Re: [PATCH 1/2] powerpc/watchdog: prevent printk and send IPI while holding the wd lock
Le 27/10/2021 à 11:49, Nicholas Piggin a écrit :
Excerpts from Nicholas Piggin's message of October 27, 2021 1:29 pm:
Excerpts from Laurent Dufour's message of October 27, 2021 2:27 am:
When handling the Watchdog interrupt, long processing should not be done
while holding the __wd_smp_lock. This prevents the other CPUs to grab it
and to process Watchdog timer interrupts. Furhtermore, this could lead to
the following situation:
CPU x detect lockup on CPU y and grab the __wd_smp_lock
in watchdog_smp_panic()
CPU y caught the watchdog interrupt and try to grab the __wd_smp_lock
in soft_nmi_interrupt()
CPU x wait for CPU y to catch the IPI for 1s in __smp_send_nmi_ipi()
CPU y should get the IPI here if it's a NMI IPI (which will be true for
= POWER9 64s).
That said, not all platforms support it and the console lock problem
seems real, so okay.
CPU x will timeout and so has spent 1s waiting while holding the
__wd_smp_lock.
A deadlock may also happen between the __wd_smp_lock and the console_owner
'lock' this way:
CPU x grab the console_owner
CPU y grab the __wd_smp_lock
CPU x catch the watchdog timer interrupt and needs to grab __wd_smp_lock
CPU y wants to print something and wait for console_owner
-> deadlock
Doing all the long processing without holding the _wd_smp_lock prevents
these situations.
The intention was to avoid logs getting garbled e.g., if multiple
different CPUs fire at once.
I wonder if instead we could deal with that by protecting the IPI
sending and printing stuff with a trylock, and if you don't get the
trylock then just return, and you'll come back with the next timer
interrupt.
Something like this (untested) should basically hold off concurrency on
watchdog panics. It does not serialize output from IPI targets but it
should prevent multiple CPUs trying to send IPIs at once, without
holding the lock.
Got it, I'll work this way, despite the minor comments below.
---
diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c
index 2ffeb3f8b5a7..3a0db577da56 100644
--- a/arch/powerpc/kernel/watchdog.c
+++ b/arch/powerpc/kernel/watchdog.c
@@ -85,6 +85,7 @@ static DEFINE_PER_CPU(u64, wd_timer_tb);
/* SMP checker bits */
static unsigned long __wd_smp_lock;
+static unsigned long __wd_printing;
static cpumask_t wd_smp_cpus_pending;
static cpumask_t wd_smp_cpus_stuck;
static u64 wd_smp_last_reset_tb;
@@ -131,10 +132,10 @@ static void wd_lockup_ipi(struct pt_regs *regs)
/* Do not panic from here because that can recurse into NMI IPI layer */
}
-static void set_cpumask_stuck(const struct cpumask *cpumask, u64 tb)
+static void set_cpu_stuck(int cpu, u64 tb)
{
- cpumask_or(_smp_cpus_stuck, _smp_cpus_stuck, cpumask);
- cpumask_andnot(_smp_cpus_pending, _smp_cpus_pending, cpumask);
+ cpumask_set_cpu(cpu, _smp_cpus_stuck);
+ cpumask_clear_cpu(cpu, _smp_cpus_pending);
if (cpumask_empty(_smp_cpus_pending)) {
wd_smp_last_reset_tb = tb;
cpumask_andnot(_smp_cpus_pending,
@@ -142,10 +143,6 @@ static void set_cpumask_stuck(const struct cpumask
*cpumask, u64 tb)
_smp_cpus_stuck);
}
}
-static void set_cpu_stuck(int cpu, u64 tb)
-{
- set_cpumask_stuck(cpumask_of(cpu), tb);
-}
static void watchdog_smp_panic(int cpu, u64 tb)
{
@@ -160,6 +157,10 @@ static void watchdog_smp_panic(int cpu, u64 tb)
goto out;
if (cpumask_weight(_smp_cpus_pending) == 0)
goto out;
+ if (__wd_printing)
+ goto out;
+ __wd_printing = 1;
+ wd_smp_unlock();
pr_emerg("CPU %d detected hard LOCKUP on other CPUs %*pbl\n",
cpu, cpumask_pr_args(_smp_cpus_pending));
@@ -172,24 +173,31 @@ static void watchdog_smp_panic(int cpu, u64 tb)
* Try to trigger the stuck CPUs, unless we are going to
* get a backtrace on all of them anyway.
