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

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Date:   Mon, 15 Nov 2021 14:19:59 -0500
From:   Waiman Long <longman@...hat.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Cassio Neri <cassio.neri@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Colin Ian King <colin.king@...onical.com>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH 1/2] clocksource: Avoid accidental unstable marking of
 clocksources


On 11/15/21 02:59, Feng Tang wrote:
> On Sun, Nov 14, 2021 at 10:24:56PM -0500, Waiman Long wrote:
>> On 11/14/21 21:08, Feng Tang wrote:
>>> 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.
>>>> Thank you both for the data!
>>>>
>>>>> 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.
>>>> If anything, adding that extra read would cause me to -reduce- the number
>>>> of retries to avoid increasing the per-watchdog overhead.
>>> I understand Waiman's concern here, and in our test patch, the 2
>>> consecutive watchdog read delay check is done inside this retrying
>>> loop accompanying the 'cs' read, and once an abnormal delay is found,
>>> the watchdog check is skipped without waiting for the max-retries to
>>> complete.
>>>
>>> Our test data shows the consecutive delay is not always big even when
>>> the system is much stressed, that's why I suggest to increase the
>>> retries.
>> If we need a large number of retries to avoid triggering the unstable TSC
>> message, we should consider increase the threshod instead. Right?
>>
>> That is why my patch 2 makes the max skew value a configurable option so
>> that we can tune it if necessary.
> I'm fine with it, though the ideal case I expected is with carefully
> picked values for max_retries/screw_threshhold, we could save the users
> from configuring these. But given the complexity of all HWs out there,
> it's not an easy goal.
>
> And I still suggest to put the consecutive watchdog read check inside
> the retry loop, so that it could bail out early when detecting the
> abnormal delay.
Yes, I think that may sense.
>
> Another thing is we may need to set the 'watchdog_reset_pending', as
> under the stress, there could be consecutive many times of "skipping"
> watchdog check, and the saved value of 'cs' and 'watchdog' should be
> reset.

I agree with that too. IOW, we combine the best part of the patches 
together. I will post an updated patch for that.

Cheers,
Longman