linux-kernel - Re: [PATCH 2/2] mm: memcontrol: try harder to set a new memory.high

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Message-ID: <20191023175724.GD366316@cmpxchg.org>
Date:   Wed, 23 Oct 2019 13:57:24 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 2/2] mm: memcontrol: try harder to set a new memory.high

On Wed, Oct 23, 2019 at 08:59:49AM +0200, Michal Hocko wrote:
> On Tue 22-10-19 16:15:18, Johannes Weiner wrote:
> > Setting a memory.high limit below the usage makes almost no effort to
> > shrink the cgroup to the new target size.
> > 
> > While memory.high is a "soft" limit that isn't supposed to cause OOM
> > situations, we should still try harder to meet a user request through
> > persistent reclaim.
> > 
> > For example, after setting a 10M memory.high on an 800M cgroup full of
> > file cache, the usage shrinks to about 350M:
> > 
> > + cat /cgroup/workingset/memory.current
> > 841568256
> > + echo 10M
> > + cat /cgroup/workingset/memory.current
> > 355729408
> > 
> > This isn't exactly what the user would expect to happen. Setting the
> > value a few more times eventually whittles the usage down to what we
> > are asking for:
> > 
> > + echo 10M
> > + cat /cgroup/workingset/memory.current
> > 104181760
> > + echo 10M
> > + cat /cgroup/workingset/memory.current
> > 31801344
> > + echo 10M
> > + cat /cgroup/workingset/memory.current
> > 10440704
> > 
> > To improve this, add reclaim retry loops to the memory.high write()
> > callback, similar to what we do for memory.max, to make a reasonable
> > effort that the usage meets the requested size after the call returns.
> 
> That suggests that the reclaim couldn't meet the given reclaim target
> but later attempts just made it through. Is this due to amount of dirty
> pages or what prevented the reclaim to do its job?
> 
> While I am not against the reclaim retry loop I would like to understand
> the underlying issue. Because if this is really about dirty memory then
> we should probably be more pro-active in flushing it. Otherwise the
> retry might not be of any help.

All the pages in my test case are clean cache. But they are active,
and they need to go through the inactive list before reclaiming. The
inactive list size is designed to pre-age just enough pages for
regular reclaim targets, i.e. pages in the SWAP_CLUSTER_MAX ballpark,
In this case, the reclaim goal for a single invocation is 790M and the
inactive list is a small funnel to put all that through, and we need
several iterations to accomplish that.

But 790M is not a reasonable reclaim target to ask of a single reclaim
invocation. And it wouldn't be reasonable to optimize the reclaim code
for it. So asking for the full size but retrying is not a bad choice
here: we express our intent, and benefit if reclaim becomes better at
handling larger requests, but we also acknowledge that some of the
deltas we can encounter in memory_high_write() are just too
ridiculously big for a single reclaim invocation to manage.