*/
- for_each_cpu(c, _smp_cpus_pending) {
+ for_each_online_cpu(c) {
if (c == cpu)
continue;
+ if (!cpumask_test_cpu(cpu, _smp_cpus_pending))
+ continue;
+ wd_smp_lock();
+ if (!cpumask_test_cpu(cpu, _smp_cpus_pending)) {
+ wd_smp_unlock();
+ continue;
+ }
+ /* Take the stuck CPU out of the watch group */
+ set_cpu_stuck(cpu, tb);
+ wd_smp_unlock();
+
smp_send_nmi_ipi(c, wd_lockup_ipi, 100);
}
}
- /* Take the stuck CPUs out of the watch group */
- set_cpumask_stuck(_smp_cpus_pending, tb);
-
- wd_smp_unlock();
-
if (sysctl_hardlockup_all_cpu_backtrace)
Re: [PATCH 1/2] powerpc/watchdog: prevent printk and send IPI while holding the wd lock
Excerpts from Nicholas Piggin's message of October 27, 2021 1:29 pm:
> Excerpts from Laurent Dufour's message of October 27, 2021 2:27 am:
>> When handling the Watchdog interrupt, long processing should not be done
>> while holding the __wd_smp_lock. This prevents the other CPUs to grab it
>> and to process Watchdog timer interrupts. Furhtermore, this could lead to
>> the following situation:
>>
>> CPU x detect lockup on CPU y and grab the __wd_smp_lock
>> in watchdog_smp_panic()
>> CPU y caught the watchdog interrupt and try to grab the __wd_smp_lock
>> in soft_nmi_interrupt()
>> CPU x wait for CPU y to catch the IPI for 1s in __smp_send_nmi_ipi()
>
> CPU y should get the IPI here if it's a NMI IPI (which will be true for
>>= POWER9 64s).
>
> That said, not all platforms support it and the console lock problem
> seems real, so okay.
>
>> CPU x will timeout and so has spent 1s waiting while holding the
>> __wd_smp_lock.
>>
>> A deadlock may also happen between the __wd_smp_lock and the console_owner
>> 'lock' this way:
>> CPU x grab the console_owner
>> CPU y grab the __wd_smp_lock
>> CPU x catch the watchdog timer interrupt and needs to grab __wd_smp_lock
>> CPU y wants to print something and wait for console_owner
>> -> deadlock
>>
>> Doing all the long processing without holding the _wd_smp_lock prevents
>> these situations.
>
> The intention was to avoid logs getting garbled e.g., if multiple
> different CPUs fire at once.
>
> I wonder if instead we could deal with that by protecting the IPI
> sending and printing stuff with a trylock, and if you don't get the
> trylock then just return, and you'll come back with the next timer
> interrupt.
Something like this (untested) should basically hold off concurrency on
watchdog panics. It does not serialize output from IPI targets but it
should prevent multiple CPUs trying to send IPIs at once, without
holding the lock.
