linux-kernel - Re: [PATCH v3] mm: memcg: Use larger batches for proactive reclaim

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Message-ID: <ZcH0wBPvOjqayjAD@tiehlicka>
Date: Tue, 6 Feb 2024 09:58:40 +0100
From: Michal Hocko <mhocko@...e.com>
To: "T.J. Mercier" <tjmercier@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Shakeel Butt <shakeelb@...gle.com>,
	Muchun Song <muchun.song@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Efly Young <yangyifei03@...ishou.com>, android-mm@...gle.com,
	yuzhao@...gle.com, mkoutny@...e.com,
	Yosry Ahmed <yosryahmed@...gle.com>, cgroups@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm: memcg: Use larger batches for proactive reclaim

On Mon 05-02-24 20:01:40, T.J. Mercier wrote:
> On Mon, Feb 5, 2024 at 1:16 PM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Mon 05-02-24 12:47:47, T.J. Mercier wrote:
> > > On Mon, Feb 5, 2024 at 12:36 PM Michal Hocko <mhocko@...e.com> wrote:
> > [...]
> > > > This of something like
> > > > timeout $TIMEOUT echo $TARGET > $MEMCG_PATH/memory.reclaim
> > > > where timeout acts as a stop gap if the reclaim cannot finish in
> > > > TIMEOUT.
> > >
> > > Yeah I get the desired behavior, but using sc->nr_reclaimed to achieve
> > > it is what's bothering me.
> >
> > I am not really happy about this subtlety. If we have a better way then
> > let's do it. Better in its own patch, though.
> >
> > > It's already wired up that way though, so if you want to make this
> > > change now then I can try to test for the difference using really
> > > large reclaim targets.
> >
> > Yes, please. If you want it a separate patch then no objection from me
> > of course. If you do no like the nr_to_reclaim bailout then maybe we can
> > go with a simple break out flag in scan_control.
> >
> > Thanks!
> 
> It's a bit difficult to test under the too_many_isolated check, so I
> moved the fatal_signal_pending check outside and tried with that.
> Performing full reclaim on the /uid_0 cgroup with a 250ms delay before
> SIGKILL, I got an average of 16ms better latency with
> sc->nr_to_reclaim across 20 runs ignoring one 1s outlier with
> SWAP_CLUSTER_MAX.

This will obviously scale with the number of memcgs in the hierarchy but
you are right that too_many_isolated makes the whole fatal_signal_pending
check rather inefficient. I haven't missed that. The reclaim path is
rather convoluted so this will likely be more complex than I
anticipated. I will think about that some more.

In order to not delay your patch, please repost with suggested updates
to the changelog. This needs addressing IMO but I do not think this is
critical at this stage.

-- 
Michal Hocko
SUSE Labs