linux-kernel - Re: [PATCH RFC 1/4] fs/locks: Fix file lock cache accounting, again

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Date: Wed, 17 Jan 2024 13:02:19 -0800
From: Shakeel Butt <shakeelb@...gle.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, Vlastimil Babka <vbabka@...e.cz>, 
	Jeff Layton <jlayton@...nel.org>, Chuck Lever <chuck.lever@...cle.com>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org, 
	Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>, Vasily Averin <vasily.averin@...ux.dev>, 
	Michal Koutny <mkoutny@...e.com>, Waiman Long <longman@...hat.com>, 
	Muchun Song <muchun.song@...ux.dev>, Jiri Kosina <jikos@...nel.org>, cgroups@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCH RFC 1/4] fs/locks: Fix file lock cache accounting, again

On Wed, Jan 17, 2024 at 12:21 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Wed, 17 Jan 2024 at 11:39, Josh Poimboeuf <jpoimboe@...nel.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.
>

Roman last looked into optimizing this code path. I suspect
mod_objcg_state() to be more costly than obj_cgroup_charge(). I will
try to measure this path and see if I can improve it.