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
+ * @xtime_sec: Current CLOCK_REALTIME time in seconds
+ * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
+ * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
+ * @offs_real: Offset clock monotonic -> clock realtime
+ * @offs_boot: Offset clock monotonic -> clock boottime
+ * @offs_tai: Offset clock monotonic -> clock tai
+ * @tai_offset: The current UTC to TAI offset in seconds
+ * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
+ * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
+ * @clock_was_set_seq: The sequence number of clock was set events
+ * @cs_was_changed_seq: The sequence number of clocksource change events
+ * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
+ * @cycle_interval: Number of clock cycles in one NTP interval
+ * @xtime_interval: Number of clock shifted nano seconds in one NTP
+ * interval.
+ * @xtime_remainder: Shifted nano seconds left over when rounding
+ * @cycle_interval
+ * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
+ * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
+ * @ntp_tick: The ntp_tick_length() value currently being
+ * used. This cached copy ensures we consistently
+ * apply the tick length for an entire tick, as
+ * ntp_tick_length may change mid-tick, and we don't
+ * want to apply that new value to the tick in
+ * progress.
+ * @ntp_error: Difference between accumulated time and NTP time in ntp
+ * shifted nano seconds.
+ * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
+ * ntp shifted nano seconds.
+ * @ntp_err_mult: Multiplication factor for scaled math conversion
+ * @skip_second_overflow: Flag used to avoid updating NTP twice with same second
+ * @last_warning: Warning ratelimiter (DEBUG_TIMEKEEPING)
+ * @underflow_seen: Underflow warning flag (DEBUG_TIMEKEEPING)
+ * @overflow_seen: Overflow warning flag (DEBUG_TIMEKEEPING)
*
* Note: For timespec(64) based interfaces wall_to_monotonic is what
* we need to add to xtime (or xtime corrected for sub jiffy times)
@@ -88,10 +96,28 @@ struct tk_read_base {
*
* @monotonic_to_boottime is a timespec64 representation of @offs_boot to
* accelerate the VDSO update for CLOCK_BOOTTIME.
+ *
+ * The cacheline ordering of the structure is optimized for in kernel usage of
+ * the ktime_get() and ktime_get_ts64() family of time accessors. Struct
+ * timekeeper is prepended in the core timekeeping code with a sequence count,
+ * which results in the following cacheline layout:
+ *
+ * 0: seqcount, tkr_mono
+ * 1: xtime_sec ... tai_offset
+ * 2: tkr_raw, raw_sec
+ * 3,4: Internal variables
+ *
+ * Cacheline 0,1 contain the data which is used for accessing
+ * CLOCK_MONOTONIC/REALTIME/BOOTTIME/TAI, while cacheline 2 contains the
+ * data for accessing CLOCK_MONOTONIC_RAW. Cacheline 3,4 are internal
+ * variables which are only accessed during timekeeper updates once per
+ * tick.
*/
struct timekeeper {
+ /* Cacheline 0 (together with prepended seqcount of timekeeper core): */
struct tk_read_base tkr_mono;
- struct tk_read_base tkr_raw;
+
+ /* Cacheline 1: */
u64 xtime_sec;
unsigned long ktime_sec;
struct timespec64 wall_to_monotonic;
@@ -99,31 +125,29 @@ struct timekeeper {
ktime_t offs_boot;
ktime_t offs_tai;
s32 tai_offset;
+
+ /* Cacheline 2: */
+ struct tk_read_base tkr_raw;
+ u64 raw_sec;
+
+ /* Cachline 3 and 4 (timekeeping internal variables): */
unsigned int clock_was_set_seq;
u8 cs_was_changed_seq;
- ktime_t next_leap_ktime;
- u64 raw_sec;
+
struct timespec64 monotonic_to_boot;
- /* The following members are for timekeeping internal use */
u64 cycle_interval;
u64 xtime_interval;
s64 xtime_remainder;
u64 raw_interval;
- /* The ntp_tick_length() value currently being used.
- * This cached copy ensures we consistently apply the tick
- * length for an entire tick, as ntp_tick_length may change
- * mid-tick, and we don't want to apply that new value to
- * the tick in progress.
