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

[Linus Torvalds]

On Wed, 17 Jan 2024 at 11:39, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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