linux-kernel - Re: [PATCH] acpi_pm: Reduce PMTMR counter read contention

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Date:   Sun, 10 Feb 2019 22:08:53 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Zhenzhong Duan <zhenzhong.duan@...cle.com>
cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Waiman Long <longman@...hat.com>,
        Srinivas Eeda <srinivas.eeda@...cle.com>, kin.cho@...cle.com
Subject: Re: [PATCH] acpi_pm: Reduce PMTMR counter read contention

On Sat, 2 Feb 2019, Zhenzhong Duan wrote:
> On 2019/1/31 22:26, Thomas Gleixner wrote:

> > > > I'm not against the change per se, but I really want to understand
> > > > why we need all the complexity for something which should never be
> > > > used in a real world deployment.
> > > >
> > > Hmm, it's a strong word of "never be used". Customers may happen to
> > > use nohpet(sanity test?) and report bug to us. Sometimes they does
> > > report a bug that reproduce with their customed config. There may
> > > also be BIOS setting HPET disabled.
> > 
> > And because the customer MAY do completely nonsensical things (and there
> > are a lot more than the HPET) the kernel has to handle all of them?
> 
> Ok, then. I don't have more suggestion to convince you.

You give up too fast :)

The point is, that we really want proper justifications for changes like
this. Some 'may, could and more handwaving' simply does not cut it.

So if you can just describe a realistic scenario, which does not involve
thoughtless flipping of BIOS options, then this becomes way more palatable.

> I just think of a simple fix as below. I think it will work for both hpet
> and pmtmr. We will test it when the env is available.

> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -1353,6 +1353,7 @@ static int change_clocksource(void *data)
> 
>         write_seqcount_end(&tk_core.seq);
>         raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
> +       tick_clock_notify();
> 
>         return 0;
>  }
> @@ -1371,7 +1372,6 @@ int timekeeping_notify(struct clocksource *clock)
>         if (tk->tkr_mono.clock == clock)
>                 return 0;
>         stop_machine(change_clocksource, clock, NULL);
> -       tick_clock_notify();
>         return tk->tkr_mono.clock == clock ? 0 : -1;
>  }

This won't resolve the concurrency issues of HPET or PMTIMER in any
way. Instead it breaks the careful orchestrated mechanism of clocksource
change.

Thanks,

	tglx