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

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Message-ID: <alpine.DEB.2.21.1901311525360.8200@nanos.tec.linutronix.de>
Date:   Thu, 31 Jan 2019 15:26:56 +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>
Subject: Re: [PATCH] acpi_pm: Reduce PMTMR counter read contention

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?

Thanks,

	tglx