---
diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c
index 2ffeb3f8b5a7..3a0db577da56 100644
--- a/arch/powerpc/kernel/watchdog.c
+++ b/arch/powerpc/kernel/watchdog.c
@@ -85,6 +85,7 @@ static DEFINE_PER_CPU(u64, wd_timer_tb);
/* SMP checker bits */
static unsigned long __wd_smp_lock;
+static unsigned long __wd_printing;
static cpumask_t wd_smp_cpus_pending;
static cpumask_t wd_smp_cpus_stuck;
static u64 wd_smp_last_reset_tb;
@@ -131,10 +132,10 @@ static void wd_lockup_ipi(struct pt_regs *regs)
/* Do not panic from here because that can recurse into NMI IPI layer */
}
-static void set_cpumask_stuck(const struct cpumask *cpumask, u64 tb)
+static void set_cpu_stuck(int cpu, u64 tb)
{
- cpumask_or(_smp_cpus_stuck, _smp_cpus_stuck, cpumask);
- cpumask_andnot(_smp_cpus_pending, _smp_cpus_pending, cpumask);
+ cpumask_set_cpu(cpu, _smp_cpus_stuck);
+ cpumask_clear_cpu(cpu, _smp_cpus_pending);
if (cpumask_empty(_smp_cpus_pending)) {
wd_smp_last_reset_tb = tb;
cpumask_andnot(_smp_cpus_pending,
@@ -142,10 +143,6 @@ static void set_cpumask_stuck(const struct cpumask
*cpumask, u64 tb)
_smp_cpus_stuck);
}
}
-static void set_cpu_stuck(int cpu, u64 tb)
-{
- set_cpumask_stuck(cpumask_of(cpu), tb);
-}
static void watchdog_smp_panic(int cpu, u64 tb)
{
@@ -160,6 +157,10 @@ static void watchdog_smp_panic(int cpu, u64 tb)
goto out;
if (cpumask_weight(_smp_cpus_pending) == 0)
goto out;
+ if (__wd_printing)
+ goto out;
+ __wd_printing = 1;
+ wd_smp_unlock();
pr_emerg("CPU %d detected hard LOCKUP on other CPUs %*pbl\n",
cpu, cpumask_pr_args(_smp_cpus_pending));
@@ -172,24 +173,31 @@ static void watchdog_smp_panic(int cpu, u64 tb)
* Try to trigger the stuck CPUs, unless we are going to
* get a backtrace on all of them anyway.
*/
- for_each_cpu(c, _smp_cpus_pending) {
+ for_each_online_cpu(c) {
if (c == cpu)
continue;
+ if (!cpumask_test_cpu(cpu, _smp_cpus_pending))
+ continue;
+ wd_smp_lock();
+ if (!cpumask_test_cpu(cpu, _smp_cpus_pending)) {
+ wd_smp_unlock();
+ continue;
+ }
+ /* Take the stuck CPU out of the watch group */
+ set_cpu_stuck(cpu, tb);
+ wd_smp_unlock();
+
smp_send_nmi_ipi(c, wd_lockup_ipi, 100);
}
}
- /* Take the stuck CPUs out of the watch group */
- set_cpumask_stuck(_smp_cpus_pending, tb);
-
- wd_smp_unlock();
-
if (sysctl_hardlockup_all_cpu_backtrace)
trigger_allbutself_cpu_backtrace();
Re: [PATCH 1/2] powerpc/watchdog: prevent printk and send IPI while holding the wd lock
Excerpts from Laurent Dufour's message of October 27, 2021 6:14 pm: > Le 27/10/2021 à 05:29, Nicholas Piggin a écrit : >> Excerpts from Laurent Dufour's message of October 27, 2021 2:27 am: >>> When handling the Watchdog interrupt, long processing should not be done >>> while holding the __wd_smp_lock. This prevents the other CPUs to grab it >>> and to process Watchdog timer interrupts. Furhtermore, this could lead to >>> the following situation: >>> >>> CPU x detect lockup on CPU y and grab the __wd_smp_lock >>>in watchdog_smp_panic() >>> CPU y caught the watchdog interrupt and try to grab the __wd_smp_lock >>>in soft_nmi_interrupt() >>> CPU x wait for CPU y to catch the IPI for 1s in __smp_send_nmi_ipi() >> >> CPU y should get the IPI here if it's a NMI IPI (which will be true for >>> = POWER9 64s). >> >> That said, not all platforms support it and the console lock problem >> seems real, so okay. >> >>> CPU x will timeout and so has spent 1s waiting while holding the >>>__wd_smp_lock. >>> >>> A deadlock may also happen between the __wd_smp_lock and the console_owner >>> 'lock' this way: >>> CPU x grab the console_owner >>> CPU y grab the __wd_smp_lock >>> CPU x catch the watchdog timer interrupt and needs to grab __wd_smp_lock >>> CPU y wants to print something and wait for console_owner >>> -> deadlock >>> >>> Doing all the long processing without holding the _wd_smp_lock prevents >>> these situations. >> >> The intention was to avoid logs getting garbled e.g., if multiple >> different CPUs