From: "Paul E. McKenney" <paulmck@kernel.org>
To: Feng Tang <feng.tang@intel.com>
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, 28 Oct 2022 10:52:07 -0700 [thread overview]
Message-ID: <20221028175207.GM5600@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <Y1Hr6PNy9EJk245f@feng-clx>
On Fri, Oct 21, 2022 at 08:46:32AM +0800, Feng Tang wrote:
> 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.
Here is the patch, which just adds comments. (The exponential-backoff
patch is on its way.)
Thoughts?
Thanx, Paul
------------------------------------------------------------------------
commit 59d9db36dc15b3b40a30d7a3d733dbd412c8557a
Author: Paul E. McKenney <paulmck@kernel.org>
Date: Thu Oct 27 11:58:27 2022 -0700
clocksource: Add comments to classify bogus measurements
An extremely busy system can delay the clocksource watchdog, so that
the corresponding too-long bogus-measurement error does not necessarily
imply an error in the system. However, a too-short bogus-measurement
error likely indicates a bug in hardware, firmware or software.
Therefore, add comments clarifying these bogus-measurement pr_warn()s.
Reported-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: John Stultz <jstultz@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Waiman Long <longman@redhat.com>
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index dcaf38c062161..3f5317faf891f 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -443,10 +443,12 @@ static void clocksource_watchdog(struct timer_list *unused)
/* Check for bogus measurements. */
wdi = jiffies_to_nsecs(WATCHDOG_INTERVAL);
if (wd_nsec < (wdi >> 2)) {
+ /* This usually indicates broken timer code or hardware. */
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 (wd_nsec > (wdi << 2)) {
+ /* This can happen on busy systems, which can delay the watchdog. */
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;
}
next prev parent reply other threads:[~2022-10-28 17:52 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
2022-10-28 17:52 ` Paul E. McKenney [this message]
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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