- */
+
+ ktime_t next_leap_ktime;
u64 ntp_tick;
- /* Difference between accumulated time and NTP time in ntp
- * shifted nano seconds. */
s64 ntp_error;
u32 ntp_error_shift;
u32 ntp_err_mult;
- /* Flag used to avoid updating NTP twice with same second */
u32 skip_second_overflow;
+
#ifdef CONFIG_DEBUG_TIMEKEEPING
long last_warning;
/*The following commit has been merged into the timers/core branch of tip:
Commit-ID: 3b7b1f9c33bc25ace98ee5be766d0c8b23332c14
Gitweb: https://git.kernel.org/tip/3b7b1f9c33bc25ace98ee5be766d0c8b23332c14
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Tue, 15 Oct 2024 12:08:39 +02:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Fri, 25 Oct 2024 16:41:11 +02:00
timekeeping: Reorder struct timekeeper
struct timekeeper is ordered suboptimal vs. cachelines. The layout,
including the preceding seqcount (see struct tk_core in timekeeper.c) is:
cacheline 0: seqcount, tkr_mono
cacheline 1: tkr_raw, xtime_sec
cacheline 2: ktime_sec ... tai_offset, internal variables
cacheline 3: next_leap_ktime, raw_sec, internal variables
cacheline 4: internal variables
So any access to via ktime_get*() except for access to CLOCK_MONOTONIC_RAW
will use either cachelines 0 + 1 or cachelines 0 + 2. Access to
CLOCK_MONOTONIC_RAW uses cachelines 0 + 1 + 3.
Reorder the members so that the result is more efficient:
cacheline 0: seqcount, tkr_mono
cacheline 1: xtime_sec, ktime_sec ... tai_offset
cacheline 2: tkr_raw, raw_sec
cacheline 3: internal variables
cacheline 4: internal variables
That means ktime_get*() will access cacheline 0 + 1 and CLOCK_MONOTONIC_RAW
access will use cachelines 0 + 2.
Update kernel-doc and fix formatting issues while at it. Also fix a typo
in struct tk_read_base kernel-doc.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/all/20241015100839.12702-1-anna-maria@linutronix.de
---
include/linux/timekeeper_internal.h | 106 ++++++++++++++++-----------
1 file changed, 65 insertions(+), 41 deletions(-)
diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
index 902c20e..a3b6380 100644
--- a/include/linux/timekeeper_internal.h
+++ b/include/linux/timekeeper_internal.h
@@ -26,7 +26,7 @@
* occupies a single 64byte cache line.
*
* The struct is separate from struct timekeeper as it is also used
- * for a fast NMI safe accessors.
+ * for the fast NMI safe accessors.
*
* @base_real is for the fast NMI safe accessor to allow reading clock
* realtime from any context.
@@ -44,33 +44,41 @@ struct tk_read_base {
/**
* struct timekeeper - Structure holding internal timekeeping values.
- * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
- * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
- * @xtime_sec: Current CLOCK_REALTIME time in seconds
- * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
- * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
- * @offs_real: Offset clock monotonic -> clock realtime
- * @offs_boot: Offset clock monotonic -> clock boottime
- * @offs_tai: Offset clock monotonic -> clock tai
- * @tai_offset: The current UTC to TAI offset in seconds
- * @clock_was_set_seq: The sequence number of clock was set events
- * @cs_was_changed_seq: The sequence number of clocksource change events
- * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
- * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
- * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
- * @cycle_interval: Number of clock cycles in one NTP interval
- * @xtime_interval: Number of clock shifted nano seconds in one NTP
- * interval.
- * @xtime_remainder: Shifted nano seconds left over when rounding
- * @cycle_interval
- * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
- * @ntp_error: Difference between accumulated time and NTP time in ntp
- * shifted nano seconds.
- * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
- * ntp shifted nano seconds.
- * @last_warning: Warning ratelimiter (DEBUG_TIMEKEEPING)
- * @underflow_seen: Underflow warning flag (DEBUG_TIMEKEEPING)
- * @overflow_seen: Overflow warning flag (DEBUG_TIMEKEEPING)
+ * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
+ * @xtime_sec: Current CLOCK_REALTIME time in seconds
+ * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
+ * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
+ * @offs_real: Offset clock monotonic -> clock realtime
+ * @offs_boot: Offset clock monotonic -> clock boottime
+ * @offs_tai: Offset clock monotonic -> clock tai
+ * @tai_offset: The current UTC to TAI offset in seconds
+ * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
+ * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
+ * @clock_was_set_seq: The sequence number of clock was set events
+ * @cs_was_changed_seq: The sequence number of clocksource change events
+ * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
+ * @cycle_interval: Number of clock cycles in one NTP interval
+ * @xtime_interval: Number of clock shifted nano seconds in one NTP
+ * interval.
