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Date: Sat, 2 Feb 2019 10:52:36 +0800 From: Zhenzhong Duan <zhenzhong.duan@...cle.com> To: Thomas Gleixner <tglx@...utronix.de> 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 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
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