Messages in this thread | | | Date | Tue, 20 Dec 2022 22:26:15 -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 20:01, Feng Tang wrote: > Using correct email address of John Stultz. > > On Tue, Dec 20, 2022 at 10:34:00AM -0800, Paul E. McKenney wrote: >> On Tue, Dec 20, 2022 at 11:11:08AM -0500, Waiman Long wrote: >>> 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> >> Queued, thank you both! > Thanks for reviewing and queueing! > >> If you would like this to go in some other way: >> >> Acked-by: Paul E. McKenney <paulmck@kernel.org> >> >> And while I am remembering it... Any objections to reversing the role of >> TSC and the other timers on systems where TSC is believed to be accurate? >> So that if there is clocksource skew, HPET is marked unstable rather than >> TSC? > For the bug in commit log, I think it's the 8 sockets system with > hundreds of CPUs causing the big latency, while the HPET itself may > not be broken, and if we switched to ACPI PM_TIMER as watchdog, we > could see similar big latency. > > I used to only see this issue with stress tool like stress-ng, but > seems with larger and larger system, even the momory intensive load > can easily trigger this. > >> This would preserve the diagnostics without hammering performance >> when skew is detected. (Switching from TSC to HPET hammers performance >> enough that our automation usually notices and reboots the system.) > Yes, switching to HPET is a disaster for performance, we've seen > from 30% to 90% drop in different benchmarks.
Switching to hpet is very bad for performance. That is the main reason why I posted clocksource patches in the past to avoid this as much as possible. I think your patch is also a good countermeasure to avoid this.
Thanks, Longman
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