[PATCH v4 1/2] clocksource: Use pr_info() for "Checking clocksource synchronization" message

[PATCH v4 1/2] clocksource: Use pr_info() for "Checking clocksource synchronization" message

[Waiman Long]

The "Checking clocksource synchronization" message is normally printed
when clocksource_verify_percpu() is called for a given clocksource if
both the CLOCK_SOURCE_UNSTABLE and CLOCK_SOURCE_VERIFY_PERCPU flags
are set. It is an informational message and so pr_info() should be used
instead of pr_warn().

Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: John Stultz <jstultz@google.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/time/clocksource.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 7304d7cf47f2..77d9566d3aa6 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -382,7 +382,8 @@ void clocksource_verify_percpu(struct clocksource *cs)
 		return;
 	}
 	testcpu = smp_processor_id();
-	pr_warn("Checking clocksource %s synchronization from CPU %d to CPUs %*pbl.\n", cs->name, testcpu, cpumask_pr_args(&cpus_chosen));
+	pr_info("Checking clocksource %s synchronization from CPU %d to CPUs %*pbl.\n",
+		cs->name, testcpu, cpumask_pr_args(&cpus_chosen));
 	for_each_cpu(cpu, &cpus_chosen) {
 		if (cpu == testcpu)
 			continue;
-- 
2.48.1

[PATCH v4 2/2] clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context

[Waiman Long]

The following bug report happened with a PREEMPT_RT kernel.

[   30.957705] BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
[   30.957711] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2012, name: kwatchdog
[   30.962673] preempt_count: 1, expected: 0
[   30.962676] RCU nest depth: 0, expected: 0

It is due to the fact that clocksource_verify_choose_cpus() is invoked
with preemption disabled.  This function invokes get_random_u32()
to obtain random numbers for choosing CPUs.  The batched_entropy_32
local lock and/or the base_crng.lock spinlock in driver/char/random.c
will be acquired during the call. In PREEMPT_RT kernel, they are both
sleeping locks and so cannot be acquired in atomic context.

Fix this problem by using migrate_disable() to allow
smp_processor_id() to be reliably used without introducing atomic
context. The preempt_disable() function is then called after
clocksource_verify_choose_cpus() but before the clocksource measurement
is being run to avoid introducing unexpected latency.

Fixes: 7560c02bdffb ("clocksource: Check per-CPU clock synchronization when marked unstable")
Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/time/clocksource.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 77d9566d3aa6..2a7802ec480c 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -373,10 +373,10 @@ void clocksource_verify_percpu(struct clocksource *cs)
 	cpumask_clear(&cpus_ahead);
 	cpumask_clear(&cpus_behind);
 	cpus_read_lock();
-	preempt_disable();
+	migrate_disable();
 	clocksource_verify_choose_cpus();
 	if (cpumask_empty(&cpus_chosen)) {
-		preempt_enable();
+		migrate_enable();
 		cpus_read_unlock();
 		pr_warn("Not enough CPUs to check clocksource '%s'.\n", cs->name);
 		return;
@@ -384,6 +384,7 @@ void clocksource_verify_percpu(struct clocksource *cs)
 	testcpu = smp_processor_id();
 	pr_info("Checking clocksource %s synchronization from CPU %d to CPUs %*pbl.\n",
 		cs->name, testcpu, cpumask_pr_args(&cpus_chosen));
+	preempt_disable();
 	for_each_cpu(cpu, &cpus_chosen) {
 		if (cpu == testcpu)
 			continue;
@@ -403,6 +404,7 @@ void clocksource_verify_percpu(struct clocksource *cs)
 			cs_nsec_min = cs_nsec;
 	}
 	preempt_enable();
+	migrate_enable();
 	cpus_read_unlock();
 	if (!cpumask_empty(&cpus_ahead))
 		pr_warn("        CPUs %*pbl ahead of CPU %d for clocksource %s.\n",
-- 
2.48.1

Re: [PATCH v4 2/2] clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context

[Paul E. McKenney]

On Fri, Jan 31, 2025 at 12:33:23PM -0500, Waiman Long wrote:
> The following bug report happened with a PREEMPT_RT kernel.
> 
> [   30.957705] BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
> [   30.957711] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2012, name: kwatchdog
> [   30.962673] preempt_count: 1, expected: 0
> [   30.962676] RCU nest depth: 0, expected: 0
> 
> It is due to the fact that clocksource_verify_choose_cpus() is invoked
> with preemption disabled.  This function invokes get_random_u32()
> to obtain random numbers for choosing CPUs.  The batched_entropy_32
> local lock and/or the base_crng.lock spinlock in driver/char/random.c
> will be acquired during the call. In PREEMPT_RT kernel, they are both
> sleeping locks and so cannot be acquired in atomic context.
> 
> Fix this problem by using migrate_disable() to allow
> smp_processor_id() to be reliably used without introducing atomic
> context. The preempt_disable() function is then called after
> clocksource_verify_choose_cpus() but before the clocksource measurement
> is being run to avoid introducing unexpected latency.
> 
> Fixes: 7560c02bdffb ("clocksource: Check per-CPU clock synchronization when marked unstable")
> Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Waiman Long <longman@redhat.com>

