Re: [PATCH] clocksource: Make clocksource watchdog check with WATCHDOG_INTERVAL period

[Michael Larabel]

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@xxxxxxxxxxxx>
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   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