Re: [PATCH v3] lib/raid6: Reduce high latency by using migrate instead of preempt

[Daniel Vacek]

On Fri, Dec 17, 2021 at 10:57 PM Song Liu <song@xxxxxxxxxx> wrote:
>
> On Fri, Dec 17, 2021 at 5:42 AM Sebastian Andrzej Siewior
> <bigeasy@xxxxxxxxxxxxx> wrote:
> >
> > On 2021-12-17 10:16:10 [+0800], Yajun Deng wrote:
> > > We found an abnormally high latency when executing modprobe raid6_pq, the
> > > latency is greater than 1.2s when CONFIG_PREEMPT_VOLUNTARY=y, greater than
> > > 67ms when CONFIG_PREEMPT=y, and greater than 16ms when CONFIG_PREEMPT_RT=y.
> > >
> > > How to reproduce:
> > >  - Install cyclictest
> > >      sudo apt install rt-tests
> > >  - Run cyclictest example in one terminal
> > >      sudo cyclictest -S -p 95 -d 0 -i 1000 -D 24h -m
> > >  - Modprobe raid6_pq in another terminal
> > >      sudo modprobe raid6_pq
> > >
> > > This is caused by ksoftirqd fail to scheduled due to disable preemption,
> > > this time is too long and unreasonable.
> > >
> > > Reduce high latency by using migrate_disabl()/emigrate_enable() instead of
> > > preempt_disable()/preempt_enable(), the latency won't greater than 100us.
> > >
> > > This patch beneficial for CONFIG_PREEMPT=y or CONFIG_PREEMPT_RT=y, but no
> > > effect for CONFIG_PREEMPT_VOLUNTARY=y.
> >
> > Why does it matter? This is only during boot-up/ module loading or do I
> > miss something?
>
> Yes this only happens on boot-up and module loading.I don't know RT well
> enough to tell whether latency during module loading is an issue.

Nope. It is not.

> > 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.
>
> With current code, the delay _should be_ 16 jiffies. However, the experiment
> hits way longer latencies. I agree this may cause inaccurate benchmark results
> and thus suboptimal RAID algorithm.

I explained this in the original thread. All the observed latencies
are really expected.

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

Again, it does not matter at all and here it is rather desired by design.

Drop this, please.

--nX

> Thanks,
> Song