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From: Yu Zhao <yuzhao@google.com>
Date: Fri, 26 Jan 2024 23:17:26 -0700
Message-ID: <CAOUHufa68Tm0OpWkzC_VQYAEGtgaAwkJQqztOFwBqYrDD=1T+g@mail.gmail.com>
Subject: Re: [PATCH] Revert "mm:vmscan: fix inaccurate reclaim during
 proactive reclaim"
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: "T.J. Mercier" <tjmercier@google.com>, Michal Hocko <mhocko@suse.com>, 
	Roman Gushchin <roman.gushchin@linux.dev>, Shakeel Butt <shakeelb@google.com>, 
	Muchun Song <muchun.song@linux.dev>, Andrew Morton <akpm@linux-foundation.org>, android-mm@google.com, 
	yangyifei03@kuaishou.com, cgroups@vger.kernel.org, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org
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On Tue, Jan 23, 2024 at 9:48=E2=80=AFAM Johannes Weiner <hannes@cmpxchg.org=
> wrote:
>
> The revert isn't a straight-forward solution.
>
> The patch you're reverting fixed conventional reclaim and broke
> MGLRU. Your revert fixes MGLRU and breaks conventional reclaim.

This is not true -- the patch reverted regressed the active/inactive
LRU too, on execution time.

Quoting the commit message: "completion times for proactive reclaim on
much smaller non-root cgroups take ~30% longer (with or without
MGLRU)."

And I wouldn't call the original patch a fix -- it shifted the problem
from space to time, which at best is a tradeoff.

> On Tue, Jan 23, 2024 at 05:58:05AM -0800, T.J. Mercier wrote:
> > They both are able to make progress. The main difference is that a
> > single iteration of try_to_free_mem_cgroup_pages with MGLRU ends soon
> > after it reclaims nr_to_reclaim, and before it touches all memcgs. So
> > a single iteration really will reclaim only about SWAP_CLUSTER_MAX-ish
> > pages with MGLRU. WIthout MGLRU the memcg walk is not aborted
> > immediately after nr_to_reclaim is reached, so a single call to
> > try_to_free_mem_cgroup_pages can actually reclaim thousands of pages
> > even when sc->nr_to_reclaim is 32. (I.E. MGLRU overreclaims less.)
> > https://lore.kernel.org/lkml/20221201223923.873696-1-yuzhao@google.com/
>
> Is that a feature or a bug?
>
>  * 1. Memcg LRU only applies to global reclaim, and the round-robin incre=
menting
>  *    of their max_seq counters ensures the eventual fairness to all elig=
ible
>  *    memcgs. For memcg reclaim, it still relies on mem_cgroup_iter().
>
> If it bails out exactly after nr_to_reclaim, it'll overreclaim
> less. But with steady reclaim in a complex subtree, it will always hit
> the first cgroup returned by mem_cgroup_iter() and then bail. This
> seems like a fairness issue.
>
> We should figure out what the right method for balancing fairness with
> overreclaim is, regardless of reclaim implementation. Because having
> two different approaches and reverting dependent things back and forth
> doesn't make sense.
>
> Using an LRU to rotate through memcgs over multiple reclaim cycles
> seems like a good idea. Why is this specific to MGLRU? Shouldn't this
> be a generic piece of memcg infrastructure?
>
> Then there is the question of why there is an LRU for global reclaim,
> but not for subtree reclaim. Reclaiming a container with multiple
> subtrees would benefit from the fairness provided by a container-level
> LRU order just as much; having fairness for root but not for subtrees
> would produce different reclaim and pressure behavior, and can cause
> regressions when moving a service from bare-metal into a container.
>
> Figuring out these differences and converging on a method for cgroup
> fairness would be the better way of fixing this. Because of the
> regression risk to the default reclaim implementation, I'm inclined to
> NAK this revert.