From: Vlastimil Babka <vbabka@suse.cz>
To: Linus Torvalds <torvalds@linux-foundation.org>,
Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Jeff Layton <jlayton@kernel.org>,
Chuck Lever <chuck.lever@oracle.com>,
Shakeel Butt <shakeelb@google.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
Subject: Re: [PATCH RFC 1/4] fs/locks: Fix file lock cache accounting, again
Date: Wed, 17 Jan 2024 22:19:48 +0100 [thread overview]
Message-ID: <f1e43327-30ce-4a73-8a24-c813a516f97f@suse.cz> (raw)
In-Reply-To: <CAHk-=wg_CoTOfkREgaQQA6oJ5nM9ZKYrTn=E1r-JnvmQcgWpSg@mail.gmail.com>
On 1/17/24 21:20, Linus Torvalds 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.
Counters are one thing (afaik some batching happens on the memcg side via
"stocks"), but another is associating the memcg with the allocated objects
in slab pages, so kmem_cache_free() knows which counter to decrement. We'll
have to see where the overhead is today.
If there's overhead due to calls between mm/slub.c and mm/memcontrol.c we
can now reduce that with SLAB gone.
> 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.
Roman and Shakeel are already Cc'd. Roman recently did
https://lore.kernel.org/lkml/20231019225346.1822282-1-roman.gushchin@linux.dev/
which is mentioned in the cover letter and was merged in 6.7, but cover says
it didn't help much, too bad. So is it still 33% or how much?
> 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
next prev parent reply other threads:[~2024-01-17 21:19 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-17 16:14 [PATCH RFC 0/4] Fix file lock cache accounting, again Josh Poimboeuf
2024-01-17 16:14 ` [PATCH RFC 1/4] fs/locks: " Josh Poimboeuf
2024-01-17 19:00 ` Jeff Layton
2024-01-17 19:39 ` Josh Poimboeuf
2024-01-17 20:20 ` Linus Torvalds
2024-01-17 21:02 ` Shakeel Butt
2024-01-17 22:20 ` Roman Gushchin
2024-01-17 22:56 ` Shakeel Butt
2024-01-22 5:10 ` Linus Torvalds
2024-01-22 17:38 ` Shakeel Butt
2024-01-26 9:50 ` Vlastimil Babka
2024-01-30 11:04 ` Vlastimil Babka
2024-01-19 7:47 ` Shakeel Butt
2024-01-17 21:19 ` Vlastimil Babka [this message]
2024-01-17 21:50 ` Roman Gushchin
2024-01-18 9:49 ` Michal Hocko
2024-01-17 16:14 ` [PATCH RFC 2/4] fs/locks: Add CONFIG_FLOCK_ACCOUNTING Josh Poimboeuf
2024-01-17 16:14 ` [PATCH RFC 3/4] mitigations: Expand 'mitigations=off' to include optional software mitigations Josh Poimboeuf
2024-01-17 16:14 ` [PATCH RFC 4/4] mitigations: Add flock cache accounting to 'mitigations=off' Josh Poimboeuf
2024-01-18 9:04 ` Michal Koutný
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