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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 21 Jun 2023 12:31:45 -0700
Message-ID: <CAHk-=wgXoyxy99HnEFcvf+eUZAS5=ekWt_y84LC3P+0osxh6Jw@mail.gmail.com>
Subject: Re: [PATCHSET v1 wq/for-6.5] workqueue: Improve unbound workqueue
 execution locality
To:     Tejun Heo <tj@kernel.org>
Cc:     Pin-yen Lin <treapking@chromium.org>,
        Brian Norris <briannorris@chromium.org>,
        jiangshanlai@gmail.com, peterz@infradead.org,
        linux-kernel@vger.kernel.org, kernel-team@meta.com,
        joshdon@google.com, brho@google.com, nhuck@google.com,
        agk@redhat.com, snitzer@kernel.org, void@manifault.com
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On Wed, 21 Jun 2023 at 12:16, Tejun Heo <tj@kernel.org> wrote:
>
> I find that perplexing given that switching to a per-cpu workqueue remedies
> the situation quite a bit, which is how this patchset came to be. #3 is the
> same as per-cpu workqueue, so if you're seeing noticeably different
> performance numbers between #3 and per-cpu workqueue, there's something
> wrong with either the code or test setup.

Or maybe there's some silly thinko in the wq code that is hidden by
the percpu code.

For example, WQ_UNBOUND triggers a lot of other overhead at least on
wq allocation and free. Maybe some of that stuff then indirectly
affects workqueue execution even when strict cpu affinity is set.

Pin-Yen Li - can you do a system-wide profile of the two cases (the
percpu case vs the "strict cpu affinity" one), to see if something
stands out?

             Linus