sched/isolation: fix boot crash when maxcpus < first-housekeeping-cpu

[PATCH] sched/isolation: fix boot crash when maxcpus < first-housekeeping-cpu

Posted by Oleg Nesterov 1 year, 10 months ago

housekeeping_setup() checks cpumask_intersects(present, online) to ensure
that the kernel will have at least one housekeeping CPU after smp_init(),
but this doesn't work if the maxcpus= kernel parameter limits the number
of processors available after bootup.

For example, the kernel with "maxcpus=2 nohz_full=0-2" parameters crashes
at boot time on my virtual machine with 4 CPUs.

Change housekeeping_setup() to use cpumask_first_and() and check that the
returned cpu number is valid and less than setup_max_cpus.

Another corner case is "nohz_full=0" on a machine with a single CPU or
with the maxcpus=1 kernel argument. In this case non_housekeeping_mask
is empty and IIUC tick_nohz_full_setup() makes no sense. And indeed, the
kernel hits the WARN_ON(tick_nohz_full_running) in tick_sched_do_timer().

And how should the kernel interpret the "nohz_full=" parameter? I think
it should be silently ignored, but currently cpulist_parse() happily
returns the empty cpumask and this leads to the same problem.

Change housekeeping_setup() to check cpumask_empty(non_housekeeping_mask)
and do nothing in this case.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/sched/isolation.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 2a262d3ecb3d..5891e715f00d 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -118,6 +118,7 @@ static void __init housekeeping_setup_type(enum hk_type type,
 static int __init housekeeping_setup(char *str, unsigned long flags)
 {
 	cpumask_var_t non_housekeeping_mask, housekeeping_staging;
+	unsigned int first_cpu;
 	int err = 0;
 
 	if ((flags & HK_FLAG_TICK) && !(housekeeping.flags & HK_FLAG_TICK)) {
@@ -138,7 +139,8 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
 	cpumask_andnot(housekeeping_staging,
 		       cpu_possible_mask, non_housekeeping_mask);
 
-	if (!cpumask_intersects(cpu_present_mask, housekeeping_staging)) {
+	first_cpu = cpumask_first_and(cpu_present_mask, housekeeping_staging);
+	if (first_cpu >= nr_cpu_ids || first_cpu >= setup_max_cpus) {
 		__cpumask_set_cpu(smp_processor_id(), housekeeping_staging);
 		__cpumask_clear_cpu(smp_processor_id(), non_housekeeping_mask);
 		if (!housekeeping.flags) {
@@ -147,6 +149,9 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
 		}
 	}
 
+	if (cpumask_empty(non_housekeeping_mask))
+		goto free_housekeeping_staging;
+
 	if (!housekeeping.flags) {
 		/* First setup call ("nohz_full=" or "isolcpus=") */
 		enum hk_type type;
-- 
2.25.1.362.g51ebf55

Re: [PATCH] sched/isolation: fix boot crash when maxcpus < first-housekeeping-cpu

Posted by Frederic Weisbecker 1 year, 9 months ago

Le Sat, Apr 13, 2024 at 04:17:46PM +0200, Oleg Nesterov a écrit :
> housekeeping_setup() checks cpumask_intersects(present, online) to ensure
> that the kernel will have at least one housekeeping CPU after smp_init(),
> but this doesn't work if the maxcpus= kernel parameter limits the number
> of processors available after bootup.
> 
> For example, the kernel with "maxcpus=2 nohz_full=0-2" parameters crashes
> at boot time on my virtual machine with 4 CPUs.
> 
> Change housekeeping_setup() to use cpumask_first_and() and check that the
> returned cpu number is valid and less than setup_max_cpus.
> 
> Another corner case is "nohz_full=0" on a machine with a single CPU or
> with the maxcpus=1 kernel argument. In this case non_housekeeping_mask
> is empty and IIUC tick_nohz_full_setup() makes no sense. And indeed, the
> kernel hits the WARN_ON(tick_nohz_full_running) in tick_sched_do_timer().
> 
> And how should the kernel interpret the "nohz_full=" parameter? I think
> it should be silently ignored, but currently cpulist_parse() happily
> returns the empty cpumask and this leads to the same problem.
> 
> Change housekeeping_setup() to check cpumask_empty(non_housekeeping_mask)
> and do nothing in this case.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Acked-by: Frederic Weisbecker <frederic@kernel.org>

