Re: [PATCH] mm: fix condition for throttle_direct_reclaim

[Shakeel Butt <shakeelb@google.com>]

On Mon, Mar 13, 2017 at 8:46 AM, Michal Hocko <mhocko@kernel.org> wrote:
> On Mon 13-03-17 08:07:15, Shakeel Butt wrote:
>> On Mon, Mar 13, 2017 at 2:02 AM, Michal Hocko <mhocko@kernel.org> wrote:
>> > On Fri 10-03-17 11:46:20, Shakeel Butt wrote:
>> >> Recently kswapd has been modified to give up after MAX_RECLAIM_RETRIES
>> >> number of unsucessful iterations. Before going to sleep, kswapd thread
>> >> will unconditionally wakeup all threads sleeping on pfmemalloc_wait.
>> >> However the awoken threads will recheck the watermarks and wake the
>> >> kswapd thread and sleep again on pfmemalloc_wait. There is a chance
>> >> of continuous back and forth between kswapd and direct reclaiming
>> >> threads if the kswapd keep failing and thus defeat the purpose of
>> >> adding backoff mechanism to kswapd. So, add kswapd_failures check
>> >> on the throttle_direct_reclaim condition.
>> >
>> > I have to say I really do not like this. kswapd_failures shouldn't
>> > really be checked outside of the kswapd context. The
>> > pfmemalloc_watermark_ok/throttle_direct_reclaim is quite complex even
>> > without putting another variable into it. I wish we rather replace this
>> > throttling by something else. Johannes had an idea to throttle by the
>> > number of reclaimers.
>> >
>>
>> Do you suspect race in accessing kswapd_failures in non-kswapd
>> context?
>
> No, this is not about race conditions. It is more about the logic of the
> code. kswapd_failures is the private thing to the kswapd daemon. Direct
> reclaimers shouldn't have any business in it - well except resetting it.
>
>> Please do let me know more about replacing this throttling.
>
> The idea behind a different throttling would be to not allow too many
> direct reclaimers on the same set of nodes/zones. Johannes would tell
> you more.
>
>> > Anyway, I am wondering whether we can hit this issue in
>> > practice? Have you seen it happening or is this a result of the code
>> > review? I would assume that that !zone_reclaimable_pages check in
>> > pfmemalloc_watermark_ok should help to some degree.
>> >
>> Yes, I have seen this issue going on for more than one hour on my
>> test. It was a simple test where the number of processes, in the
>> presence of swap, try to allocate memory more than RAM.
>
> this is an anonymous memory, right?
>
Yes.

>> The number of
>> processes are equal to the number of cores and are pinned to each
>> individual core. I am suspecting that !zone_reclaimable_pages() check
>> did not help.
>
> Hmm, interesting! I would expect the OOM killer triggering but I guess
> I see what is going on. kswapd couldn't reclaim a single page and ran
> out of its kswapd_failures attempts while no direct reclaimers could
> reclaim a single page either until we reached the throttling point when
> we are basically livelocked because neither kswapd nor _all_ direct
> reclaimers can make a forward progress. Although this sounds quite
> unlikely I think it is quite possible to happen. So we cannot really
> throttle _all_ direct reclaimers when the kswapd is out of game which I
> haven't fully realized when reviewing "mm: fix 100% CPU kswapd busyloop
> on unreclaimable nodes".
>
> The simplest thing to do would be something like you have proposed and
> do not throttle if kswapd is out of game.
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bae698484e8e..d34b1afc781a 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2791,6 +2791,9 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
>         int i;
>         bool wmark_ok;
>
> +       if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES)
> +               return true;
> +
>         for (i = 0; i <= ZONE_NORMAL; i++) {
>                 zone = &pgdat->node_zones[i];
>                 if (!managed_zone(zone))
>
> I do not like this as I've already said but it would allow to merge
> "mm: fix 100% CPU kswapd busyloop on unreclaimable nodes" without too
> many additional changes.
>
> Another option would be to cap the waiting time same as we do for
> GFP_NOFS. Not ideal either because I suspect we would just get herds
> of direct reclaimers that way.
>
> The best option would be to rethink the throttling and move it out of
> the direct reclaim path somehow.
>
Agreed.

> Thanks and sorry for not spotting the potential lockup previously.
> --
> Michal Hocko
> SUSE Labs

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