Re: [LKP] Re: [clocksource] 6c52b5f3cf: stress-ng.opcode.ops_per_sec -14.4% regression

[Feng Tang]

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