Re: [PATCH] sched/isolation: fix boot crash when maxcpus < first-housekeeping-cpu

Posted by Phil Auld 1 year, 9 months ago

On Sat, Apr 13, 2024 at 04:17:46PM +0200 Oleg Nesterov wrote:
> housekeeping_setup() checks cpumask_intersects(present, online) to ensure
> that the kernel will have at least one housekeeping CPU after smp_init(),
> but this doesn't work if the maxcpus= kernel parameter limits the number
> of processors available after bootup.
> 
> For example, the kernel with "maxcpus=2 nohz_full=0-2" parameters crashes
> at boot time on my virtual machine with 4 CPUs.
> 
> Change housekeeping_setup() to use cpumask_first_and() and check that the
> returned cpu number is valid and less than setup_max_cpus.
> 
> Another corner case is "nohz_full=0" on a machine with a single CPU or
> with the maxcpus=1 kernel argument. In this case non_housekeeping_mask
> is empty and IIUC tick_nohz_full_setup() makes no sense. And indeed, the
> kernel hits the WARN_ON(tick_nohz_full_running) in tick_sched_do_timer().
> 
> And how should the kernel interpret the "nohz_full=" parameter? I think
> it should be silently ignored, but currently cpulist_parse() happily
> returns the empty cpumask and this leads to the same problem.
> 
> Change housekeeping_setup() to check cpumask_empty(non_housekeeping_mask)
> and do nothing in this case.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Looks good to me. One less footgun. 

Reviewed-by: Phil Auld <pauld@redhat.com>

> ---
>  kernel/sched/isolation.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 2a262d3ecb3d..5891e715f00d 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -118,6 +118,7 @@ static void __init housekeeping_setup_type(enum hk_type type,
>  static int __init housekeeping_setup(char *str, unsigned long flags)
>  {
>  	cpumask_var_t non_housekeeping_mask, housekeeping_staging;
> +	unsigned int first_cpu;
>  	int err = 0;
>  
>  	if ((flags & HK_FLAG_TICK) && !(housekeeping.flags & HK_FLAG_TICK)) {
> @@ -138,7 +139,8 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
>  	cpumask_andnot(housekeeping_staging,
>  		       cpu_possible_mask, non_housekeeping_mask);
>  
> -	if (!cpumask_intersects(cpu_present_mask, housekeeping_staging)) {
> +	first_cpu = cpumask_first_and(cpu_present_mask, housekeeping_staging);
> +	if (first_cpu >= nr_cpu_ids || first_cpu >= setup_max_cpus) {
>  		__cpumask_set_cpu(smp_processor_id(), housekeeping_staging);
>  		__cpumask_clear_cpu(smp_processor_id(), non_housekeeping_mask);
>  		if (!housekeeping.flags) {
> @@ -147,6 +149,9 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
>  		}
>  	}
>  
> +	if (cpumask_empty(non_housekeeping_mask))
> +		goto free_housekeeping_staging;
> +
>  	if (!housekeeping.flags) {
>  		/* First setup call ("nohz_full=" or "isolcpus=") */
>  		enum hk_type type;
> -- 
> 2.25.1.362.g51ebf55
> 
> 
> 

--

[tip: sched/urgent] sched/isolation: Fix boot crash when maxcpus < first housekeeping CPU

Posted by tip-bot2 for Oleg Nesterov 1 year, 9 months ago

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     b6ad00418eaf376b4f2a68a1696d6368c1381310
Gitweb:        https://git.kernel.org/tip/b6ad00418eaf376b4f2a68a1696d6368c1381310
Author:        Oleg Nesterov <oleg@redhat.com>
AuthorDate:    Sat, 13 Apr 2024 16:17:46 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Wed, 24 Apr 2024 21:53:34 +02:00

sched/isolation: Fix boot crash when maxcpus < first housekeeping CPU

housekeeping_setup() checks cpumask_intersects(present, online) to ensure
that the kernel will have at least one housekeeping CPU after smp_init(),
but this doesn't work if the maxcpus= kernel parameter limits the number of
processors available after bootup.

