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

[Zhenzhong Duan]

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:
> > > >    1) There is a single PMTMR counter shared by all the CPUs.
> > > >    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)

        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;
 }


Thanks
Zhenzhong