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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>
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 <CALAqxLWgw8tA1Lrg27JUUFrGWCQqPQXmhjHyjsTRA5a4qingkg@mail.gmail.com>
From:   brookxu <brookxu.cn@gmail.com>
Message-ID: <95c1a031-6751-f90f-d003-b74fbec0e9d8@gmail.com>
Date:   Tue, 12 Oct 2021 16:06:50 +0800
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John Stultz wrote on 2021/10/12 13:29:
> On Mon, Oct 11, 2021 at 10:23 PM brookxu <brookxu.cn@gmail.com> wrote:
>> 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?
> 
> I can't say I totally understand the idea. Maybe could you clarify with a patch?
>

Sorry, it looks almost as follows:

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index b8a14d2..87f3b67 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -119,6 +119,7 @@
 static DECLARE_WORK(watchdog_work, clocksource_watchdog_work);
 static DEFINE_SPINLOCK(watchdog_lock);
 static int watchdog_running;
+static unsigned long watchdog_start_time;
 static atomic_t watchdog_reset_pending;
 
 static inline void clocksource_watchdog_lock(unsigned long *flags)
@@ -356,6 +357,7 @@ static void clocksource_watchdog(struct timer_list *unused)
 	int next_cpu, reset_pending;
 	int64_t wd_nsec, cs_nsec;
 	struct clocksource *cs;
+	unsigned long max_jiffies;
 	u32 md;
 
 	spin_lock(&watchdog_lock);
@@ -402,6 +404,10 @@ static void clocksource_watchdog(struct timer_list *unused)
 		if (atomic_read(&watchdog_reset_pending))
 			continue;
 
+		max_jiffies = nsecs_to_jiffies(cs->max_idle_ns);
+		if (time_is_before_jiffies(watchdog_start_time + max_jiffies))
+			continue;
+
 		/* Check the deviation from the watchdog clocksource. */
 		md = cs->uncertainty_margin + watchdog->uncertainty_margin;
 		if (abs(cs_nsec - wd_nsec) > md) {
@@ -474,6 +480,7 @@ static void clocksource_watchdog(struct timer_list *unused)
 	 * pair clocksource_stop_watchdog() clocksource_start_watchdog().
 	 */
 	if (!timer_pending(&watchdog_timer)) {
+		watchdog_start_time = jiffies;
 		watchdog_timer.expires += WATCHDOG_INTERVAL;
 		add_timer_on(&watchdog_timer, next_cpu);
 	}
@@ -488,6 +495,7 @@ static inline void clocksource_start_watchdog(void)
 	timer_setup(&watchdog_timer, clocksource_watchdog, 0);
 	watchdog_timer.expires = jiffies + WATCHDOG_INTERVAL;
 	add_timer_on(&watchdog_timer, cpumask_first(cpu_online_mask));
+	watchdog_start_time = jiffies;
 	watchdog_running = 1;
 }


> thanks
> -john
>