linux-kernel - Re: [PATCH 0/2] clocksource: Avoid incorrect hpet fallback

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Date:   Wed, 10 Nov 2021 20:19:58 -0500
From:   Waiman Long <longman@...hat.com>
To:     paulmck@...nel.org
Cc:     John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Cassio Neri <cassio.neri@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Feng Tang <feng.tang@...el.com>
Subject: Re: [PATCH 0/2] clocksource: Avoid incorrect hpet fallback


On 11/10/21 19:04, Paul E. McKenney wrote:
> On Wed, Nov 10, 2021 at 06:25:14PM -0500, Waiman Long wrote:
>> On 11/10/21 17:32, Paul E. McKenney wrote:
>>> On Wed, Nov 10, 2021 at 05:17:30PM -0500, Waiman Long wrote:
>>>> It was found that when an x86 system was being stressed by running
>>>> various different benchmark suites, the clocksource watchdog might
>>>> occasionally mark TSC as unstable and fall back to hpet which will
>>>> have a signficant impact on system performance.
>>>>
>>>> The current watchdog clocksource skew threshold of 50us is found to be
>>>> insufficient. So it is changed back to 100us before commit 2e27e793e280
>>>> ("clocksource: Reduce clocksource-skew threshold") in patch 1. Patch 2
>>>> adds a Kconfig option to allow kernel builder to control the actual
>>>> threshold to be used.
>>>>
>>>> Waiman Long (2):
>>>>     clocksource: Avoid accidental unstable marking of clocksources
>>>>     clocksource: Add a Kconfig option for WATCHDOG_MAX_SKEW
>>> The ability to control the fine-grained threshold seems useful, but is
>>> the TSC still marked unstable when this commit from -rcu is applied?
>>> It has passed significant testing on other workloads.
>>>
>>> 2a43fb0479aa ("clocksource: Forgive repeated long-latency watchdog clocksource reads")
>>>
>>> If the patch below takes care of your situation, my thought is to
>>> also take your second patch, which would allow people to set the
>>> cutoff more loosely or more tightly, as their situation dictates.
>>>
>>> Thoughts?
>> That is commit 14dbb29eda51 ("clocksource: Forgive repeated long-latency
>> watchdog clocksource reads") in your linux-rcu git tree. From reading the
>> patch, I believe it should be able to address the hpet fallback problem that
>> Red Hat had encountered. Your patch said it was an out-of-tree patch. Are
>> you planning to mainline it?
> Yes, I expect to submit it into the next merge window (not the current
> v5.16 merge window, but v5.17).  However, if your situation is urgent, and
> if it works for you, I could submit it as a fix for an earlier regression.

I will build a test kernel based on your patch and ask our benchmarking 
group to run their test suites. It will take a day or two to get a 
definitive answer even though I believe it should fix the issue.

Cheers,
Longman