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

From: Waiman Long
Date: Fri Nov 12 2021 - 22:43:26 EST

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@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.

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.

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.

Cheers,
Longman