Messages in this thread | | | Date | Mon, 26 Apr 2021 23:36:05 +0800 | From | Feng Tang <> | Subject | Re: [PATCH v10 clocksource 6/7] clocksource: Forgive tsc_early pre-calibration drift |
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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 >
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