+ * @xtime_remainder: Shifted nano seconds left over when rounding
+ * @cycle_interval
+ * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
+ * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
+ * @ntp_tick: The ntp_tick_length() value currently being
+ * used. This cached copy ensures we consistently
+ * apply the tick length for an entire tick, as
+ * ntp_tick_length may change mid-tick, and we don't
+ * want to apply that new value to the tick in
+ * progress.
+ * @ntp_error: Difference between accumulated time and NTP time in ntp
+ * shifted nano seconds.
+ * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
+ * ntp shifted nano seconds.
+ * @ntp_err_mult: Multiplication factor for scaled math conversion
+ * @skip_second_overflow: Flag used to avoid updating NTP twice with same second
+ * @last_warning: Warning ratelimiter (DEBUG_TIMEKEEPING)
+ * @underflow_seen: Underflow warning flag (DEBUG_TIMEKEEPING)
+ * @overflow_seen: Overflow warning flag (DEBUG_TIMEKEEPING)
*
* Note: For timespec(64) based interfaces wall_to_monotonic is what
* we need to add to xtime (or xtime corrected for sub jiffy times)
@@ -88,10 +96,28 @@ struct tk_read_base {
*
* @monotonic_to_boottime is a timespec64 representation of @offs_boot to
* accelerate the VDSO update for CLOCK_BOOTTIME.
+ *
+ * The cacheline ordering of the structure is optimized for in kernel usage of
+ * the ktime_get() and ktime_get_ts64() family of time accessors. Struct
+ * timekeeper is prepended in the core timekeeping code with a sequence count,
+ * which results in the following cacheline layout:
+ *
+ * 0: seqcount, tkr_mono
+ * 1: xtime_sec ... tai_offset
+ * 2: tkr_raw, raw_sec
+ * 3,4: Internal variables
+ *
+ * Cacheline 0,1 contain the data which is used for accessing
+ * CLOCK_MONOTONIC/REALTIME/BOOTTIME/TAI, while cacheline 2 contains the
+ * data for accessing CLOCK_MONOTONIC_RAW. Cacheline 3,4 are internal
+ * variables which are only accessed during timekeeper updates once per
+ * tick.
*/
struct timekeeper {
+ /* Cacheline 0 (together with prepended seqcount of timekeeper core): */
struct tk_read_base tkr_mono;
- struct tk_read_base tkr_raw;
+
+ /* Cacheline 1: */
u64 xtime_sec;
unsigned long ktime_sec;
struct timespec64 wall_to_monotonic;
@@ -99,31 +125,29 @@ struct timekeeper {
ktime_t offs_boot;
ktime_t offs_tai;
s32 tai_offset;
+
+ /* Cacheline 2: */
+ struct tk_read_base tkr_raw;
+ u64 raw_sec;
+
+ /* Cachline 3 and 4 (timekeeping internal variables): */
unsigned int clock_was_set_seq;
u8 cs_was_changed_seq;
- ktime_t next_leap_ktime;
- u64 raw_sec;
+
struct timespec64 monotonic_to_boot;
- /* The following members are for timekeeping internal use */
u64 cycle_interval;
u64 xtime_interval;
s64 xtime_remainder;
u64 raw_interval;
- /* The ntp_tick_length() value currently being used.
- * This cached copy ensures we consistently apply the tick
- * length for an entire tick, as ntp_tick_length may change
- * mid-tick, and we don't want to apply that new value to
- * the tick in progress.
- */
+
+ ktime_t next_leap_ktime;
u64 ntp_tick;
- /* Difference between accumulated time and NTP time in ntp
- * shifted nano seconds. */
s64 ntp_error;
u32 ntp_error_shift;
u32 ntp_err_mult;
- /* Flag used to avoid updating NTP twice with same second */
u32 skip_second_overflow;
+
#ifdef CONFIG_DEBUG_TIMEKEEPING
long last_warning;
/*© 2016 - 2026 Red Hat, Inc.