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From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     tglx@linutronix.de
Cc:     linux-kernel@vger.kernel.org, john.stultz@linaro.org,
        sboyd@kernel.org, corbet@lwn.net, Mark.Rutland@arm.com,
        maz@kernel.org, kernel-team@meta.com, neeraju@codeaurora.org,
        ak@linux.intel.com, feng.tang@intel.com, zhengjun.xing@intel.com,
        "Paul E. McKenney" <paulmck@kernel.org>,
        John Stultz <jstultz@google.com>,
        Waiman Long <longman@redhat.com>
Subject: [PATCH clocksource 1/2] clocksource: Add comments to classify bogus measurements
Date:   Wed,  2 Nov 2022 11:40:08 -0700
Message-Id: <20221102184009.1306751-1-paulmck@kernel.org>
In-Reply-To: <20221102184001.GA1306489@paulmck-ThinkPad-P17-Gen-1>
References: <20221102184001.GA1306489@paulmck-ThinkPad-P17-Gen-1>
MIME-Version: 1.0
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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>
---
 kernel/time/clocksource.c | 2 ++
 1 file changed, 2 insertions(+)

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;
 		}
-- 
2.31.1.189.g2e36527f23