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

[Josh Poimboeuf <jpoimboe@kernel.org>]

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