fire at once. >> >> I wonder if instead we could deal with that by protecting the IPI >> sending and printing stuff with a trylock, and if you don't get the >> trylock then just return, and you'll come back with the next timer >> interrupt. > > That sounds a bit risky to me, especially on large system when system goes > wrong, all the CPU may try lock here. That should be okay though, one will get through and the others will skip. > Furthermore, now operation done under the lock protection are quite fast, > there > is no more spinning like the delay loop done when sending an IPI. > > Protecting the IPI sending is a nightmare, if the target CPU is later play > with > the lock we are taking during the IPI processing, furthermore, if the target > CPU > is not responding the sending CPU is waiting for 1s, which slows all the > system > due to the lock held. > Since I do a copy of the pending CPU mask and clear it under the lock > protection, the IPI sending is safe while done without holding the lock. Protecting IPI sending basically has all the same issues in the NMI IPI layer. > > Regarding the interleaved traces, I don't think this has to be managed down > here, but rather in the printk/console path. It can't necessarily be because some of the problem is actually that a NMI handler can be interrupted by another NMI IPI because the caller can return only after handlers start running rather than complete. I don't think it would be an additional nightmare to trylock. Thanks, Nick
Re: [PATCH 1/2] powerpc/watchdog: prevent printk and send IPI while holding the wd lock
Le 27/10/2021 à 05:29, Nicholas Piggin a écrit :
Excerpts from Laurent Dufour's message of October 27, 2021 2:27 am:
When handling the Watchdog interrupt, long processing should not be done
while holding the __wd_smp_lock. This prevents the other CPUs to grab it
and to process Watchdog timer interrupts. Furhtermore, this could lead to
the following situation:
CPU x detect lockup on CPU y and grab the __wd_smp_lock
in watchdog_smp_panic()
CPU y caught the watchdog interrupt and try to grab the __wd_smp_lock
in soft_nmi_interrupt()
CPU x wait for CPU y to catch the IPI for 1s in __smp_send_nmi_ipi()
CPU y should get the IPI here if it's a NMI IPI (which will be true for
= POWER9 64s).
That said, not all platforms support it and the console lock problem
seems real, so okay.
CPU x will timeout and so has spent 1s waiting while holding the
__wd_smp_lock.
A deadlock may also happen between the __wd_smp_lock and the console_owner
'lock' this way:
CPU x grab the console_owner
CPU y grab the __wd_smp_lock
CPU x catch the watchdog timer interrupt and needs to grab __wd_smp_lock
CPU y wants to print something and wait for console_owner
-> deadlock
Doing all the long processing without holding the _wd_smp_lock prevents
these situations.
The intention was to avoid logs getting garbled e.g., if multiple
different CPUs fire at once.
I wonder if instead we could deal with that by protecting the IPI
sending and printing stuff with a trylock, and if you don't get the
trylock then just return, and you'll come back with the next timer
interrupt.
That sounds a bit risky to me, especially on large system when system goes
wrong, all the CPU may try lock here.
Furthermore, now operation done under the lock protection are quite fast, there
is no more spinning like the delay loop done when sending an IPI.
Protecting the IPI sending is a nightmare, if the target CPU is later play with
the lock we are taking during the IPI processing, furthermore, if the target CPU
is not responding the sending CPU is waiting for 1s, which slows all the system
due to the lock held.
Since I do a copy of the pending CPU mask and clear it under the lock
protection, the IPI sending is safe while done without holding the lock.
Regarding the interleaved traces, I don't think this has to be managed down
here, but rather in the printk/console path.
Cheers,
Laurent.
