linux-kernel - Re: [LKP] Re: [clocksource] 6c52b5f3cf: stress-ng.opcode.ops_per

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Date:   Mon, 26 Apr 2021 14:39:25 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Feng Tang <feng.tang@...el.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Xing Zhengjun <zhengjun.xing@...ux.intel.com>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Mark Rutland <Mark.Rutland@....com>,
        Marc Zyngier <maz@...nel.org>, Andi Kleen <ak@...ux.intel.com>,
        Chris Mason <clm@...com>, LKML <linux-kernel@...r.kernel.org>,
        lkp@...ts.01.org, lkp@...el.com
Subject: Re: [LKP] Re: [clocksource] 6c52b5f3cf: stress-ng.opcode.ops_per_sec -14.4% regression

On Sat, Apr 24 2021 at 20:29, Feng Tang wrote:
> On Fri, Apr 23, 2021 at 07:02:54AM -0700, Paul E. McKenney wrote:
> And I'm eager to know if there is any real case of an unreliable tsc
> on the 'large numbers' of x86 system which complies with our cpu feature
> check. And if there is, my 2/2 definitely should be dropped.   

Nothing prevents BIOS tinkerers from trying to be 'smart'. My most
recent encounter (3 month ago) was on a laptop where TSC drifted off on
CPU0 very slowly, but was caught due to the TSC_ADJUST check in idle.

I'm still thinking about a solution to avoid that extra timer and the
watchdog for these systems, but haven't found anything which I don't
hate with a passion yet.

Thanks,

        tglx