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

[Michael Larabel <Michael@phoronix.com>]

On 5/29/22 19:16, Waiman Long wrote:
> On 5/29/22 18:16, Michael Larabel wrote:
>> 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@phoronix.com>
>>>> Signed-off-by: Waiman Long <longman@redhat.com>
>>>> ---
>>>>   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
>>
> Thanks for the testing. Did the spamming stop after a while?
>
> It does show that your particular Milan-X CPU(s) have unreliable hpet. 
> The only way to stop the spamming is to build a kernel with a larger 
> CONFIG_CLOCKSOURCE_WATCHDOG_MAX_SKEW_US. By default, it is 100us. The 
> allowable hpet-hpet delay is only half of that. In this particular 
> case, it will have to be set to at least 500 or maybe even 600.
>
> Thanks,
> Longman
>

No, the spamming hadn't stopped. At least as of one hour into the uptime 
it was still spewing every half second. Can confirm tomorrow if it ever 
stops after a longer duration but at least as of one hour of running the 
benchmarks to verify the performance was back to expectations, I had 
shut down that server to move onto other work.

Thanks,
Michael