Re: [PATCH] mm, vmscan: do not loop on too_many_isolated for ever

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 10 Jul 2017 09:48:42 +0200 Michal Hocko <mhocko@kernel.org> wrote:

> From: Michal Hocko <mhocko@suse.com>
> 
> Tetsuo Handa has reported [1][2][3]that direct reclaimers might get stuck
> in too_many_isolated loop basically for ever because the last few pages
> on the LRU lists are isolated by the kswapd which is stuck on fs locks
> when doing the pageout or slab reclaim. This in turn means that there is
> nobody to actually trigger the oom killer and the system is basically
> unusable.
> 
> too_many_isolated has been introduced by 35cd78156c49 ("vmscan: throttle
> direct reclaim when too many pages are isolated already") to prevent
> from pre-mature oom killer invocations because back then no reclaim
> progress could indeed trigger the OOM killer too early. But since the
> oom detection rework 0a0337e0d1d1 ("mm, oom: rework oom detection")
> the allocation/reclaim retry loop considers all the reclaimable pages
> and throttles the allocation at that layer so we can loosen the direct
> reclaim throttling.
> 
> Make shrink_inactive_list loop over too_many_isolated bounded and returns
> immediately when the situation hasn't resolved after the first sleep.
> Replace congestion_wait by a simple schedule_timeout_interruptible because
> we are not really waiting on the IO congestion in this path.
> 
> Please note that this patch can theoretically cause the OOM killer to
> trigger earlier while there are many pages isolated for the reclaim
> which makes progress only very slowly. This would be obvious from the oom
> report as the number of isolated pages are printed there. If we ever hit
> this should_reclaim_retry should consider those numbers in the evaluation
> in one way or another.

Need to figure out which kernels to patch.  Maybe just 4.13-rc after a
week or two?

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1713,9 +1713,15 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
>  	int file = is_file_lru(lru);
>  	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>  	struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
> +	bool stalled = false;
>  
>  	while (unlikely(too_many_isolated(pgdat, file, sc))) {
> -		congestion_wait(BLK_RW_ASYNC, HZ/10);
> +		if (stalled)
> +			return 0;
> +
> +		/* wait a bit for the reclaimer. */
> +		schedule_timeout_interruptible(HZ/10);

a) if this task has signal_pending(), this falls straight through
   and I suspect the code breaks?

b) replacing congestion_wait() with schedule_timeout_interruptible()
   means this task no longer contributes to load average here and it's
   a (slightly) user-visible change.

c) msleep_interruptible() is nicer

d) IOW, methinks we should be using msleep() here?

> +		stalled = true;
>  
>  		/* We are about to die and free our memory. Return now. */
>  		if (fatal_signal_pending(current))

(Gets distracted by the thought that we should do
s/msleep/msleep_uninterruptible/g)