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>

> ---
>  kernel/time/clocksource.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> index 77d9566d3aa6..2a7802ec480c 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -373,10 +373,10 @@ void clocksource_verify_percpu(struct clocksource *cs)
>  	cpumask_clear(&cpus_ahead);
>  	cpumask_clear(&cpus_behind);
>  	cpus_read_lock();
> -	preempt_disable();
> +	migrate_disable();
>  	clocksource_verify_choose_cpus();
>  	if (cpumask_empty(&cpus_chosen)) {
> -		preempt_enable();
> +		migrate_enable();
>  		cpus_read_unlock();
>  		pr_warn("Not enough CPUs to check clocksource '%s'.\n", cs->name);
>  		return;
> @@ -384,6 +384,7 @@ void clocksource_verify_percpu(struct clocksource *cs)
>  	testcpu = smp_processor_id();
>  	pr_info("Checking clocksource %s synchronization from CPU %d to CPUs %*pbl.\n",
>  		cs->name, testcpu, cpumask_pr_args(&cpus_chosen));
> +	preempt_disable();
>  	for_each_cpu(cpu, &cpus_chosen) {
>  		if (cpu == testcpu)
>  			continue;
> @@ -403,6 +404,7 @@ void clocksource_verify_percpu(struct clocksource *cs)
>  			cs_nsec_min = cs_nsec;
>  	}
>  	preempt_enable();
> +	migrate_enable();
>  	cpus_read_unlock();
>  	if (!cpumask_empty(&cpus_ahead))
>  		pr_warn("        CPUs %*pbl ahead of CPU %d for clocksource %s.\n",
> -- 
> 2.48.1
> 

Re: [PATCH v4 2/2] clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context

[Sebastian Andrzej Siewior]

On 2025-01-31 12:33:23 [-0500], Waiman Long wrote:
> The following bug report happened with a PREEMPT_RT kernel.
> 
> [   30.957705] BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
> [   30.957711] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2012, name: kwatchdog
> [   30.962673] preempt_count: 1, expected: 0
> [   30.962676] RCU nest depth: 0, expected: 0
> 
> It is due to the fact that clocksource_verify_choose_cpus() is invoked
> with preemption disabled.  This function invokes get_random_u32()
> to obtain random numbers for choosing CPUs.  The batched_entropy_32
> local lock and/or the base_crng.lock spinlock in driver/char/random.c
> will be acquired during the call. In PREEMPT_RT kernel, they are both
> sleeping locks and so cannot be acquired in atomic context.
> 
> Fix this problem by using migrate_disable() to allow
> smp_processor_id() to be reliably used without introducing atomic
> context. The preempt_disable() function is then called after
> clocksource_verify_choose_cpus() but before the clocksource measurement
> is being run to avoid introducing unexpected latency.
> 
> Fixes: 7560c02bdffb ("clocksource: Check per-CPU clock synchronization when marked unstable")
> Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Waiman Long <longman@redhat.com>

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

I would have moved the preempt-disable into the for_each_cpu() loop. But
given that the clocksource is only updated on boot and by the watchdog
this does not matter at runtime.

Sebastian

[tip: timers/urgent] clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context

[tip-bot2 for Waiman Long]

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

Commit-ID:     6bb05a33337b2c842373857b63de5c9bf1ae2a09
Gitweb:        https://git.kernel.org/tip/6bb05a33337b2c842373857b63de5c9bf1ae2a09
Author:        Waiman Long <longman@redhat.com>
AuthorDate:    Fri, 31 Jan 2025 12:33:23 -05:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Mon, 03 Feb 2025 16:18:56 +01:00

clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context

The following bug report happened with a PREEMPT_RT kernel:

  BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
  in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2012, name: kwatchdog
  preempt_count: 1, expected: 0
  RCU nest depth: 0, expected: 0
  get_random_u32+0x4f/0x110
  clocksource_verify_choose_cpus+0xab/0x1a0
  clocksource_verify_percpu.part.0+0x6b/0x330
  clocksource_watchdog_kthread+0x193/0x1a0

It is due to the fact that clocksource_verify_choose_cpus() is invoked with
preemption disabled.  This function invokes get_random_u32() to obtain
random numbers for choosing CPUs.  The batched_entropy_32 local lock and/or
the base_crng.lock spinlock in driver/char/random.c will be acquired during
the call. In PREEMPT_RT kernel, they are both sleeping locks and so cannot
be acquired in atomic context.

Fix this problem by using migrate_disable() to allow smp_processor_id() to
be reliably used without introducing atomic context. preempt_disable() is
then called after clocksource_verify_choose_cpus() but before the
clocksource measurement is being run to avoid introducing unexpected
latency.

Fixes: 7560c02bdffb ("clocksource: Check per-CPU clock synchronization when marked unstable")
Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/all/20250131173323.891943-2-longman@redhat.com
---
 kernel/time/clocksource.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 77d9566..2a7802e 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -373,10 +373,10 @@ void clocksource_verify_percpu(struct clocksource *cs)
 	cpumask_clear(&cpus_ahead);
 	cpumask_clear(&cpus_behind);
 	cpus_read_lock();
-	preempt_disable();
+	migrate_disable();
 	clocksource_verify_choose_cpus();
 	if (cpumask_empty(&cpus_chosen)) {
-		preempt_enable();
+		migrate_enable();
 		cpus_read_unlock();
 		pr_warn("Not enough CPUs to check clocksource '%s'.\n", cs->name);
 		return;
@@ -384,6 +384,7 @@ void clocksource_verify_percpu(struct clocksource *cs)
 	testcpu = smp_processor_id();
 	pr_info("Checking clocksource %s synchronization from CPU %d to CPUs %*pbl.\n",
 		cs->name, testcpu, cpumask_pr_args(&cpus_chosen));
+	preempt_disable();
 	for_each_cpu(cpu, &cpus_chosen) {
 		if (cpu == testcpu)
 			continue;
@@ -403,6 +404,7 @@ void clocksource_verify_percpu(struct clocksource *cs)
 			cs_nsec_min = cs_nsec;
 	}
 	preempt_enable();
+	migrate_enable();
 	cpus_read_unlock();
 	if (!cpumask_empty(&cpus_ahead))
 		pr_warn("        CPUs %*pbl ahead of CPU %d for clocksource %s.\n",