Re: [PATCH v10 clocksource 6/7] clocksource: Forgive tsc_early pre-calibration drift

[Feng Tang]

On Mon, Apr 26, 2021 at 08:25:29AM -0700, Paul E. McKenney wrote:
> On Mon, Apr 26, 2021 at 11:01:27PM +0800, Feng Tang wrote:
> > Hi Paul,
> > 
> > On Sun, Apr 25, 2021 at 03:47:07PM -0700, Paul E. McKenney wrote:
> > > Because the x86 tsc_early clocksource is given a quick and semi-accurate
> > > calibration (by design!), it might have drift rates well in excess of
> > > the 0.1% limit that is in the process of being adopted.
> > > 
> > > Therefore, add a max_drift field to the clocksource structure that, when
> > > non-zero, specifies the maximum allowable drift rate in nanoseconds over
> > > a half-second period.  The tsc_early clocksource initializes this to five
> > > miliseconds, which corresponds to the 1% drift rate limit suggested by
> > > Xing Zhengjun.  This max_drift field is intended only for early boot,
> > > so clocksource_watchdog() splats if it encounters a non-zero value in
> > > this field more than 60 seconds after boot, inspired by a suggestion by
> > > Thomas Gleixner.
> > > 
> > > This was tested by setting the clocksource_tsc ->max_drift field to 1,
> > > which, as expected, resulted in a clock-skew event.
> > 
> > We've run the same last for this v10, and those 'unstable' thing [1] can
> > not be reproduced!
> 
> Good to hear!  ;-)
> 
> > We've reported one case that tsc can be wrongly judged as 'unstable'
> > by 'refined-jiffies' watchdog [1], while reducing the threshold could
> > make it easier to be triggered.
> > 
> > It could be reproduced on the a plaform with a 115200 serial console,
> > and hpet been disabled (several x86 platforms has this), add 
> > 'initcall_debug' cmdline parameter to get more debug message, we can
> > see:
> > 
> > [    1.134197] clocksource: timekeeping watchdog on CPU1: Marking clocksource 'tsc-early' as unstable because the skew is too large:
> > [    1.134214] clocksource:                       'refined-jiffies' wd_nesc: 500000000 wd_now: ffff8b35 wd_last: ffff8b03 mask: ffffffff
> > [    1.134217] clocksource:                       'tsc-early' cs_nsec: 507537855 cs_now: 4e63c9d09 cs_last: 4bebd81f5 mask: ffffffffffffffff
> > [    1.134220] clocksource:                       No current clocksource.
> > [    1.134222] tsc: Marking TSC unstable due to clocksource watchdog
> 
> Just to make sure I understand: "could be reproduced" as in this is the
> result from v9, and v10 avoids this, correct?

Sorry I didn't make it clear. This is a rarely happened case, and can
be reproduced with upstream kerenl, which has 62.5 ms threshold. 6/7 &
7/7 patch of reducing the threshold can make it easier to be triggered.

Thanks,
Feng


> 							Thanx, Paul
>