Messages in this thread | | | Date | Mon, 26 Apr 2021 08:25:29 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v10 clocksource 6/7] clocksource: Forgive tsc_early pre-calibration drift |
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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?
Thanx, Paul
> [1]. https://lore.kernel.org/lkml/20210420064934.GE31773@xsang-OptiPlex-9020/ > [2]. https://lore.kernel.org/lkml/20201126012421.GA92582@shbuild999.sh.intel.com/ > > Thanks, > Feng > > > Cc: John Stultz <john.stultz@linaro.org> > > Cc: Thomas Gleixner <tglx@linutronix.de> > > Cc: Stephen Boyd <sboyd@kernel.org> > > Cc: Jonathan Corbet <corbet@lwn.net> > > Cc: Mark Rutland <Mark.Rutland@arm.com> > > Cc: Marc Zyngier <maz@kernel.org> > > Cc: Andi Kleen <ak@linux.intel.com> > > Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com> > > Cc: Feng Tang <feng.tang@intel.com> > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org> > > --- > > arch/x86/kernel/tsc.c | 1 + > > include/linux/clocksource.h | 5 +++++ > > kernel/time/clocksource.c | 19 ++++++++++++++++++- > > 3 files changed, 24 insertions(+), 1 deletion(-) > > > > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c > > index 56289170753c..7192b8950322 100644 > > --- a/arch/x86/kernel/tsc.c > > +++ b/arch/x86/kernel/tsc.c > > @@ -1127,6 +1127,7 @@ static int tsc_cs_enable(struct clocksource *cs) > > static struct clocksource clocksource_tsc_early = { > > .name = "tsc-early", > > .rating = 299, > > + .max_drift = 5 * NSEC_PER_MSEC, > > .read = read_tsc, > > .mask = CLOCKSOURCE_MASK(64), > > .flags = CLOCK_SOURCE_IS_CONTINUOUS | > > diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h > > index 83a3ebff7456..07d25a158d7a 100644 > > --- a/include/linux/clocksource.h > > +++ b/include/linux/clocksource.h > > @@ -42,6 +42,10 @@ struct module; > > * @shift: Cycle to nanosecond divisor (power of two) > > * @max_idle_ns: Maximum idle time permitted by the clocksource (nsecs) > > * @maxadj: Maximum adjustment value to mult (~11%) > > + * @max_drift: Maximum drift rate in nanoseconds per half second. > > + * Zero says to use default WATCHDOG_THRESHOLD. > > + * A non-zero value for ->max_drift more than 60 seconds after boot > > + * will result in a splat. > > * @archdata: Optional arch-specific data > > * @max_cycles: Maximum safe cycle value which won't overflow on > > * multiplication > > @@ -93,6 +97,7 @@ struct clocksource { > > u32 shift; > > u64 max_idle_ns; > > u32 maxadj; > > + u32 max_drift; > > #ifdef CONFIG_ARCH_CLOCKSOURCE_DATA > > struct arch_clocksource_data archdata; > > #endif > > diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c > > index f71f375df544..070b09dab815 100644 > > --- a/kernel/time/clocksource.c > > +++ b/kernel/time/clocksource.c > > @@ -126,6 +126,7 @@ static void __clocksource_change_rating(struct clocksource *cs, int rating); > > */ > > #define WATCHDOG_INTERVAL (HZ >> 1) > > #define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4) > > +#define WATCHDOG_SYNC_FORGIVENESS (HZ * 60UL) > > > > /* > > * Maximum permissible delay between two readouts of the watchdog > > @@ -377,6 +378,7 @@ static void clocksource_watchdog(struct timer_list *unused) > > int next_cpu, reset_pending; > > int64_t wd_nsec, cs_nsec; > > struct clocksource *cs; > > + u32 md; > > > > spin_lock(&watchdog_lock); > > if (!watchdog_running) > > @@ -423,7 +425,22 @@ static void clocksource_watchdog(struct timer_list *unused) > > continue; > > > > /* Check the deviation from the watchdog clocksource. */ > > - if (abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD) { > > + if (!cs->max_drift) { > > + md = WATCHDOG_THRESHOLD; > > + } else { > > + static unsigned long first_jiffies; > > + static bool beenhere; > > + > > + if (beenhere) { > > + WARN_ON_ONCE(time_after(jiffies, > > + first_jiffies + WATCHDOG_SYNC_FORGIVENESS)); > > + } else { > > + beenhere = true; > > + first_jiffies = jiffies; > > + } > > + md = cs->max_drift; > > + } > > + 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", > > -- > > 2.31.1.189.g2e36527f23
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