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

[Mel Gorman <mgorman@techsingularity.net>]

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.

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.

-- 
Mel Gorman
SUSE Labs