Re: [PATCH v7 clocksource 3/5] clocksource: Check per-CPU clock synchronization when marked unstable

[Thomas Gleixner]

Paul,

On Mon, Apr 12 2021 at 11:20, Paul E. McKenney wrote:
> On Mon, Apr 12, 2021 at 03:08:16PM +0200, Thomas Gleixner wrote:
>> The reason for irqsave is again historical AFAICT and nobody bothered to
>> clean it up. spin_lock_bh() should be sufficient to serialize against
>> the watchdog timer, though I haven't looked at all possible scenarios.
>
> Though if BH is disabled, there is not so much advantage to
> invoking it from __clocksource_watchdog_kthread().  Might as
> well just invoke it directly from clocksource_watchdog().
>
>> > 2.	Invoke clocksource_verify_percpu() from its original
>> > 	location in clocksource_watchdog(), just before the call to
>> > 	__clocksource_unstable().  This relies on the fact that
>> > 	clocksource_watchdog() acquires watchdog_lock without
>> > 	disabling interrupts.
>> 
>> That should be fine, but this might cause the softirq to 'run' for a
>> very long time which is not pretty either.
>> 
>> Aside of that, do we really need to check _all_ online CPUs? What you
>> are trying to figure out is whether the wreckage is CPU local or global,
>> right?
>> 
>> Wouldn't a shirt-sleeve approach of just querying _one_ CPU be good
>> enough? Either the other CPU has the same wreckage, then it's global or
>> it hasn't which points to a per CPU local issue.
>> 
>> Sure it does not catch the case where a subset (>1) of all CPUs is
>> affected, but I'm not seing how that really buys us anything.
>
> Good point!  My thought is to randomly pick eight CPUs to keep the
> duration reasonable while having a good chance of hitting "interesting"
> CPU choices in multicore and multisocket systems.
>
> However, if a hard-to-reproduce problem occurred, it would be good to take
> the hit and scan all the CPUs.  Additionally, there are some workloads
> for which the switch from TSC to HPET is fatal anyway due to increased
> overhead.  For these workloads, the full CPU scan is no additional pain.
>
> So I am thinking in terms of a default that probes eight randomly selected
> CPUs without worrying about duplicates (as in there would be some chance
> that fewer CPUs would actually be probed), but with a boot-time flag
> that does all CPUs.  I would add the (default) random selection as a
> separate patch.

You can't do without making it complex, right? Keep it simple is not an
option for a RCU hacker it seems :)

> I will send a new series out later today, Pacific Time.

Can you do me a favour and send it standalone and not as yet another
reply to this existing thread maze. A trivial lore link to the previous
version gives enough context.

Thanks,

        tglx