Re: [PATCH] acpi_pm: Reduce PMTMR counter read contention

[Kin Cho]

Zhenzhong,
The machine is running test this weekend, I'll try your simple fix next 
week.

All,
We're not aware of a specific customer need for acpi_pm/PMTMR,
but if we must keep acpi_pm/PMTMR in the kernel, let's fix it so it
actually works, even on machine like ours.
On our hardware currently it's broken both during clocksource selection
and as a permanent clocksource.

Thanks,

-kin

On 2/1/19 6:52 PM, Zhenzhong Duan wrote:
> On 2019/1/31 22:26, Thomas Gleixner wrote:
> > On Thu, 31 Jan 2019, Zhenzhong Duan wrote:
> >
> > > On 2019/1/30 16:06, Thomas Gleixner wrote:
> > > > On Tue, 22 Jan 2019, Zhenzhong Duan wrote:
> > > >
> > > > > On a large system with many CPUs, using PMTMR as the clock source can
> > > > > have a significant impact on the overall system performance because
> > > > > of the following reasons:
> > > > > ÂÂ 1) There is a single PMTMR counter shared by all the CPUs.
> > > > > ÂÂ 2) PMTMR counter reading is a very slow operation.
> > > > >
> > > > > Using PMTMR as the default clock source may happen when, for example,
> > > > > the TSC clock calibration exceeds the allowable tolerance and HPET
> > > > > disabled by nohpet on kernel command line. Sometimes the performance
> > > >
> > > > The question is why would anyone disable HPET on a larger machine 
> > > > when the
> > > > TSC is wreckaged?
> > >
> > > There may be broken hardware where TSC is wreckaged.
> >
> > I know that.
> >
> > > > 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. 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)
>
> ÂÂÂÂÂÂÂ write_seqcount_end(&tk_core.seq);
> ÂÂÂÂÂÂÂ raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
> +ÂÂÂÂÂÂ tick_clock_notify();
>
> ÂÂÂÂÂÂÂ return 0;
> Â}
> @@ -1371,7 +1372,6 @@ int timekeeping_notify(struct clocksource *clock)
> ÂÂÂÂÂÂÂ if (tk->tkr_mono.clock == clock)
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ return 0;
> ÂÂÂÂÂÂÂ stop_machine(change_clocksource, clock, NULL);
> -ÂÂÂÂÂÂ tick_clock_notify();
> ÂÂÂÂÂÂÂ return tk->tkr_mono.clock == clock ? 0 : -1;
> Â}
>
>
> Thanks
> Zhenzhong