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

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Message-ID: <20240117193915.urwueineol7p4hg7@treble>
Date: Wed, 17 Jan 2024 11:39:15 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Jeff Layton <jlayton@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Chuck Lever <chuck.lever@...cle.com>,
	Shakeel Butt <shakeelb@...gle.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 02:00:55PM -0500, Jeff Layton wrote:
> I'm really not a fan of tunables or different kconfig options,
> especially for something niche like this.
> 
> I also question whether this accounting will show up under any real-
> world workloads, and whether it was just wrong to revert those patches
> back in 2021.
> 
> File locking is an activity where we inherently expect to block. Ideally
> we don't if the lock is uncontended of course, but it's always a
> possibility.
> 
> The benchmark that prompted the regression basically just tries to
> create and release a bunch of file locks as quickly as possible.
> Legitimate applications that do a lot of very rapid locking like this
> benchmark are basically non-existent. Usually the pattern is:
> 
>     acquire lock
>     do some (relatively slow) I/O
>     release lock
> 
> In that sort of scenario, is this memcg accounting more than just line
> noise? I wonder whether we should just bite the bullet and see whether
> there are any real workloads that suffer due to SLAB_ACCOUNT being
> enabled on these caches?

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?

-- 
Josh