linux-kernel - Re: [PATCH v3 0/8] make slab shrink lockless

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Date:   Tue, 28 Feb 2023 19:40:57 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Qi Zheng <zhengqi.arch@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>, tkhai@...ru,
        hannes@...xchg.org, shakeelb@...gle.com, roman.gushchin@...ux.dev,
        muchun.song@...ux.dev, david@...hat.com, shy828301@...il.com,
        sultan@...neltoast.com, dave@...olabs.net,
        penguin-kernel@...ove.sakura.ne.jp, paulmck@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/8] make slab shrink lockless

On Mon 27-02-23 17:08:30, Mike Rapoport wrote:
[...]
> The results you present do show improvement in IPC for an artificial test
> script. But more interesting would be to see how a real world workloads
> benefit from your changes.

It's been quite some time ago (2018ish) when we have seen bug report
where mount got stalled when racing with memory reclaim. This was
nasty because the said mount was a part of login chain and users simply
had to wait for a long time to get loged in in that particular
deployment.

The mount was blocked on a shrinker registration and the reclaim was
stalled in a slab shrinker IIRC. I do not remember all the details but
the underlying problem was that a shrinker callback took a long time
because there were too many objects to scan or it had to sync with other
fs operation. I believe we ended up using Minchan's break out from slab
shrinking if the shrinker semaphore was contended and that helped to
some degree but there were still some corner cases where a single slab
shrinker could take a noticeable amount of time.

In general using a "big" lock like shrinker_rwsem from the reclaim and
potentially block many unrelated subsystems that just want to register
or unregister shrinkers is a potential source of hard to predict
problems. So this is a very welcome change.
-- 
Michal Hocko
SUSE Labs