Messages in this thread | | | Date | Sun, 29 May 2022 19:43:01 -0500 | Subject | Re: [PATCH] clocksource: Make clocksource watchdog check with WATCHDOG_INTERVAL period | From | Michael Larabel <> |
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On 5/29/22 19:16, Waiman Long wrote: > On 5/29/22 18:16, Michael Larabel wrote: >> On 5/27/22 21:07, Waiman Long wrote: >>> On 5/27/22 21:57, Waiman Long wrote: >>>> Since commit c86ff8c55b8a ("clocksource: Avoid accidental unstable >>>> marking of clocksource"), a new WD_READ_SKIP value was introduced >>>> as a possible return value of cs_watchdog_read() to skip the current >>>> check. However, this has an undesriable side effect of extending the >>>> time gap between csnow and cs_last to more than one WATCHDOG_INTERVAL >>>> (0.5s) in case of intermittent WD_READ_SKIP's. >>>> >>>> There was an instance of reported clocksource watchdog failure with >>>> the time skew of 485us where the uncertainly threshold is 400us. In >>>> that particular case, the (now - last) gap was about 2s. Looking at >>>> the dmesg log, it was clear there was a successful cs_watchdog_read() >>>> followed by 3 skips and then another successful cs_watchdog_read(). >>>> >>>> If there is an existing skew between the hpet (watchdog) and tsc >>>> clocksource, enlarging the period by 4x will certainly increase the >>>> measured skew causing it to exceed the threshold in this case. Fix >>>> this variable period problem by resetting the CLOCK_SOURCE_WATCHDOG >>>> bit >>>> after each WD_READ_SKIP to force the reloading of wd_last and cs_last >>>> in the next round. This ensures that we have two consecutive >>>> successful >>>> cs_watchdog_read()'s before checking the clock skew. >>>> >>>> Fixes: c86ff8c55b8a ("clocksource: Avoid accidental unstable >>>> marking of clocksource") >>>> Reported-by: Michael Larabel <Michael@phoronix.com> >>>> Signed-off-by: Waiman Long <longman@redhat.com> >>>> --- >>>> kernel/time/clocksource.c | 11 ++++++++++- >>>> 1 file changed, 10 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c >>>> index cee5da1e54c4..173e052c12b6 100644 >>>> --- a/kernel/time/clocksource.c >>>> +++ b/kernel/time/clocksource.c >>>> @@ -411,9 +411,18 @@ 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) >>>> + if (read_ret == WD_READ_UNSTABLE) { >>>> /* Clock readout unreliable, so give it up. */ >>>> __clocksource_unstable(cs); >>>> + } else { /* WD_READ_SKIP */ >>>> + /* >>>> + * Watchdog clock unstable at the moment, >>>> + * discard the stored wd_last and cs_last to >>>> + * make sure the gap between now and last >>>> + * is always one WATCHDOG_INTERVAL. >>>> + */ >>>> + cs->flags &= ~CLOCK_SOURCE_WATCHDOG; >>>> + } >>>> continue; >>>> } >>> >>> Sorry, I accidentally use the old email address for John. >>> >>> Cheers, >>> Longman >> >> >> I've tested this patch on the affected Daytona + Milan-X system and >> can confirm it does fix the performance problem that led to this >> issue. Though it is spamming the kernel log now every half-second >> with clocksource messages, not sure if that is intended/desirable >> behavior? >> >> >> [ 0.000000] tsc: Fast TSC calibration using PIT >> [ 0.000000] tsc: Detected 2195.990 MHz processor >> [ 1.238759] clocksource: tsc-early: mask: 0xffffffffffffffff >> max_cycles: 0x1fa766bc6ba, max_idle_ns: 440795275714 ns >> [ 2.769608] clocksource: Switched to clocksource tsc-early >> [ 3.263925] clocksource: wd-tsc-early-wd read-back delay of >> 292215ns, clock-skew test skipped! >> [ 3.743804] clocksource: wd-tsc-early-wd read-back delay of >> 268469ns, clock-skew test skipped! >> [ 3.935663] tsc: Refined TSC clocksource calibration: 2195.274 MHz >> [ 3.935844] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: >> 0x1fa4c255513, max_idle_ns: 440795289702 ns >> [ 3.936449] clocksource: Switched to clocksource tsc >> [ 4.255932] clocksource: wd-tsc-wd read-back delay of 260228ns, >> clock-skew test skipped! >> [ 4.767892] clocksource: wd-tsc-wd read-back delay of 272520ns, >> clock-skew test skipped! >> [ 5.247581] clocksource: wd-tsc-wd read-back delay of 200444ns, >> clock-skew test skipped! >> [ 5.759560] clocksource: wd-tsc-wd read-back delay of 165942ns, >> clock-skew test skipped! >> [ 6.239687] clocksource: wd-tsc-wd read-back delay of 232222ns, >> clock-skew test skipped! >> [ 7.264014] clocksource: wd-tsc-wd read-back delay of 282927ns, >> clock-skew test skipped! >> [ 7.743864] clocksource: wd-tsc-wd read-back delay of 288374ns, >> clock-skew test skipped! >> [ 8.255590] clocksource: wd-tsc-wd read-back delay of 206730ns, >> clock-skew test skipped! >> [ 8.767778] clocksource: wd-tsc-wd read-back delay of 267771ns, >> clock-skew test skipped! >> [ 9.247870] clocksource: wd-tsc-wd read-back delay of 224469ns, >> clock-skew test skipped! >> [ 10.239340] clocksource: wd-tsc-wd read-back delay of 109720ns, >> clock-skew test skipped! >> [ 12.255276] clocksource: wd-tsc-wd read-back delay of 104692ns, >> clock-skew test skipped! >> [ 16.255362] clocksource: wd-tsc-wd read-back delay of 122780ns, >> clock-skew test skipped! >> [ 17.759335] clocksource: wd-tsc-wd read-back delay of 155885ns, >> clock-skew test skipped! >> [ 18.239500] clocksource: wd-tsc-wd read-back delay of 176558ns, >> clock-skew test skipped! >> [ 18.751341] clocksource: wd-tsc-wd read-back delay of 157352ns, >> clock-skew test skipped! >> [ 19.263618] clocksource: wd-tsc-wd read-back delay of 177606ns, >> clock-skew test skipped! >> [ 19.743487] clocksource: wd-tsc-wd read-back delay of 157841ns, >> clock-skew test skipped! >> [ 20.255482] clocksource: wd-tsc-wd read-back delay of 157701ns, >> clock-skew test skipped! >> [ 20.767634] clocksource: wd-tsc-wd read-back delay of 173136ns, >> clock-skew test skipped! >> [ 21.247405] clocksource: wd-tsc-wd read-back delay of 175441ns, >> clock-skew test skipped! >> ... >> >> Thanks, >> Michael >> > Thanks for the testing. Did the spamming stop after a while? > > It does show that your particular Milan-X CPU(s) have unreliable hpet. > The only way to stop the spamming is to build a kernel with a larger > CONFIG_CLOCKSOURCE_WATCHDOG_MAX_SKEW_US. By default, it is 100us. The > allowable hpet-hpet delay is only half of that. In this particular > case, it will have to be set to at least 500 or maybe even 600. > > Thanks, > Longman >
No, the spamming hadn't stopped. At least as of one hour into the uptime it was still spewing every half second. Can confirm tomorrow if it ever stops after a longer duration but at least as of one hour of running the benchmarks to verify the performance was back to expectations, I had shut down that server to move onto other work.
Thanks, Michael
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