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From: Feng Tang <feng.tang@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
	Xing Zhengjun <zhengjun.xing@linux.intel.com>,
	John Stultz <john.stultz@linaro.org>,
	Stephen Boyd <sboyd@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
	Mark Rutland <Mark.Rutland@arm.com>,
	Marc Zyngier <maz@kernel.org>, Andi Kleen <ak@linux.intel.com>,
	Chris Mason <clm@fb.com>, LKML <linux-kernel@vger.kernel.org>,
	"lkp@lists.01.org" <lkp@lists.01.org>, lkp <lkp@intel.com>
Subject: Re: [LKP] Re: [clocksource] 6c52b5f3cf: stress-ng.opcode.ops_per_sec -14.4% regression
Date: Mon, 26 Apr 2021 23:12:49 +0800	[thread overview]
Message-ID: <20210426151249.GA89018@shbuild999.sh.intel.com> (raw)
In-Reply-To: <87fszdt8sk.ffs@nanos.tec.linutronix.de>

On Mon, Apr 26, 2021 at 04:33:31PM +0200, Thomas Gleixner wrote:
> On Mon, Apr 26 2021 at 22:05, Feng Tang wrote:
> > On Mon, Apr 26, 2021 at 08:39:25PM +0800, Thomas Gleixner wrote:
> >> 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.
> >
> > Thanks for sharing the info! So this laptop can still work with the
> > tsc_adjust check and restore, without triggering the 'unstable' alarm.
> >
> > Why are those BIOSes playing the trick? Maybe some other OS has hard limit
> > for SMI's maxim handling time, so they try to hide the time?
> 
> Years ago someone admitted that it was the attempt to hide the
> (substantial) time wasted in SMIs from being detectable via tracing, but
> obviously that backfired because TSC got out of sync.
> 
> Since then this has mostly vanished but for some reasons it's coming
> back every now and then. Rarely, but it happens still.
 
I see now.

> >> 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.
> >
> > I see. So should I hold my two patches (tsc_adjust timer and tsc watchdog
> > check lifting) for a while?
> 
> I have them on my list anyway, but yes we want to avoid the timer
> because that's what the HPC / NOHZ full people are going to complain
> about anyway.

Got it. Thanks for the clarification!

- Feng

> Thanks,
> 
>         tglx

  reply	other threads:[~2021-04-26 15:12 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-20  6:49 [clocksource] 6c52b5f3cf: stress-ng.opcode.ops_per_sec -14.4% regression kernel test robot
2021-04-20 13:43 ` Paul E. McKenney
2021-04-20 14:05   ` Paul E. McKenney
2021-04-21  6:07     ` [LKP] " Xing, Zhengjun
2021-04-21 13:42       ` Paul E. McKenney
2021-04-22  6:58         ` Xing Zhengjun
2021-04-22  7:41           ` Feng Tang
2021-04-22 14:24             ` Paul E. McKenney
2021-04-22 16:57               ` Paul E. McKenney
2021-04-23  6:11                 ` Feng Tang
2021-04-23 14:02                   ` Paul E. McKenney
2021-04-24 12:29                     ` Feng Tang
2021-04-24 17:53                       ` Paul E. McKenney
2021-04-25  2:14                         ` Feng Tang
2021-04-25  3:14                           ` Feng Tang
2021-04-25 19:15                             ` Paul E. McKenney
2021-04-25 19:14                           ` Paul E. McKenney
2021-04-26 12:39                       ` Thomas Gleixner
2021-04-26 14:05                         ` Feng Tang
2021-04-26 14:33                           ` Thomas Gleixner
2021-04-26 15:12                             ` Feng Tang [this message]
2021-04-23  2:15               ` Xing Zhengjun
2021-04-23  4:12                 ` Paul E. McKenney
2021-04-23 19:14               ` Thomas Gleixner
2021-04-23 21:14                 ` Paul E. McKenney
2021-04-23 23:39                   ` Paul E. McKenney
2021-04-23 19:09             ` Thomas Gleixner

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