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From: Shakeel Butt <shakeelb@google.com>
Date: Wed, 17 Jan 2024 13:02:19 -0800
Message-ID: <CALvZod6LgX-FQOGgNBmoRACMBK4GB+K=a+DYrtExcuGFH=J5zQ@mail.gmail.com>
Subject: Re: [PATCH RFC 1/4] fs/locks: Fix file lock cache accounting, again
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>, Vlastimil Babka <vbabka@suse.cz>, 
	Jeff Layton <jlayton@kernel.org>, Chuck Lever <chuck.lever@oracle.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, Jan 17, 2024 at 12:21=E2=80=AFPM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> 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/whe=
n
> > 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-902=
0/
>
> 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.