From: Feng Tang <feng.tang@intel.com>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: <linux-kernel@vger.kernel.org>, <clm@meta.com>,
<jstultz@google.com>, <tglx@linutronix.de>, <sboyd@kernel.org>,
<longman@redhat.com>
Subject: Re: [PATCH clocksource] Reject bogus watchdog clocksource measurements
Date: Fri, 21 Oct 2022 08:46:32 +0800 [thread overview]
Message-ID: <Y1Hr6PNy9EJk245f@feng-clx> (raw)
In-Reply-To: <20221020140944.GK5600@paulmck-ThinkPad-P17-Gen-1>
On Thu, Oct 20, 2022 at 07:09:44AM -0700, Paul E. McKenney wrote:
[...]
> > > static void clocksource_watchdog(struct timer_list *unused)
> > > {
> > > - u64 csnow, wdnow, cslast, wdlast, delta;
> > > + u64 csnow, wdnow, cslast, wdlast, delta, wdi;
> > > int next_cpu, reset_pending;
> > > int64_t wd_nsec, cs_nsec;
> > > struct clocksource *cs;
> > > @@ -440,6 +440,17 @@ static void clocksource_watchdog(struct timer_list *unused)
> > > if (atomic_read(&watchdog_reset_pending))
> > > continue;
> > >
> > > + /* Check for bogus measurements. */
> > > + wdi = jiffies_to_nsecs(WATCHDOG_INTERVAL);
> > > + if (wd_nsec < (wdi >> 2)) {
> > > + pr_warn("timekeeping watchdog on CPU%d: Watchdog clocksource '%s' advanced only %lld ns during %d-jiffy time interval, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL);
> > > + continue;
> > > + }
> >
> > If this happens (500ms timer happens only after less than 125ms),
> > there is some severe problem with timer/interrupt system.
>
> Should I add ", suspect timer/interrupt bug" just after "jiffy time
> interval"? Or would a comment before that pr_warn() work better for you?
Both are fine for me.
> > > + if (wd_nsec > (wdi << 2)) {
> > > + pr_warn("timekeeping watchdog on CPU%d: Watchdog clocksource '%s' advanced an excessive %lld ns during %d-jiffy time interval, probable CPU overutilization, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL);
> > > + continue;
> > > + }
> >
> > I agree with Waiman that some rate limiting may be needed. As there
> > were reports of hundreds of seconds of delay, 2 seconds delay could
> > easily happen if a system is too busy or misbehave to trigger this
> > problem.
>
> Good points, thank you both!
>
> Left to myself, I would use a capped power-of-two backoff that was reset
> any time that the interval was within bounds. Maybe a cap of 10 minutes?
Yep, that's more informative. Thanks!
- Feng
> Or is there a better way to do this?
>
> Thanx, Paul
next prev parent reply other threads:[~2022-10-21 0:47 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-19 23:09 [PATCH clocksource] Reject bogus watchdog clocksource measurements Paul E. McKenney
2022-10-20 3:01 ` Waiman Long
2022-10-20 8:09 ` Feng Tang
2022-10-20 14:09 ` Paul E. McKenney
2022-10-20 17:29 ` Waiman Long
2022-10-21 0:46 ` Feng Tang [this message]
2022-10-28 17:52 ` Paul E. McKenney
2022-10-28 17:52 ` Paul E. McKenney
2022-10-31 5:59 ` Feng Tang
2022-10-31 17:42 ` Paul E. McKenney
2022-11-01 5:43 ` Feng Tang
2022-11-01 19:06 ` Paul E. McKenney
2022-11-02 2:58 ` Feng Tang
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