Re: [PATCH 1/2] clocksource: Avoid accidental unstable marking of clocksources

[Paul E. McKenney]

On Fri, Nov 12, 2021 at 01:44:17PM +0800, 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@xxxxxxxxx>
> > 
> > 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.

Much appreciated!

> > 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.

OK, given your data, it might indeed be best to keep things simple for
the time being.

So when you suggested adding more details about root cause, what did you
have in mind?  More details on Waiman's commit log?  Some other strategy
for extracting additional latency information?  Something else entirely?

							Thanx, Paul