Messages in this thread | | | Date | Tue, 20 Dec 2022 11:11:08 -0500 | Subject | Re: [RFC PATCH] clocksource: Suspend the watchdog temporarily when high read lantency detected | From | Waiman Long <> |
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On 12/20/22 03:25, Feng Tang wrote: > There were bug reported on 8 sockets x86 machines that TSC was wrongly > disabled when system is under heavy workload. > > [ 818.380354] clocksource: timekeeping watchdog on CPU336: hpet wd-wd read-back delay of 1203520ns > [ 818.436160] clocksource: wd-tsc-wd read-back delay of 181880ns, clock-skew test skipped! > [ 819.402962] clocksource: timekeeping watchdog on CPU338: hpet wd-wd read-back delay of 324000ns > [ 819.448036] clocksource: wd-tsc-wd read-back delay of 337240ns, clock-skew test skipped! > [ 819.880863] clocksource: timekeeping watchdog on CPU339: hpet read-back delay of 150280ns, attempt 3, marking unstable > [ 819.936243] tsc: Marking TSC unstable due to clocksource watchdog > [ 820.068173] TSC found unstable after boot, most likely due to broken BIOS. Use 'tsc=unstable'. > [ 820.092382] sched_clock: Marking unstable (818769414384, 1195404998) > [ 820.643627] clocksource: Checking clocksource tsc synchronization from CPU 267 to CPUs 0,4,25,70,126,430,557,564. > [ 821.067990] clocksource: Switched to clocksource hpet > > This can be reproduced when system is running memory intensive 'stream' > test, or some stress-ng subcases like 'ioport'. > > The reason is when system is under heavy load, the read latency of > clocksource can be very high, it can be seen even with lightweight > TSC read, and is much worse on MMIO or IO port read based external > clocksource. Causing the watchdog check to be inaccurate. > > As the clocksource watchdog is a lifetime check with frequency of > twice a second, there is no need to rush doing it when the system > is under heavy load and the clocksource read latency is very high, > suspend the watchdog timer for 5 minutes. > > Signed-off-by: Feng Tang <feng.tang@intel.com> > --- > kernel/time/clocksource.c | 45 ++++++++++++++++++++++++++++----------- > 1 file changed, 32 insertions(+), 13 deletions(-) > > diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c > index 9cf32ccda715..8cd74b89d577 100644 > --- a/kernel/time/clocksource.c > +++ b/kernel/time/clocksource.c > @@ -384,6 +384,15 @@ void clocksource_verify_percpu(struct clocksource *cs) > } > EXPORT_SYMBOL_GPL(clocksource_verify_percpu); > > +static inline void clocksource_reset_watchdog(void) > +{ > + struct clocksource *cs; > + > + list_for_each_entry(cs, &watchdog_list, wd_list) > + cs->flags &= ~CLOCK_SOURCE_WATCHDOG; > +} > + > + > static void clocksource_watchdog(struct timer_list *unused) > { > u64 csnow, wdnow, cslast, wdlast, delta; > @@ -391,6 +400,7 @@ static void clocksource_watchdog(struct timer_list *unused) > int64_t wd_nsec, cs_nsec; > struct clocksource *cs; > enum wd_read_status read_ret; > + unsigned long extra_wait = 0; > u32 md; > > spin_lock(&watchdog_lock); > @@ -410,13 +420,30 @@ static void clocksource_watchdog(struct timer_list *unused) > > read_ret = cs_watchdog_read(cs, &csnow, &wdnow); > > - if (read_ret != WD_READ_SUCCESS) { > - if (read_ret == WD_READ_UNSTABLE) > - /* Clock readout unreliable, so give it up. */ > - __clocksource_unstable(cs); > + if (read_ret == WD_READ_UNSTABLE) { > + /* Clock readout unreliable, so give it up. */ > + __clocksource_unstable(cs); > continue; > } > > + /* > + * When WD_READ_SKIP is returned, it means the system is likely > + * under very heavy load, where the latency of reading > + * watchdog/clocksource is very big, and affect the accuracy of > + * watchdog check. So give system some space and suspend the > + * watchdog check for 5 minutes. > + */ > + if (read_ret == WD_READ_SKIP) { > + /* > + * As the watchdog timer will be suspended, and > + * cs->last could keep unchanged for 5 minutes, reset > + * the counters. > + */ > + clocksource_reset_watchdog(); > + extra_wait = HZ * 300; > + break; > + } > + > /* Clocksource initialized ? */ > if (!(cs->flags & CLOCK_SOURCE_WATCHDOG) || > atomic_read(&watchdog_reset_pending)) { > @@ -512,7 +539,7 @@ static void clocksource_watchdog(struct timer_list *unused) > * pair clocksource_stop_watchdog() clocksource_start_watchdog(). > */ > if (!timer_pending(&watchdog_timer)) { > - watchdog_timer.expires += WATCHDOG_INTERVAL; > + watchdog_timer.expires += WATCHDOG_INTERVAL + extra_wait; > add_timer_on(&watchdog_timer, next_cpu); > } > out: > @@ -537,14 +564,6 @@ static inline void clocksource_stop_watchdog(void) > watchdog_running = 0; > } > > -static inline void clocksource_reset_watchdog(void) > -{ > - struct clocksource *cs; > - > - list_for_each_entry(cs, &watchdog_list, wd_list) > - cs->flags &= ~CLOCK_SOURCE_WATCHDOG; > -} > - > static void clocksource_resume_watchdog(void) > { > atomic_inc(&watchdog_reset_pending);
It looks reasonable to me. Thanks for the patch.
Acked-by: Waiman Long <longman@redhat.com>
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