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

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Message-ID: <alpine.DEB.2.21.1901300903350.1950@nanos.tec.linutronix.de>
Date:   Wed, 30 Jan 2019 09:06:02 +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 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?

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.

Thanks,

	tglx