Re: [PATCH] kernel/watchdog: flush all printk nmi buffers when hardlockup detected

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

On 11/02/2020 11.14, Kirill Tkhai wrote:
> Hi, Konstantin,
> 
> On 10.02.2020 12:48, Konstantin Khlebnikov wrote:
>> In NMI context printk() could save messages into per-cpu buffers and
>> schedule flush by irq_work when IRQ are unblocked. This means message
>> about hardlockup appears in kernel log only when/if lockup is gone.
>>
>> Comment in irq_work_queue_on() states that remote IPI aren't NMI safe
>> thus printk() cannot schedule flush work to another cpu.
>>
>> This patch adds simple atomic counter of detected hardlockups and
>> flushes all per-cpu printk buffers in context softlockup watchdog
>> at any other cpu when it sees changes of this counter.
>>
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>> ---
>>   include/linux/nmi.h   |    1 +
>>   kernel/watchdog.c     |   22 ++++++++++++++++++++++
>>   kernel/watchdog_hld.c |    1 +
>>   3 files changed, 24 insertions(+)
>>
>> diff --git a/include/linux/nmi.h b/include/linux/nmi.h
>> index 9003e29cde46..8406df72ae5a 100644
>> --- a/include/linux/nmi.h
>> +++ b/include/linux/nmi.h
>> @@ -84,6 +84,7 @@ static inline void reset_hung_task_detector(void) { }
>>   #if defined(CONFIG_HARDLOCKUP_DETECTOR)
>>   extern void hardlockup_detector_disable(void);
>>   extern unsigned int hardlockup_panic;
>> +extern atomic_t hardlockup_detected;
>>   #else
>>   static inline void hardlockup_detector_disable(void) {}
>>   #endif
>> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
>> index b6b1f54a7837..9f5c68fababe 100644
>> --- a/kernel/watchdog.c
>> +++ b/kernel/watchdog.c
>> @@ -92,6 +92,26 @@ static int __init hardlockup_all_cpu_backtrace_setup(char *str)
>>   }
>>   __setup("hardlockup_all_cpu_backtrace=", hardlockup_all_cpu_backtrace_setup);
>>   # endif /* CONFIG_SMP */
>> +
>> +atomic_t hardlockup_detected = ATOMIC_INIT(0);
>> +
>> +static inline void flush_hardlockup_messages(void)
>> +{
>> +	static atomic_t flushed = ATOMIC_INIT(0);
>> +
>> +	/* flush messages from hard lockup detector */
>> +	if (atomic_read(&hardlockup_detected) != atomic_read(&flushed)) {
>> +		atomic_set(&flushed, atomic_read(&hardlockup_detected));
>> +		printk_safe_flush();
>> +	}
>> +}
> 
> Do we really need two variables here? They may come into two different
> cache lines, and there will be double cache pollution just because of
> this simple check. Why not the below?

I don't think anybody could notice read-only access to second variable.
This executes once in several seconds.

Watchdogs already use same pattern (monotonic counter + snapshot) in
couple places. So code looks more clean in this way.

> 
> 	if (atomic_read(&hardlockup_detected)) {
> 		atomic_set(&hardlockup_detected, 0);
> 		printk_safe_flush();
> 	}
> 
> Or even, since atomic is not needed here, as it does not give any ordering guarantees.
> static inline void flush_hardlockup_messages(void)
> {
> 	if (READ_ONCE(&hardlockup_detected)) {
> 		WRITE_ONCE(&hardlockup_detected, 0);
> 		printk_safe_flush();
> 	}
> }
> 
> watchdog_timer_fn()
> {
> 	...
> 	WRITE_ONCE(&hardlockup_detected, 1);
> 	...
> }
> 
> Kirill
>