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From:   Nathan Huckleberry <nhuck@google.com>
Date:   Mon, 13 Mar 2023 16:07:59 -0700
Message-ID: <CAJkfWY5eg2wLMUKUL5whU6zoHFogiUna1NPE+W8fPHm8YS-ReA@mail.gmail.com>
Subject: Re: [PATCH v2] fsverity: Remove WQ_UNBOUND from fsverity read workqueue
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     "Theodore Y. Ts'o" <tytso@mit.edu>, fsverity@lists.linux.dev,
        linux-kernel@vger.kernel.org
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Thanks Eric.

On Mon, Mar 13, 2023 at 3:59=E2=80=AFPM Eric Biggers <ebiggers@kernel.org> =
wrote:
>
> On Fri, Mar 10, 2023 at 11:33:25AM -0800, Nathan Huckleberry wrote:
> > WQ_UNBOUND causes significant scheduler latency on ARM64/Android.  This
> > is problematic for latency sensitive workloads, like I/O
> > post-processing.
> >
> > Removing WQ_UNBOUND gives a 96% reduction in fsverity workqueue related
> > scheduler latency and improves app cold startup times by ~30ms.
> > WQ_UNBOUND was also removed from the dm-verity workqueue for the same
> > reason [1].
> >
> > This code was tested by running Android app startup benchmarks and
> > measuring how long the fsverity workqueue spent in the runnable state.
> >
> > Before
> > Total workqueue scheduler latency: 553800us
> > After
> > Total workqueue scheduler latency: 18962us
> >
> > [1]: https://lore.kernel.org/all/20230202012348.885402-1-nhuck@google.c=
om/
> >
> > Signed-off-by: Nathan Huckleberry <nhuck@google.com>
> > ---
> > Changelog:
> > v1 -> v2:
> > - Added comment about WQ_UNBOUND
> > - Added info about related dm-verity patches in commit message
> >
> >  fs/verity/verify.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/verity/verify.c b/fs/verity/verify.c
> > index f50e3b5b52c9..782b8b4a24c1 100644
> > --- a/fs/verity/verify.c
> > +++ b/fs/verity/verify.c
> > @@ -387,15 +387,15 @@ EXPORT_SYMBOL_GPL(fsverity_enqueue_verify_work);
> >  int __init fsverity_init_workqueue(void)
> >  {
> >       /*
> > -      * Use an unbound workqueue to allow bios to be verified in paral=
lel
> > -      * even when they happen to complete on the same CPU.  This sacri=
fices
> > -      * locality, but it's worthwhile since hashing is CPU-intensive.
> > -      *
> >        * Also use a high-priority workqueue to prioritize verification =
work,
> >        * which blocks reads from completing, over regular application t=
asks.
> > +      *
> > +      * This workqueue is not marked as unbound for performance reason=
s.
> > +      * Using an unbound workqueue for crypto operations causes excess=
ive
> > +      * scheduler latency on ARM64.
> >        */
> >       fsverity_read_workqueue =3D alloc_workqueue("fsverity_read_queue"=
,
> > -                                               WQ_UNBOUND | WQ_HIGHPRI=
,
> > +                                               WQ_HIGHPRI,
> >                                                 num_online_cpus());
>
> Applied to https://git.kernel.org/pub/scm/fs/fsverity/linux.git/log/?h=3D=
for-next
>
> I adjusted the comment slightly so that the first paragraph doesn't start=
 with
> "Also".
>
> - Eric