Messages in this thread | | | Date | Sun, 14 Nov 2021 07:54:07 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/2] clocksource: Avoid accidental unstable marking of clocksources |
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On Fri, Nov 12, 2021 at 10:43:15PM -0500, Waiman Long wrote: > On 11/12/21 00:44, Feng Tang wrote: > > On Thu, Nov 11, 2021 at 06:43:11AM -0800, Paul E. McKenney wrote: > > > On Thu, Nov 11, 2021 at 12:57:03PM +0800, Feng Tang wrote: > > > > On Wed, Nov 10, 2021 at 05:17:31PM -0500, Waiman Long wrote: > > > > > Since commit db3a34e17433 ("clocksource: Retry clock read if long delays > > > > > detected") and commit 2e27e793e280 ("clocksource: Reduce clocksource-skew > > > > > threshold"), it is found that tsc clocksource fallback to hpet can > > > > > sometimes happen on both Intel and AMD systems especially when they are > > > > > running stressful benchmarking workloads. Of the 23 systems tested with > > > > > a v5.14 kernel, 10 of them have switched to hpet clock source during > > > > > the test run. > > > > > > > > > > The result of falling back to hpet is a drastic reduction of performance > > > > > when running benchmarks. For example, the fio performance tests can > > > > > drop up to 70% whereas the iperf3 performance can drop up to 80%. > > > > > > > > > > 4 hpet fallbacks happened during bootup. They were: > > > > > > > > > > [ 8.749399] clocksource: timekeeping watchdog on CPU13: hpet read-back delay of 263750ns, attempt 4, marking unstable > > > > > [ 12.044610] clocksource: timekeeping watchdog on CPU19: hpet read-back delay of 186166ns, attempt 4, marking unstable > > > > > [ 17.336941] clocksource: timekeeping watchdog on CPU28: hpet read-back delay of 182291ns, attempt 4, marking unstable > > > > > [ 17.518565] clocksource: timekeeping watchdog on CPU34: hpet read-back delay of 252196ns, attempt 4, marking unstable > > > > > > > > > > Other fallbacks happen when the systems were running stressful > > > > > benchmarks. For example: > > > > > > > > > > [ 2685.867873] clocksource: timekeeping watchdog on CPU117: hpet read-back delay of 57269ns, attempt 4, marking unstable > > > > > [46215.471228] clocksource: timekeeping watchdog on CPU8: hpet read-back delay of 61460ns, attempt 4, marking unstable > > > > > > > > > > Commit 2e27e793e280 ("clocksource: Reduce clocksource-skew threshold"), > > > > > changed the skew margin from 100us to 50us. I think this is too small > > > > > and can easily be exceeded when running some stressful workloads on > > > > > a thermally stressed system. So it is switched back to 100us. On > > > > > the other hand, it doesn't look like we need to increase the minimum > > > > > uncertainty margin. So it is kept the same at 100us too. > > > > > > > > > > Even a maximum skew margin of 100us may be too small in for some systems > > > > > when booting up especially if those systems are under thermal stress. To > > > > > eliminate the case that the large skew is due to the system being too > > > > > busy slowing down the reading of both the watchdog and the clocksource, > > > > > a final check is done by reading watchdog time again and comparing the > > > > > consecutive watchdog timing read delay against WATCHDOG_MAX_SKEW/2. If > > > > > that delay exceeds the limit, we assume that the system is just too > > > > > busy. A warning will be printed to the console and the watchdog check > > > > > is then skipped for this round. For example: > > > > > > > > > > [ 8.789316] clocksource: timekeeping watchdog on CPU13: hpet consecutive read-back delay of 174541ns, system too busy > > > > > > > > I think it may be better to add more details about the root cause, other > > > > than that it looks good to me, as we tested similar patch on our test > > > > platforms. > > > > > > > > Reviewed-by: Feng Tang <feng.tang@intel.com> > > > Thank you both! > > > > > > I agree on the bit about root cause. Would it make sense to compare the > > > difference between HPET reads 1 and 2 (containing the read of the TSC) > > > and the difference between HPET reads 2 and 3? If the 2-1 difference was > > > no more than (say) 8/7ths of the 3-2 difference, or the 2-1 difference > > > was no more than (say) 20 microseconds more than the 3-2 difference, > > > this could be considered a good-as-it-gets read, ending the retry loop. > > > Then if the 3-1 difference was greater than the default (100 microseconds > > > in current -rcu), that difference could be substituted for that particular > > > clocksource watchdog check. With a console message noting the unusually > > > high overhead (but not a splat). > > > > > > So if it took 75 microseconds for each HPET read and 1 microsecond for > > > the TSC read, then 226 microseconds would be substituted for the default > > > of 100 microseconds for that cycle's skew cutoff. Unless the previous > > > skew cutoff was larger, in which case the previous cutoff should be > > > used instead. Either way, the current cutoff is recorded for comparison > > > for the next clocksource watchdog check. > > > > > > If the 3-1 difference was greater than 62.5 milliseconds, a warning should > > > probably be emitted anyway. > > I can test the patch with our cases that could reproduce the problem. > > > > > 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.
Thoughts?
Thanx, Paul
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