Thanks,
Nick
Signed-off-by: Laurent Dufour
---
arch/powerpc/kernel/watchdog.c | 31 +--
1 file changed, 17 insertions(+), 14 deletions(-)
diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c
index f9ea0e5357f9..bc7411327066 100644
--- a/arch/powerpc/kernel/watchdog.c
+++ b/arch/powerpc/kernel/watchdog.c
@@ -149,6 +149,8 @@ static void set_cpu_stuck(int cpu, u64 tb)
static void watchdog_smp_panic(int cpu, u64 tb)
{
+ cpumask_t cpus_pending_copy;
+ u64 last_reset_tb_copy;
unsigned long flags;
int c;
@@ -161,29 +163,32 @@ static void watchdog_smp_panic(int cpu, u64 tb)
if (cpumask_weight(_smp_cpus_pending) == 0)
goto out;
+ cpumask_copy(_pending_copy, _smp_cpus_pending);
+ last_reset_tb_copy = wd_smp_last_reset_tb;
+
+ /* Take the stuck CPUs out of the watch group */
+ set_cpumask_stuck(_smp_cpus_pending, tb);
+
+ wd_smp_unlock();
+
pr_emerg("CPU %d detected hard LOCKUP on other CPUs %*pbl\n",
-cpu, cpumask_pr_args(_smp_cpus_pending));
+cpu, cpumask_pr_args(_pending_copy));
pr_emerg("CPU %d TB:%lld, last SMP heartbeat TB:%lld (%lldms ago)\n",
-cpu, tb, wd_smp_last_reset_tb,
-tb_to_ns(tb - wd_smp_last_reset_tb) / 100);
+cpu, tb, last_reset_tb_copy,
+tb_to_ns(tb - last_reset_tb_copy) / 100);
if (!sysctl_hardlockup_all_cpu_backtrace) {
/*
* Try to trigger the stuck CPUs, unless we are going to
* get a backtrace on all of them anyway.
*/
- for_each_cpu(c, _smp_cpus_pending) {
+ for_each_cpu(c, _pending_copy) {
if (c == cpu)
continue;
smp_send_nmi_ipi(c, wd_lockup_ipi, 100);
}
}
- /* Take the stuck CPUs out of the watch group */
- set_cpumask_stuck(_smp_cpus_pending, tb);
-
- wd_smp_unlock();
-
if (sysctl_hardlockup_all_cpu_backtrace)
trigger_allbutself_cpu_backtrace();
@@ -204,6 +209,8 @@ static void wd_smp_clear_cpu_pending(int cpu, u64 tb)
unsigned long flags;
wd_smp_lock();
+ cpumask_clear_cpu(cpu, _smp_cpus_stuck);
+ wd_smp_unlock();
pr_emerg("CPU %d became unstuck TB:%lld\n",
cpu, tb);
@@ -212,9 +219,6 @@ static
Re: [PATCH 1/2] powerpc/watchdog: prevent printk and send IPI while holding the wd lock
Excerpts from Laurent Dufour's message of October 27, 2021 2:27 am:
> When handling the Watchdog interrupt, long processing should not be done
> while holding the __wd_smp_lock. This prevents the other CPUs to grab it
> and to process Watchdog timer interrupts. Furhtermore, this could lead to
> the following situation:
>
> CPU x detect lockup on CPU y and grab the __wd_smp_lock
> in watchdog_smp_panic()
> CPU y caught the watchdog interrupt and try to grab the __wd_smp_lock
> in soft_nmi_interrupt()
> CPU x wait for CPU y to catch the IPI for 1s in __smp_send_nmi_ipi()
CPU y should get the IPI here if it's a NMI IPI (which will be true for
>= POWER9 64s).
That said, not all platforms support it and the console lock problem
seems real, so okay.
> CPU x will timeout and so has spent 1s waiting while holding the
> __wd_smp_lock.
>
> A deadlock may also happen between the __wd_smp_lock and the console_owner
> 'lock' this way:
> CPU x grab the console_owner
> CPU y grab the __wd_smp_lock
> CPU x catch the watchdog timer interrupt and needs to grab __wd_smp_lock
> CPU y wants to print something and wait for console_owner
> -> deadlock
>
> Doing all the long processing without holding the _wd_smp_lock prevents
> these situations.