For example, a kernel with "maxcpus=2 nohz_full=0-2" parameters crashes at
boot time on a virtual machine with 4 CPUs.

Change housekeeping_setup() to use cpumask_first_and() and check that the
returned CPU number is valid and less than setup_max_cpus.

Another corner case is "nohz_full=0" on a machine with a single CPU or with
the maxcpus=1 kernel argument. In this case non_housekeeping_mask is empty
and tick_nohz_full_setup() makes no sense. And indeed, the kernel hits the
WARN_ON(tick_nohz_full_running) in tick_sched_do_timer().

And how should the kernel interpret the "nohz_full=" parameter? It should
be silently ignored, but currently cpulist_parse() happily returns the
empty cpumask and this leads to the same problem.

Change housekeeping_setup() to check cpumask_empty(non_housekeeping_mask)
and do nothing in this case.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Phil Auld <pauld@redhat.com>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20240413141746.GA10008@redhat.com
---
 kernel/sched/isolation.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 2a262d3..5891e71 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -118,6 +118,7 @@ static void __init housekeeping_setup_type(enum hk_type type,
 static int __init housekeeping_setup(char *str, unsigned long flags)
 {
 	cpumask_var_t non_housekeeping_mask, housekeeping_staging;
+	unsigned int first_cpu;
 	int err = 0;
 
 	if ((flags & HK_FLAG_TICK) && !(housekeeping.flags & HK_FLAG_TICK)) {
@@ -138,7 +139,8 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
 	cpumask_andnot(housekeeping_staging,
 		       cpu_possible_mask, non_housekeeping_mask);
 
-	if (!cpumask_intersects(cpu_present_mask, housekeeping_staging)) {
+	first_cpu = cpumask_first_and(cpu_present_mask, housekeeping_staging);
+	if (first_cpu >= nr_cpu_ids || first_cpu >= setup_max_cpus) {
 		__cpumask_set_cpu(smp_processor_id(), housekeeping_staging);
 		__cpumask_clear_cpu(smp_processor_id(), non_housekeeping_mask);
 		if (!housekeeping.flags) {
@@ -147,6 +149,9 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
 		}
 	}
 
+	if (cpumask_empty(non_housekeeping_mask))
+		goto free_housekeeping_staging;
+
 	if (!housekeeping.flags) {
 		/* First setup call ("nohz_full=" or "isolcpus=") */
 		enum hk_type type;

Re: [tip: sched/urgent] sched/isolation: Fix boot crash when maxcpus < first housekeeping CPU

Posted by Ingo Molnar 1 year, 9 months ago

* tip-bot2 for Oleg Nesterov <tip-bot2@linutronix.de> wrote:

> Another corner case is "nohz_full=0" on a machine with a single CPU or with
> the maxcpus=1 kernel argument. In this case non_housekeeping_mask is empty
> and tick_nohz_full_setup() makes no sense. And indeed, the kernel hits the
> WARN_ON(tick_nohz_full_running) in tick_sched_do_timer().
> 
> And how should the kernel interpret the "nohz_full=" parameter? It should
> be silently ignored, but currently cpulist_parse() happily returns the
> empty cpumask and this leads to the same problem.
> 
> Change housekeeping_setup() to check cpumask_empty(non_housekeeping_mask)
> and do nothing in this case.

So arguably the user meant NOHZ_FULL to be turned off - but it is de-facto 
already turned off by the fact that there's only a single CPU available, 
right?

Thanks,

	Ingo

Re: [tip: sched/urgent] sched/isolation: Fix boot crash when maxcpus < first housekeeping CPU

Posted by Oleg Nesterov 1 year, 9 months ago

On 04/28, Ingo Molnar wrote:
>
> * tip-bot2 for Oleg Nesterov <tip-bot2@linutronix.de> wrote:
>
> > Another corner case is "nohz_full=0" on a machine with a single CPU or with
> > the maxcpus=1 kernel argument. In this case non_housekeeping_mask is empty
> > and tick_nohz_full_setup() makes no sense. And indeed, the kernel hits the
> > WARN_ON(tick_nohz_full_running) in tick_sched_do_timer().
> >
> > And how should the kernel interpret the "nohz_full=" parameter? It should
> > be silently ignored, but currently cpulist_parse() happily returns the
> > empty cpumask and this leads to the same problem.
> >
> > Change housekeeping_setup() to check cpumask_empty(non_housekeeping_mask)
> > and do nothing in this case.
>
> So arguably the user meant NOHZ_FULL to be turned off - but it is de-facto
> already turned off by the fact that there's only a single CPU available,
> right?

