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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 17 Jan 2024 12:20:59 -0800
Message-ID: <CAHk-=wg_CoTOfkREgaQQA6oJ5nM9ZKYrTn=E1r-JnvmQcgWpSg@mail.gmail.com>
Subject: Re: [PATCH RFC 1/4] fs/locks: Fix file lock cache accounting, again
To: Josh Poimboeuf <jpoimboe@kernel.org>, Vlastimil Babka <vbabka@suse.cz>
Cc: Jeff Layton <jlayton@kernel.org>, Chuck Lever <chuck.lever@oracle.com>, 
	Shakeel Butt <shakeelb@google.com>, Roman Gushchin <roman.gushchin@linux.dev>, 
	Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@kernel.org>, linux-kernel@vger.kernel.org, 
	Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>, Vasily Averin <vasily.averin@linux.dev>, 
	Michal Koutny <mkoutny@suse.com>, Waiman Long <longman@redhat.com>, 
	Muchun Song <muchun.song@linux.dev>, Jiri Kosina <jikos@kernel.org>, cgroups@vger.kernel.org, 
	linux-mm@kvack.org
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On Wed, 17 Jan 2024 at 11:39, Josh Poimboeuf <jpoimboe@kernel.org> wrote:
>
> That's a good point.  If the microbenchmark isn't likely to be even
> remotely realistic, maybe we should just revert the revert until if/when
> somebody shows a real world impact.
>
> Linus, any objections to that?

We use SLAB_ACCOUNT for much more common allocations like queued
signals, so I would tend to agree with Jeff that it's probably just
some not very interesting microbenchmark that shows any file locking
effects from SLAB_ALLOC, not any real use.

That said, those benchmarks do matter. It's very easy to say "not
relevant in the big picture" and then the end result is that
everything is a bit of a pig.

And the regression was absolutely *ENORMOUS*. We're not talking "a few
percent". We're talking a 33% regression that caused the revert:

   https://lore.kernel.org/lkml/20210907150757.GE17617@xsang-OptiPlex-9020/

I wish our SLAB_ACCOUNT wasn't such a pig. Rather than account every
single allocation, it would be much nicer to account at a bigger
granularity, possibly by having per-thread counters first before
falling back to the obj_cgroup_charge. Whatever.

It's kind of stupid to have a benchmark that just allocates and
deallocates a file lock in quick succession spend lots of time
incrementing and decrementing cgroup charges for that repeated
alloc/free.

However, that problem with SLAB_ACCOUNT is not the fault of file
locking, but more of a slab issue.

End result: I think we should bring in Vlastimil and whoever else is
doing SLAB_ACCOUNT things, and have them look at that side.

And then just enable SLAB_ACCOUNT for file locks. But very much look
at silly costs in SLAB_ACCOUNT first, at least for trivial
"alloc/free" patterns..

Vlastimil? Who would be the best person to look at that SLAB_ACCOUNT
thing? See commit 3754707bcc3e (Revert "memcg: enable accounting for
file lock caches") for the history here.

                 Linus