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From:   Nathan Huckleberry <nhuck@google.com>
Date:   Wed, 18 Jan 2023 18:01:04 -0800
Message-ID: <CAJkfWY4Az45dNkPu5JpDsiMV-gRLe2VjVuixQd9xNG7zdLb4jA@mail.gmail.com>
Subject: Re: [PATCH] workqueue: Add WQ_SCHED_FIFO
To:     Tejun Heo <tj@kernel.org>
Cc:     Sandeep Dhavale <dhavale@google.com>,
        Daeho Jeong <daehojeong@google.com>,
        Eric Biggers <ebiggers@kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Jonathan Corbet <corbet@lwn.net>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org
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On Fri, Jan 13, 2023 at 1:11 PM Tejun Heo <tj@kernel.org> wrote:
>
> On Fri, Jan 13, 2023 at 01:07:02PM -0800, Nathan Huckleberry wrote:
> > Add a WQ flag that allows workqueues to use SCHED_FIFO with the least
> > imporant RT priority.  This can reduce scheduler latency for IO
> > post-processing when the CPU is under load without impacting other RT
> > workloads.  This has been shown to improve app startup time on Android
> > [1].
> >
> > Scheduler latency affects several drivers as evidenced by [1], [2], [3],
> > [4].  Some of these drivers have moved post-processing into IRQ context.
> > However, this can cause latency spikes for real-time threads and jitter
> > related jank on Android.  Using a workqueue with SCHED_FIFO improves
> > scheduler latency without causing latency problems for RT threads.
> >
> > [1]:
> > https://lore.kernel.org/linux-erofs/20230106073502.4017276-1-dhavale@google.com/
> > [2]:
> > https://lore.kernel.org/linux-f2fs-devel/20220802192437.1895492-1-daeho43@gmail.com/
> > [3]:
> > https://lore.kernel.org/dm-devel/20220722093823.4158756-4-nhuck@google.com/
> > [4]:
> > https://lore.kernel.org/dm-crypt/20200706173731.3734-1-ignat@cloudflare.com/
> >
> > This change has been tested on dm-verity with the following fio config:
> >
> > [global]
> > time_based
> > runtime=120
> >
> > [do-verify]
> > ioengine=sync
> > filename=/dev/testing
> > rw=randread
> > direct=1
> >
> > [burn_8x90%_qsort]
> > ioengine=cpuio
> > cpuload=90
> > numjobs=8
> > cpumode=qsort
> >
> > Before:
> > clat (usec): min=13, max=23882, avg=29.56, stdev=113.29 READ:
> > bw=122MiB/s (128MB/s), 122MiB/s-122MiB/s (128MB/s-128MB/s), io=14.3GiB
> > (15.3GB), run=120001-120001msec
> >
> > After:
> > clat (usec): min=13, max=23137, avg=19.96, stdev=105.71 READ:
> > bw=180MiB/s (189MB/s), 180MiB/s-180MiB/s (189MB/s-189MB/s), io=21.1GiB
> > (22.7GB), run=120012-120012msec
>

Hi Tejun,

> Given that its use case mostly intersects with WQ_HIGHPRI, would it make
> more sense to add a switch to alter its behavior instead? I don't really
> like the idea of pushing the decision between WQ_HIGHPRI and WQ_SCHED_FIFO
> to each user.

Do you think something similar should be done for WQ_UNBOUND? In most
places where WQ_HIGHPRI is used, WQ_UNBOUND is also used because it
boosts performance. However, I suspect that most of these benchmarks
were done on x86-64. I've found that WQ_UNBOUND significantly reduces
performance on arm64/Android.

From the documentation, using WQ_UNBOUND for performance doesn't seem
correct. It's only supposed to be used for long-running work. It might
make more sense to get rid of WQ_UNBOUND altogether and only move work
to unbound worker pools once it has stuck around for long enough.

Android will probably need to remove WQ_UNBOUND from all of these
performance critical users.

If there are performance benefits to using unbinding workqueues from
CPUs on x86-64, that should probably be a config flag, not controlled
by every user.

Thanks,
Huck

>
> Thanks.
>
> --
> tejun