Messages in this thread | | | Date | Mon, 26 Apr 2021 20:46:40 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v10 clocksource 6/7] clocksource: Forgive tsc_early pre-calibration drift |
| |
On Tue, Apr 27, 2021 at 09:13:55AM +0800, Feng Tang wrote: > On Mon, Apr 26, 2021 at 11:26:52AM -0700, Paul E. McKenney wrote: > > On Mon, Apr 26, 2021 at 11:36:05PM +0800, Feng Tang wrote: > > > 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. > > > > Ah, OK, so this could be considered to be a benefit of this series, then. > > > > Does this happen only for tsc-early, or for tsc as well? > > > > Has it already been triggered on v10 of this series? (I understand that > > it certainly should be easier to trigger, just curious whether this has > > already happened.) > > Yes, it has. The upper log is from v10 (actually it's the 'dev' branch > of your linux-rcu git, which I didn't find obvious difference) on a > client platform > > [ 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 > > The deviation is 7537855 ns (7.5 ms). And as said before, it needs many > pre-conditions to be triggered. > > Also I found the debug patch is useful, which prints out the direct > nanoseconds info when 'unstable' is detected.
Looks good to me!
If you give me a Signed-off-by, I would be happy to queue it.
Thanx, Paul
> kernel/time/clocksource.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c > index a374cf7b6336..5370f0c84981 100644 > --- a/kernel/time/clocksource.c > +++ b/kernel/time/clocksource.c > @@ -443,10 +443,10 @@ static void clocksource_watchdog(struct timer_list *unused) > if (abs(cs_nsec - wd_nsec) > md) { > pr_warn("timekeeping watchdog on CPU%d: Marking clocksource '%s' as unstable because the skew is too large:\n", > smp_processor_id(), cs->name); > - pr_warn(" '%s' wd_now: %llx wd_last: %llx mask: %llx\n", > - watchdog->name, wdnow, wdlast, watchdog->mask); > - pr_warn(" '%s' cs_now: %llx cs_last: %llx mask: %llx\n", > - cs->name, csnow, cslast, cs->mask); > + pr_warn(" '%s' wd_nesc: %lld wd_now: %llx wd_last: %llx mask: %llx\n", > + watchdog->name, wd_nsec, wdnow, wdlast, watchdog->mask); > + pr_warn(" '%s' cs_nsec: %lld cs_now: %llx cs_last: %llx mask: %llx\n", > + cs->name, cs_nsec, csnow, cslast, cs->mask); > if (curr_clocksource == cs) > pr_warn(" '%s' is current clocksource.\n", cs->name); > else if (curr_clocksource) > -- > 2.27.0 > > Thanks, > Feng > > > > > > > Thanx, Paul
| |