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Subject: Re: [PATCH] Clocksource: Avoid misjudgment of clocksource
To:     John Stultz <john.stultz@linaro.org>
Cc:     yanghui <yanghui.def@bytedance.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Stephen Boyd <sboyd@kernel.org>,
        lkml <linux-kernel@vger.kernel.org>
References: <20211008080305.13401-1-yanghui.def@bytedance.com>
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 <CALAqxLVgQ6QEThWaN65nOW9F_XCh7885n9RigAQDU+OgDntS5g@mail.gmail.com>
From:   brookxu <brookxu.cn@gmail.com>
Message-ID: <6b715fb7-9850-04f3-4ab8-1a2a8a2cdfbf@gmail.com>
Date:   Tue, 12 Oct 2021 13:21:57 +0800
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John Stultz wrote on 2021/10/12 12:52 下午:
> On Sat, Oct 9, 2021 at 7:04 AM brookxu <brookxu.cn@gmail.com> wrote:
>>
>> hello
>>
>> John Stultz wrote on 2021/10/9 7:45:
>>> On Fri, Oct 8, 2021 at 1:03 AM yanghui <yanghui.def@bytedance.com> wrote:
>>>>
>>>> clocksource_watchdog is executed every WATCHDOG_INTERVAL(0.5s) by
>>>> Timer. But sometimes system is very busy and the Timer cannot be
>>>> executed in 0.5sec. For example,if clocksource_watchdog be executed
>>>> after 10sec, the calculated value of abs(cs_nsec - wd_nsec) will
>>>> be enlarged. Then the current clocksource will be misjudged as
>>>> unstable. So we add conditions to prevent the clocksource from
>>>> being misjudged.
>>>>
>>>> Signed-off-by: yanghui <yanghui.def@bytedance.com>
>>>> ---
>>>>  kernel/time/clocksource.c | 6 +++++-
>>>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
>>>> index b8a14d2fb5ba..d535beadcbc8 100644
>>>> --- a/kernel/time/clocksource.c
>>>> +++ b/kernel/time/clocksource.c
>>>> @@ -136,8 +136,10 @@ static void __clocksource_change_rating(struct clocksource *cs, int rating);
>>>>
>>>>  /*
>>>>   * Interval: 0.5sec.
>>>> + * MaxInterval: 1s.
>>>>   */
>>>>  #define WATCHDOG_INTERVAL (HZ >> 1)
>>>> +#define WATCHDOG_MAX_INTERVAL_NS (NSEC_PER_SEC)
>>>>
>>>>  static void clocksource_watchdog_work(struct work_struct *work)
>>>>  {
>>>> @@ -404,7 +406,9 @@ static void clocksource_watchdog(struct timer_list *unused)
>>>>
>>>>                 /* Check the deviation from the watchdog clocksource. */
>>>>                 md = cs->uncertainty_margin + watchdog->uncertainty_margin;
>>>> -               if (abs(cs_nsec - wd_nsec) > md) {
>>>> +               if ((abs(cs_nsec - wd_nsec) > md) &&
>>>> +                       cs_nsec < WATCHDOG_MAX_INTERVAL_NS &&
>>>
>>> Sorry, it's been awhile since I looked at this code, but why are you
>>> bounding the clocksource delta here?
>>> It seems like if the clocksource being watched was very wrong (with a
>>> delta larger than the MAX_INTERVAL_NS), we'd want to throw it out.
>>>
>>>> +                       wd_nsec < WATCHDOG_MAX_INTERVAL_NS) {
>>>
>>> Bounding the watchdog interval on the check does seem reasonable.
>>> Though one may want to keep track that if we are seeing too many of
>>> these delayed watchdog checks we provide some feedback via dmesg.
>>
>> For some fast timeout timers, such as acpi-timer, checking wd_nsec should not
>> make much sense, because when wacthdog is called, the timer may overflow many
>> times.
> 
> Indeed. But in that case we can't tell which way is up. This is what I
> was fretting about when I said:
>> So I do worry these watchdog robustness fixes are papering over a
>> problem, pushing expectations closer to the edge of how far the system
>> should tolerate bad behavior. Because at some point we'll fall off. :)
> 
> If the timer is delayed long enough for the watchdog to wrap, we're
> way out of tolerable behavior. There's not much we can do because we
> can't even tell what happened.
> 
> But in the case where the watchdog has not wrapped, I don't see a
> major issue with trying to be a bit more robust in the face of just
> slightly delayed timers.
> (And yes, we can't really distinguish between slightly delayed and
> watchdog-wrap-interval + slight delay, but in either case we can
> probably skip disqualifying the clocksource as we know something seems
> off)

If we record the watchdog's start_time in clocksource_start_watchdog(), and then
when we verify cycles in clocksource_watchdog(), check whether the clocksource
watchdog is blocked. Due to MSB verification, if the blocked time is greater than
half of the watchdog timer max_cycles, then we can safely ignore the current
verification? Do you think this idea is okay?

> thanks
> -john
>