The intention was to avoid logs getting garbled e.g., if multiple
different CPUs fire at once.
I wonder if instead we could deal with that by protecting the IPI
sending and printing stuff with a trylock, and if you don't get the
trylock then just return, and you'll come back with the next timer
interrupt.
Thanks,
Nick
>
> Signed-off-by: Laurent Dufour
> ---
> arch/powerpc/kernel/watchdog.c | 31 +--
> 1 file changed, 17 insertions(+), 14 deletions(-)
>
> diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c
> index f9ea0e5357f9..bc7411327066 100644
> --- a/arch/powerpc/kernel/watchdog.c
> +++ b/arch/powerpc/kernel/watchdog.c
> @@ -149,6 +149,8 @@ static void set_cpu_stuck(int cpu, u64 tb)
>
> static void watchdog_smp_panic(int cpu, u64 tb)
> {
> + cpumask_t cpus_pending_copy;
> + u64 last_reset_tb_copy;
> unsigned long flags;
> int c;
>
> @@ -161,29 +163,32 @@ static void watchdog_smp_panic(int cpu, u64 tb)
> if (cpumask_weight(_smp_cpus_pending) == 0)
> goto out;
>
> + cpumask_copy(_pending_copy, _smp_cpus_pending);
> + last_reset_tb_copy = wd_smp_last_reset_tb;
> +
> + /* Take the stuck CPUs out of the watch group */
> + set_cpumask_stuck(_smp_cpus_pending, tb);
> +
> + wd_smp_unlock();
> +
> pr_emerg("CPU %d detected hard LOCKUP on other CPUs %*pbl\n",
> - cpu, cpumask_pr_args(_smp_cpus_pending));
> + cpu, cpumask_pr_args(_pending_copy));
> pr_emerg("CPU %d TB:%lld, last SMP heartbeat TB:%lld (%lldms ago)\n",
> - cpu, tb, wd_smp_last_reset_tb,
> - tb_to_ns(tb - wd_smp_last_reset_tb) / 100);
> + cpu, tb, last_reset_tb_copy,
> + tb_to_ns(tb - last_reset_tb_copy) / 100);
>
> if (!sysctl_hardlockup_all_cpu_backtrace) {
> /*
>* Try to trigger the stuck CPUs, unless we are going to
>* get a backtrace on all of them anyway.
>*/
> - for_each_cpu(c, _smp_cpus_pending) {
> + for_each_cpu(c, _pending_copy) {
> if (c == cpu)
> continue;
> smp_send_nmi_ipi(c, wd_lockup_ipi, 100);
> }
> }
>
> - /* Take the stuck CPUs out of the watch group */
> - set_cpumask_stuck(_smp_cpus_pending, tb);
> -
> - wd_smp_unlock();
> -
> if (sysctl_hardlockup_all_cpu_backtrace)
> trigger_allbutself_cpu_backtrace();
>
> @@ -204,6 +209,8 @@ static void wd_smp_clear_cpu_pending(int cpu, u64 tb)
> unsigned long flags;
>
> wd_smp_lock();
> + cpumask_clear_cpu(cpu, _smp_cpus_stuck);
> + wd_smp_unlock();
>
> pr_emerg("CPU %d became unstuck TB:%lld\n",
>cpu, tb);
> @@ -212,9 +219,6 @@ static void wd_smp_clear_cpu_pending(int cpu, u64 tb)
> show_regs(regs);
> else
> dump_stack();
> -
> - cpumask_clear_cpu(cpu, _smp_cpus_stuck);
> - wd_smp_unlock();
> }
> return;
> }
> @@ -267,6 +271,7 @@ DEFINE_INTERRUPT_HANDLER_NMI(soft_nmi_interrupt)
> return 0;
> }
> set_cpu_stuck(cpu, tb);
> + wd_smp_unlock();
>
> pr_emerg("CPU %d self-detected hard LOCKUP @ %pS\n",
>cpu, (void *)regs->nip);
> @@ -277,8 +282,6 @@ DEFINE_INTERRUPT_HANDLER_NMI(soft_nmi_interrupt)
>
