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Date:	Mon, 22 Dec 2014 20:33:33 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Vladimir Davydov <vdavydov@...allels.com>,
	Michal Hocko <mhocko@...e.cz>
CC:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>, stable@...r.kernel.org,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 1/2] mm, vmscan: prevent kswapd livelock due to pfmemalloc-throttled
 process being killed

On 22.12.2014 17:25, Vladimir Davydov wrote:
>
>>> E.g. suppose processes are
>>> governed by FIFO and kswapd happens to have a higher prio than the
>>> process killed by OOM. Then after cond_resched kswapd will be picked for
>>> execution again, and the killing process won't have a chance to remove
>>> itself from the wait queue.
>> Except that kswapd runs as SCHED_NORMAL with 0 priority.
>>
>>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>>> index 744e2b491527..2a123634c220 100644
>>>>> --- a/mm/vmscan.c
>>>>> +++ b/mm/vmscan.c
>>>>> @@ -2984,6 +2984,9 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
>>>>>   	if (remaining)
>>>>>   		return false;
>>>>>   
>>>>> +	if (!pgdat_balanced(pgdat, order, classzone_idx))
>>>>> +		return false;
>>>>> +
>>>> What would be consequences of not waking up pfmemalloc waiters while the
>>>> node is not balanced?
>>> They will get woken up a bit later in balanced_pgdat. This might result
>>> in latency spikes though. In order not to change the original behaviour
>>> we could always wake all pfmemalloc waiters no matter if we are going to
>>> sleep or not:
>>>
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index 744e2b491527..a21e0bd563c3 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -2993,10 +2993,7 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
>>>   	 * so wake them now if necessary. If necessary, processes will wake
>>>   	 * kswapd and get throttled again
>>>   	 */
>>> -	if (waitqueue_active(&pgdat->pfmemalloc_wait)) {
>>> -		wake_up(&pgdat->pfmemalloc_wait);
>>> -		return false;
>>> -	}
>>> +	wake_up_all(&pgdat->pfmemalloc_wait);
>>>   
>>>   	return pgdat_balanced(pgdat, order, classzone_idx);
>> So you are relying on scheduling points somewhere down the
>> balance_pgdat. That should be sufficient. I am still quite surprised
>> that we have an OOM victim still on the queue and balanced pgdat here
>> because OOM victim didn't have chance to free memory. So somebody else
>> must have released a lot of memory after OOM.
>>
>> This patch seems better than the one from Vlastimil. Care to post it
>> with the full changelog, please?
> Attached below (merged with 2/2). I haven't checked that it does fix the
> issue, because I don't have the reproducer, so it should be committed
> only if Vlastimil approves it.

I agree it's the right fix, thanks a lot. We only have a synthetic 
reproducer,
as the real scenario would be hard to trigger reliably. I can test it 
later, but
I think it's reasonably clear the patch will help.
I would just personaly keep the comment clarification in the patch, but it's
not a critical issue.

Vlastimil
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