Re: [PATCH v2] mm: terminate shrink_slab loop if signal is pending

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Dec 8, 2017 at 1:02 PM, David Rientjes <rientjes@google.com> wrote:
> On Thu, 7 Dec 2017, Suren Baghdasaryan wrote:
>
>> Slab shrinkers can be quite time consuming and when signal
>> is pending they can delay handling of the signal. If fatal
>> signal is pending there is no point in shrinking that process
>> since it will be killed anyway. This change checks for pending
>> fatal signals inside shrink_slab loop and if one is detected
>> terminates this loop early.
>>
>
> I've proposed a similar patch in the past, but for a check on TIF_MEMDIE,
> which would today be a tsk_is_oom_victim(current), since we had observed
> lengthy stalls in reclaim that would have been prevented if the oom victim
> had exited out, returned back to the page allocator, allocated with
> ALLOC_NO_WATERMARKS, and proceeded to quickly exit.
>
> I'm not sure that all fatal_signal_pending() tasks should get the same
> treatment, but I understand the point that the task is killed and should
> free memory when it fully exits.  How much memory is unknown.
>

Thanks for the input. For my particular use case TIF_MEMDIE check
would not help because I'm trying to kill a process before OOM kicks
in, however the approach is interesting and provides food for thought.

>  > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>>
>> ---
>> V2:
>> Sergey Senozhatsky:
>>   - Fix missing parentheses
>> ---
>>  mm/vmscan.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index c02c850ea349..28e4bdc72c16 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -486,6 +486,13 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>>                       .memcg = memcg,
>>               };
>>
>> +             /*
>> +              * We are about to die and free our memory.
>> +              * Stop shrinking which might delay signal handling.
>> +              */
>> +             if (unlikely(fatal_signal_pending(current)))
>> +                     break;
>> +
>>               /*
>>                * If kernel memory accounting is disabled, we ignore
>>                * SHRINKER_MEMCG_AWARE flag and call all shrinkers