Re: [PATCH RFC] ipc/mqueue: introduce msg cache

From: Shakeel Butt <shakeelb@google.com>
To: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Vlastimil Babka <vbabka@suse.cz>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@kernel.org>,
	Muchun Song <muchun.song@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>,
	Waiman Long <longman@redhat.com>,
	Sven Luther <Sven.Luther@windriver.com>
Subject: Re: [PATCH RFC] ipc/mqueue: introduce msg cache
Date: Tue, 20 Dec 2022 15:28:11 -0800	[thread overview]
Message-ID: <CALvZod7q1jcpVyFUSzfL8-mSFa24peqrKDmqnMLwNe2=dNEpaw@mail.gmail.com> (raw)
In-Reply-To: <Y6IiPCg7c1A5fhsC@P9FQF9L96D.corp.robot.car>

On Tue, Dec 20, 2022 at 12:59 PM Roman Gushchin
<roman.gushchin@linux.dev> wrote:
>
> On Tue, Dec 20, 2022 at 11:53:25AM -0800, Shakeel Butt wrote:
> > +Vlastimil
> >
> > On Tue, Dec 20, 2022 at 10:48 AM Roman Gushchin
> > <roman.gushchin@linux.dev> wrote:
> > >
> > > Sven Luther reported a regression in the posix message queues
> > > performance caused by switching to the per-object tracking of
> > > slab objects introduced by patch series ending with the
> > > commit 10befea91b61 ("mm: memcg/slab: use a single set of kmem_caches for all
> > > allocations").
> > >
> > > To mitigate the regression cache allocated mqueue messages on a small
> > > percpu cache instead of releasing and re-allocating them every time.
> > >
> >
> > Seems fine with me but I am wondering what is stopping us to do this
> > caching in the slab layer for all accounted allocations? Does this
> > only make sense for specific scenarios/use-cases?
>
> It's far from trivial, unfortunately. Here we have an mqueue object to associate
> a percpu cache with and the hit rate is expected to be high, assuming the mqueue
> will be used to pass a lot of messages.
>
> With a generic slab cache we return to the necessity of managing
> the per-cgroup x per-slab-cache x per-cpu free list (or some other data structure),
> which is already far from trivial, based on the previous experience. It can
> easily lead to a significant memory waste, which will fully compensate all perf
> wins.
>
> So probably we need some heuristics to allocate caches only for really hot slab
> caches and use some sort of a hash map (keyed by cgroup and slab cache) to
> access freelists. What I miss to commit more time to this project (aside from not
> having it), is the lack of real workloads which will noticeably win from this work.
>
> Sven provided a good example and benchmark to reproduce the regression, so it
> was easy to justify the work.
>

Thanks for the explanation. I think we should add this to the commit
message as well. I do think we should have a general framework for
such caching as there are other users (e.g. io_uring) doing the same
and some future users can take advantage as well e.g. I think this
type of caching will be helpful for filelock_cache as well. Anyways
that can be done in future.