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From:   Nathan Huckleberry <nhuck@google.com>
Date:   Thu, 2 Feb 2023 11:11:16 -0800
Message-ID: <CAJkfWY5sYxPe1ArxSe3fkwqkOFSxxz9HDdF6XoL_Wc9g070hRg@mail.gmail.com>
Subject: Re: dm-verity: Remove WQ_UNBOUND.
To:     Mike Snitzer <snitzer@kernel.org>
Cc:     mpatocka@redhat.com, linux-kernel@vger.kernel.org,
        Eric Biggers <ebiggers@kernel.org>, dm-devel@redhat.com,
        Sami Tolvanen <samitolvanen@google.com>,
        Alasdair Kergon <agk@redhat.com>
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Looks good. Thanks.

On Thu, Feb 2, 2023 at 8:41 AM Mike Snitzer <snitzer@kernel.org> wrote:
>
> On Wed, Feb 01 2023 at  8:23P -0500,
> Nathan Huckleberry <nhuck@google.com> wrote:
>
> > Setting WQ_UNBOUND increases scheduler latency on ARM64.  This is likely
> > due to the asymmetric architecture of ARM64 processors.
> >
> > I've been unable to reproduce the results that claim WQ_UNBOUND gives a
> > performance boost on x86-64.
> >
> > This flag is causing performance issues for multiple subsystems within
> > Android.  Notably, the same slowdown exists for decompression with
> > EROFS.
> >
> > | open-prebuilt-camera  | WQ_UNBOUND | ~WQ_UNBOUND   |
> > |-----------------------|------------|---------------|
> > | verity wait time (us) | 11746      | 119 (-98%)    |
> > | erofs wait time (us)  | 357805     | 174205 (-51%) |
> >
> > | sha256 ramdisk random read | WQ_UNBOUND    | ~WQ_UNBOUND |
> > |----------------------------|-----------=---|-------------|
> > | arm64 (accelerated)        | bw=42.4MiB/s  | bw=212MiB/s |
> > | arm64 (generic)            | bw=16.5MiB/s  | bw=48MiB/s  |
> > | x86_64 (generic)           | bw=233MiB/s   | bw=230MiB/s |
> >
> > Cc: Sami Tolvanen <samitolvanen@google.com>
> > Cc: Eric Biggers <ebiggers@kernel.org>
> > Signed-off-by: Nathan Huckleberry <nhuck@google.com>
> > ---
> >  drivers/md/dm-verity-target.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> > index ccf5b852fbf7..020fd2341025 100644
> > --- a/drivers/md/dm-verity-target.c
> > +++ b/drivers/md/dm-verity-target.c
> > @@ -1399,8 +1399,8 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
> >               goto bad;
> >       }
> >
> > -     /* WQ_UNBOUND greatly improves performance when running on ramdisk */
> > -     wq_flags = WQ_MEM_RECLAIM | WQ_UNBOUND;
> > +     wq_flags = WQ_MEM_RECLAIM;
> > +
> >       /*
> >        * Using WQ_HIGHPRI improves throughput and completion latency by
> >        * reducing wait times when reading from a dm-verity device.
> > --
> > 2.39.1.456.gfc5497dd1b-goog
>
> Hi,
>
> I've discussed with Mikulas, tweaked your patch slightly but accepted
> your change, please see:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-6.3&id=6f30cc248507ee96c22ff4c3cbc86099ff12b7a9
>
> Thanks,
> Mike