Re: [PATCH] Force processes to non-realtime before mm_exit

[Brian Silverman <brian@peloton-tech.com>]

Thanks for looking at this. :)

On Wed, May 25, 2016 at 9:33 AM, Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
> * Brian Silverman | 2016-05-16 14:05:54 [-0700]:
>
> Thank you for the explanation.
>
>>It happens when realtime (SCHED_RR/SCHED_FIFO) processes which have
>>called mlockall(MCL_CURRENT | MCL_FUTURE) exit.
>
> Why can't the application drop the RT priority before its exit? Wouldn't
> that be appropriate?

If it crashes or gets killed, it doesn't have a chance to drop priority.

>>One of them:
>> queue_cyclictes-5298  [000] ....1..   290.366030: remove_vma <-exit_mmap
>> queue_cyclictes-5298  [000] ....1..   290.366032: __might_sleep <-remove_vma
>> queue_cyclictes-5298  [000] ....1..   290.366033: ___might_sleep
>><-__might_sleep
>> queue_cyclictes-5298  [000] ....1..   290.366035: kmem_cache_free <-remove_vma
>> queue_cyclictes-5298  [000] ....1..   290.366036: preempt_count_add
>><-kmem_cache_free
>> queue_cyclictes-5298  [000] ....2..   290.366037: preempt_count_sub
>><-kmem_cache_free
>> queue_cyclictes-5298  [000] ....1..   290.366039: __slab_free <-kmem_cache_free
>> queue_cyclictes-5298  [000] d...1..   290.366040: preempt_count_add
>><-__slab_free
>> queue_cyclictes-5298  [000] d...2..   290.366041: preempt_count_sub
>><-__slab_free
>> queue_cyclictes-5298  [000] ....1..   290.366043: kmem_cache_free:
>>call_site=c0147054 ptr=ebb93cb8
>
> In this trace and the others you attached, you have a preempt count of
> at least one. Is this a function trace? For events I recently figure out
> that the counter in event tracing is recorded wrongly [0]. So my
> question here is basically is this a long preempt disable section or
> not.
>
> [0] http://permalink.gmane.org/gmane.linux.kernel/2227774

Yes, they are function traces without that patch applied. Every single
line in the file has a preempt count of at least 1.

Brian