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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 23 May 2023 10:59:33 -0700
Message-ID: <CAHk-=wiH9JLQ0AN98aLKpxiAL4ssbnnq_=3R_qvy7AEGNT_iJg@mail.gmail.com>
Subject: Re: [PATCHSET v1 wq/for-6.5] workqueue: Improve unbound workqueue
 execution locality
To:     Tejun Heo <tj@kernel.org>, Sandeep Dhavale <dhavale@google.com>
Cc:     jiangshanlai@gmail.com, peterz@infradead.org,
        linux-kernel@vger.kernel.org, kernel-team@meta.com,
        joshdon@google.com, brho@google.com, briannorris@chromium.org,
        nhuck@google.com, agk@redhat.com, snitzer@kernel.org,
        void@manifault.com
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On Mon, May 22, 2023 at 6:51=E2=80=AFPM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> Ok, looks good, consider me convinced.

Ugh, and I just got the erofs pull, which made me aware of how there's
*another* hack for "worker threads don't work well on Android", which
now doubled down on setting special scheduler flags for them too.

See commit cf7f2732b4b8 ("erofs: use HIPRI by default if per-cpu
kthreads are enabled"), but the whole deeper problem goes back much
further (commit 3fffb589b9a6: "erofs: add per-cpu threads for
decompression as an option").

See also

    https://lore.kernel.org/lkml/CAB=3DBE-SBtO6vcoyLNA9F-9VaN5R0t3o_Zn+FW8G=
bO6wyUqFneQ@mail.gmail.com/

I really hate how we have random drivers and filesystems doing random
workarounds for "kthread workers don't work well enough, so add random
tweaks".

> I still would like to hear from the actual arm crowd that caused this
> all and have those odd BIG.little cases.

Sandeep, any chance that you could try out Tejun's series with plain
workers, and compare it to the percpu threads?

See

     https://lore.kernel.org/lkml/20230519001709.2563-1-tj@kernel.org/

for the beginning of this thread. The aim really is to try to figure
out - and hopefully fix - the fact that Android loads seem to trigger
all these random hacks that don't make any sense on a high level, and
seem to be workarounds rather than real fixes. Scheduling percpu
workers is a horrible horrible fix and not at all good in general, and
it would be much better if we could just improve workqueue behavior in
the odd Android situation.

              Linus