Messages in this thread | | | Date | Mon, 15 Nov 2021 14:19:59 -0500 | Subject | Re: [PATCH 1/2] clocksource: Avoid accidental unstable marking of clocksources | From | Waiman Long <> |
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On 11/15/21 02:59, Feng Tang wrote: > On Sun, Nov 14, 2021 at 10:24:56PM -0500, Waiman Long wrote: >> On 11/14/21 21:08, Feng Tang wrote: >>> Or did you have something else in mind? >>>>>> I'm not sure the detail in Waiman's cases, and in our cases (stress-ng) >>>>>> the delay between watchdog's (HPET here) read were not linear, that >>>>>> from debug data, sometimes the 3-2 difference could be bigger or much >>>>>> bigger than the 2-1 difference. >>>>>> >>>>>> The reason could be the gap between 2 reads depends hugely on the system >>>>>> pressure at that time that 3 HPET read happens. On our test box (a >>>>>> 2-Socket Cascade Lake AP server), the 2-1 and 3-2 difference are stably >>>>>> about 2.5 us, while under the stress it could be bumped to from 6 us >>>>>> to 2800 us. >>>>>> >>>>>> So I think checking the 3-2 difference plus increasing the max retries >>>>>> to 10 may be a simple way, if the watchdog read is found to be >>>>>> abnormally long, we skip this round of check. >>>>> On one of the test system, I had measured that normal delay >>>>> (hpet->tsc->hpet) was normally a bit over 2us. It was a bit more than 4us at >>>>> bootup time. However, the same system under stress could have a delay of >>>>> over 200us at bootup time. When I measured the consecutive hpet delay, it >>>>> was about 180us. So hpet read did dominate the total clocksource read delay. >>>> Thank you both for the data! >>>> >>>>> I would not suggest increasing the max retries as it may still fail in most >>>>> cases because the system stress will likely not be going away within a short >>>>> time. So we are likely just wasting cpu times. I believe we should just skip >>>>> it if it is the watchdog read that is causing most of the delay. >>>> If anything, adding that extra read would cause me to -reduce- the number >>>> of retries to avoid increasing the per-watchdog overhead. >>> I understand Waiman's concern here, and in our test patch, the 2 >>> consecutive watchdog read delay check is done inside this retrying >>> loop accompanying the 'cs' read, and once an abnormal delay is found, >>> the watchdog check is skipped without waiting for the max-retries to >>> complete. >>> >>> Our test data shows the consecutive delay is not always big even when >>> the system is much stressed, that's why I suggest to increase the >>> retries. >> If we need a large number of retries to avoid triggering the unstable TSC >> message, we should consider increase the threshod instead. Right? >> >> That is why my patch 2 makes the max skew value a configurable option so >> that we can tune it if necessary. > I'm fine with it, though the ideal case I expected is with carefully > picked values for max_retries/screw_threshhold, we could save the users > from configuring these. But given the complexity of all HWs out there, > it's not an easy goal. > > And I still suggest to put the consecutive watchdog read check inside > the retry loop, so that it could bail out early when detecting the > abnormal delay. Yes, I think that may sense. > > Another thing is we may need to set the 'watchdog_reset_pending', as > under the stress, there could be consecutive many times of "skipping" > watchdog check, and the saved value of 'cs' and 'watchdog' should be > reset.
I agree with that too. IOW, we combine the best part of the patches together. I will post an updated patch for that.
Cheers, Longman
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