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

[Roman Gushchin <roman.gushchin@linux.dev>]

On Wed, Jan 17, 2024 at 01:02:19PM -0800, Shakeel Butt wrote:
> On Wed, Jan 17, 2024 at 12:21 PM 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/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.

It's roughly an equal split between mod_objcg_state() and obj_cgroup_charge().
And each is comparable (by order of magnitude) to the slab allocation cost
itself. On the free() path a significant cost comes simple from reading
the objcg pointer (it's usually a cache miss).

So I don't see how we can make it really cheap (say, less than 5% overhead)
without caching pre-accounted objects.

I thought about merging of charge and stats handling paths, which _maybe_ can
shave off another 20-30%, but there still will be a double-digit% accounting
overhead.

I'm curious to hear other ideas and suggestions.

Thanks!