linux-kernel - Re: [PATCH] clocksource: Make clocksource watchdog check with WATCHDOG

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Message-ID: <b8ae204b-0e92-a87e-5ae0-0b38d2adb33a@redhat.com>
Date:   Sun, 29 May 2022 20:16:59 -0400
From:   Waiman Long <longman@...hat.com>
To:     Michael Larabel <Michael@...ronix.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Feng Tang <feng.tang@...el.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        John Stultz <jstultz@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Joe Mario <jmario@...hat.com>,
        Michey Mehta <mimehta@...hat.com>
Subject: Re: [PATCH] clocksource: Make clocksource watchdog check with
 WATCHDOG_INTERVAL period

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@...ronix.com>
>>> Signed-off-by: Waiman Long <longman@...hat.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