Or the user passes the empty "nohz_full=" mask on a multi-CPU machine.

In both cases (before this patch) housekeeping_setup() calls
tick_nohz_full_setup(non_housekeeping_mask) which sets
tick_nohz_full_running = true even if tick_nohz_full_mask is empty.

This doesn't look right to me and triggers the "this should not happen"
warning in tick_sched_do_timer().

But let me repeat, I know nothing about nohz/etc.

Oleg.

[tip: sched/urgent] sched/isolation: Fix boot crash when maxcpus < first housekeeping CPU

Posted by tip-bot2 for Oleg Nesterov 1 year, 9 months ago

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     257bf89d84121280904800acd25cc2c444c717ae
Gitweb:        https://git.kernel.org/tip/257bf89d84121280904800acd25cc2c444c717ae
Author:        Oleg Nesterov <oleg@redhat.com>
AuthorDate:    Sat, 13 Apr 2024 16:17:46 +02:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Sun, 28 Apr 2024 10:08:21 +02:00

sched/isolation: Fix boot crash when maxcpus < first housekeeping CPU

housekeeping_setup() checks cpumask_intersects(present, online) to ensure
that the kernel will have at least one housekeeping CPU after smp_init(),
but this doesn't work if the maxcpus= kernel parameter limits the number of
processors available after bootup.

For example, a kernel with "maxcpus=2 nohz_full=0-2" parameters crashes at
boot time on a virtual machine with 4 CPUs.

Change housekeeping_setup() to use cpumask_first_and() and check that the
returned CPU number is valid and less than setup_max_cpus.

Another corner case is "nohz_full=0" on a machine with a single CPU or with
the maxcpus=1 kernel argument. In this case non_housekeeping_mask is empty
and tick_nohz_full_setup() makes no sense. And indeed, the kernel hits the
WARN_ON(tick_nohz_full_running) in tick_sched_do_timer().

And how should the kernel interpret the "nohz_full=" parameter? It should
be silently ignored, but currently cpulist_parse() happily returns the
empty cpumask and this leads to the same problem.

Change housekeeping_setup() to check cpumask_empty(non_housekeeping_mask)
and do nothing in this case.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Phil Auld <pauld@redhat.com>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20240413141746.GA10008@redhat.com
---
 kernel/sched/isolation.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 2a262d3..5891e71 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -118,6 +118,7 @@ static void __init housekeeping_setup_type(enum hk_type type,
 static int __init housekeeping_setup(char *str, unsigned long flags)
 {
 	cpumask_var_t non_housekeeping_mask, housekeeping_staging;
+	unsigned int first_cpu;
 	int err = 0;
 
 	if ((flags & HK_FLAG_TICK) && !(housekeeping.flags & HK_FLAG_TICK)) {
@@ -138,7 +139,8 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
 	cpumask_andnot(housekeeping_staging,
 		       cpu_possible_mask, non_housekeeping_mask);
 
-	if (!cpumask_intersects(cpu_present_mask, housekeeping_staging)) {
+	first_cpu = cpumask_first_and(cpu_present_mask, housekeeping_staging);
+	if (first_cpu >= nr_cpu_ids || first_cpu >= setup_max_cpus) {
 		__cpumask_set_cpu(smp_processor_id(), housekeeping_staging);
 		__cpumask_clear_cpu(smp_processor_id(), non_housekeeping_mask);
 		if (!housekeeping.flags) {
@@ -147,6 +149,9 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
 		}
 	}
 
+	if (cpumask_empty(non_housekeeping_mask))
+		goto free_housekeeping_staging;
+
 	if (!housekeeping.flags) {
 		/* First setup call ("nohz_full=" or "isolcpus=") */
 		enum hk_type type;