Date:   Fri, 17 Dec 2021 19:17:58 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Song Liu <song@kernel.org>
Cc:     Yajun Deng <yajun.deng@linux.dev>, masahiroy@kernel.org,
        williams@redhat.com, Paul Menzel <pmenzel@molgen.mpg.de>,
        open list <linux-kernel@vger.kernel.org>,
        linux-rt-users@vger.kernel.org,
        linux-raid <linux-raid@vger.kernel.org>, stable@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v3] lib/raid6: Reduce high latency by using migrate
 instead of preempt
Message-ID: <YbzUVgPH9KDAMpA1@linutronix.de>
References: <20211217021610.12801-1-yajun.deng@linux.dev>
 <YbyTuRWkB0gYbn7x@linutronix.de>
 <CAPhsuW7GhYyfNOQg3VovU7cqC0nnRTbm1B7bFkWWa75k8YgHew@mail.gmail.com>
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On 2021-12-17 09:25:25 [-0800], Song Liu wrote:
> > The delay is a jiffy so it depends on CONFIG_HZ. You do benchmark for
> > the best algorithm and if you get preempted during that period then your
> > results may be wrong and you make a bad selection.
>=20
> With current code, the delay _should be_ 16 jiffies. However, the experim=
ent
> hits way longer latencies. I agree this may cause inaccurate benchmark re=
sults
> and thus suboptimal RAID algorithm.

Everything less than CONFIG_PREEMPT does not have an explicit
requirement for preemption so higher latencies are not unusual. *If*
this is a problem on <=3D PREEMPT_VOLUNTARY then a cond_resched() between
loops would be the usual thing to do. But only *if* it is a real problem
which I doubt. It is not a preemtible kernel after all=E2=80=A6

> I guess the key question is whether long latency at module loading time m=
atters.
> If that doesn't matter, we should just drop this.

Correct. And should this be problematic on PREEMPT_RT then I would
restrict CONFIG_RAID6_PQ_BENCHMARK to !PREEMPT_RT.

> Thanks,
> Song

Sebastian