From: Anna-Maria Behnsen <anna-maria@linutronix.de>

Architecture related vdso data is required in fastpath when acquiring
CLOCK_MONOTONIC or CLOCK_REALTIME. At the moment, this information is
located at the end of the vdso_time_data structure. The whole structure has
to be loaded into cache to be able to access this information.

To minimize the number of required cachelines, the architecture specific
vdso data struct is moved right before the basetime (basetime information
is required anyway). This change does not have an impact on architectures
with CONFIG_ARCH_HAS_VDSO_DATA=n. All other architectures could spare
reading unnecessary cachelines.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 include/vdso/datapage.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/vdso/datapage.h b/include/vdso/datapage.h
index 1df22e8bb9b31153546b72b1e8b8c8aeaed7d9e3..bcd19c223783be7c22f90120330e7dddd0496f1a 100644
--- a/include/vdso/datapage.h
+++ b/include/vdso/datapage.h
@@ -70,6 +70,8 @@ struct vdso_timestamp {
 
 /**
  * struct vdso_time_data - vdso datapage representation
+ * @arch_data:		architecture specific data (optional, defaults
+ *			to an empty struct)
  * @seq:		timebase sequence counter
  * @clock_mode:		clock mode
  * @cycle_last:		timebase at clocksource init
@@ -83,8 +85,6 @@ struct vdso_timestamp {
  * @tz_dsttime:		type of DST correction
  * @hrtimer_res:	hrtimer resolution
  * @__unused:		unused
- * @arch_data:		architecture specific data (optional, defaults
- *			to an empty struct)
  *
  * vdso_time_data will be accessed by 64 bit and compat code at the same time
  * so we should be careful before modifying this structure.
@@ -105,6 +105,8 @@ struct vdso_timestamp {
  * offset must be zero.
  */
 struct vdso_time_data {
+	struct arch_vdso_time_data arch_data;
+
 	u32			seq;
 
 	s32			clock_mode;
@@ -125,8 +127,6 @@ struct vdso_time_data {
 	s32			tz_dsttime;
 	u32			hrtimer_res;
 	u32			__unused;
-
-	struct arch_vdso_time_data arch_data;
 } ____cacheline_aligned;
 
 #define vdso_clock vdso_time_data

-- 
2.48.1

The following commit has been merged into the timers/vdso branch of tip:

Commit-ID:     97a5a90ca234eaa3de8a6aa44d43de2827393019
Gitweb:        https://git.kernel.org/tip/97a5a90ca234eaa3de8a6aa44d43de2827393019
Author:        Anna-Maria Behnsen <anna-maria@linutronix.de>
AuthorDate:    Mon, 03 Mar 2025 12:11:20 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Sat, 08 Mar 2025 14:37:41 +01:00

vdso: Move architecture related data before basetime data

Architecture related vdso data is required in the fast path when reading
CLOCK_MONOTONIC or CLOCK_REALTIME. At the moment, this information is
located at the end of the vdso_time_data structure, which is a suboptimal
cache layout.

Move the architecture specific VDSO data right before the basetime
information, which is always required. This change does not have an impact
on architectures with CONFIG_ARCH_HAS_VDSO_DATA=n. Architectures, which
have it enabled, gain a better cache layout.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250303-vdso-clock-v1-18-c1b5c69a166f@linutronix.de

---
 include/vdso/datapage.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/vdso/datapage.h b/include/vdso/datapage.h
index 1df22e8..bcd19c2 100644
--- a/include/vdso/datapage.h
+++ b/include/vdso/datapage.h
@@ -70,6 +70,8 @@ struct vdso_timestamp {
 
 /**
  * struct vdso_time_data - vdso datapage representation
+ * @arch_data:		architecture specific data (optional, defaults
+ *			to an empty struct)
  * @seq:		timebase sequence counter
  * @clock_mode:		clock mode
  * @cycle_last:		timebase at clocksource init
@@ -83,8 +85,6 @@ struct vdso_timestamp {
  * @tz_dsttime:		type of DST correction
  * @hrtimer_res:	hrtimer resolution
  * @__unused:		unused
- * @arch_data:		architecture specific data (optional, defaults
- *			to an empty struct)
  *
  * vdso_time_data will be accessed by 64 bit and compat code at the same time
  * so we should be careful before modifying this structure.
@@ -105,6 +105,8 @@ struct vdso_timestamp {
  * offset must be zero.
  */
 struct vdso_time_data {
+	struct arch_vdso_time_data arch_data;
+
 	u32			seq;
 
 	s32			clock_mode;
@@ -125,8 +127,6 @@ struct vdso_time_data {
 	s32			tz_dsttime;
 	u32			hrtimer_res;
 	u32			__unused;
-
-	struct arch_vdso_time_data arch_data;
 } ____cacheline_aligned;
 
 #define vdso_clock vdso_time_data