Re: [PATCH 1/2] mm, vmscan: prevent kswapd livelock due to pfmemalloc-throttled process being killed

[Michal Hocko]

On Fri 19-12-14 21:28:15, Vladimir Davydov wrote:
> Hi,
> 
> On Fri, Dec 19, 2014 at 04:57:47PM +0100, Michal Hocko wrote:
> > On Fri 19-12-14 14:01:55, Vlastimil Babka wrote:
> > > Charles Shirron and Paul Cassella from Cray Inc have reported kswapd stuck
> > > in a busy loop with nothing left to balance, but kswapd_try_to_sleep() failing
> > > to sleep. Their analysis found the cause to be a combination of several
> > > factors:
> > > 
> > > 1. A process is waiting in throttle_direct_reclaim() on pgdat->pfmemalloc_wait
> > > 
> > > 2. The process has been killed (by OOM in this case), but has not yet been
> > >    scheduled to remove itself from the waitqueue and die.
> > 
> > pfmemalloc_wait is used as wait_event and that one uses
> > autoremove_wake_function for wake ups so the task shouldn't stay on the
> > queue if it was woken up. Moreover pfmemalloc_wait sleeps are killable
> > by the OOM killer AFAICS.
> > 
> > $ git grep "wait_event.*pfmemalloc_wait"
> > mm/vmscan.c:
> > wait_event_interruptible_timeout(pgdat->pfmemalloc_wait,
> > mm/vmscan.c:    wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,))
> > 
> > So OOM killer would wake it up already and kswapd shouldn't see this
> > task on the waitqueue anymore.
> 
> OOM killer will wake up the process, but it won't remove it from the
> pfmemalloc_wait queue. Therefore, if kswapd gets scheduled before the
> dying process, it will see the wait queue being still active, but won't
> be able to wake anyone up, because the waiting process has already been
> woken by SIGKILL. I think this is what Vlastimil means.

OK, I see the point now. I didn't realize that autoremove_wake_function
doesn't remove the waiter from the queue if the state doesn't change.

> So AFAIU the problem does exist. However, I think it could be fixed by
> simply waking up all processes waiting on pfmemalloc_wait before putting
> kswapd to sleep:

I think that a simple cond_resched() in kswapd_try_to_sleep should be
sufficient and less risky fix, so basically what Vlastimil was proposing
in the beginning.

> 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?

>  	/*
>  	 * There is a potential race between when kswapd checks its watermarks
>  	 * and a process gets throttled. There is also a potential race if
> @@ -2993,12 +2996,9 @@ 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);
> +	return true;
>  }
>  
>  /*

-- 
Michal Hocko
SUSE Labs
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