Re: [PATCH v4 1/1] mm: vmscan: Reduce throttling due to a failure to make progress

[Shakeel Butt <shakeelb@google.com>]

On Thu, Dec 2, 2021 at 8:52 AM Mel Gorman <mgorman@techsingularity.net> wrote:
>
> On Thu, Dec 02, 2021 at 08:30:51AM -0800, Shakeel Butt wrote:
> > Hi Mel,
> >
> > On Thu, Dec 2, 2021 at 7:07 AM Mel Gorman <mgorman@techsingularity.net> wrote:
> > >
> > > Mike Galbraith, Alexey Avramov and Darrick Wong all reported similar
> > > problems due to reclaim throttling for excessive lengths of time.
> > > In Alexey's case, a memory hog that should go OOM quickly stalls for
> > > several minutes before stalling. In Mike and Darrick's cases, a small
> > > memcg environment stalled excessively even though the system had enough
> > > memory overall.
> > >
> > > Commit 69392a403f49 ("mm/vmscan: throttle reclaim when no progress is being
> > > made") introduced the problem although commit a19594ca4a8b ("mm/vmscan:
> > > increase the timeout if page reclaim is not making progress") made it
> > > worse. Systems at or near an OOM state that cannot be recovered must
> > > reach OOM quickly and memcg should kill tasks if a memcg is near OOM.
> > >
> >
> > Is there a reason we can't simply revert 69392a403f49 instead of adding
> > more code/heuristics? Looking more into 69392a403f49, I don't think the
> > code and commit message are in sync.
> >
> > For the memcg reclaim, instead of just removing congestion_wait or
> > replacing it with schedule_timeout in mem_cgroup_force_empty(), why
> > change the behavior of all memcg reclaim. Also this patch effectively
> > reverts that behavior of 69392a403f49.
> >
>
> It doesn't fully revert it but I did consider reverting it. The reason
> why I preserved it because the intent originally was to throttle somewhat
> when progress is not being made to avoid a premature OOM and I wanted to
> preserve that charactersistic. Right now, this is the least harmful way
> of doing it.

If I understand correctly, the original intent of 69392a403f49 which
you want to preserve is "avoid premature OOMs when reclaim is not
making progress". Were there any complaints or bug reports on these
premature OOMs?

>
> As more memcg, I removed the NOTHROTTLE because the primary reason why a
> memcg might fail to make progress is excessive writeback and that should
> still throttle. Completely failing to make progress in a memcg is most
> likely due to a memcg-OOM.
>
> > For direct reclaimers under global pressure, why is page allocator a bad
> > place for stalling on no progress reclaim? IMHO the callers of the
> > reclaim should decide what to do if reclaim is not making progress.
>
> Because it's a layering violation and the caller has little direct control
> over the reclaim retry logic. The page allocator has no visibility on
> why reclaim failed only that it did fail.
>

Isn't it better that the reclaim returns why it is failing instead of
littering the reclaim code with 'is this global reclaim', 'is this
memcg reclaim', 'am I kswapd' which is also a layering violation. IMO
this is the direction we should be going towards though not asking to
do this now.

Regarding this patch and 69392a403f49, I am still confused on the main
motivation behind 69392a403f49 to change the behavior of 'direct
reclaimers from page allocator'.

thanks,
Shakeel