watchdog/perf: properly initialize the turbo mode timestamp and rearm…

… counter

For systems on which the performance counter can expire early due to turbo
modes the watchdog handler has a safety net in place which validates that
since the last watchdog event there has at least 4/5th of the watchdog
period elapsed.

This works reliably only after the first watchdog event because the per
CPU variable which holds the timestamp of the last event is never
initialized.

So a first spurious event will validate against a timestamp of 0 which
results in a delta which is likely to be way over the 4/5 threshold of the
period.  As this might happen before the first watchdog hrtimer event
increments the watchdog counter, this can lead to false positives.

Fix this by initializing the timestamp before enabling the hardware event.
Reset the rearm counter as well, as that might be non zero after the
watchdog was disabled and reenabled.

Link: https://lkml.kernel.org/r/87frsfu15a.ffs@tglx
Fixes: 7edaeb6 ("kernel/watchdog: Prevent false positives with turbo modes")
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Arjan van de Ven <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

kernel/watchdog_perf.c

@@ -75,11 +75,15 @@ static bool watchdog_check_timestamp(void)
 	__this_cpu_write(last_timestamp, now);
 	return true;
 }
-#else
-static inline bool watchdog_check_timestamp(void)
+
+static void watchdog_init_timestamp(void)
 {
-	return true;
+	__this_cpu_write(nmi_rearmed, 0);
+	__this_cpu_write(last_timestamp, ktime_get_mono_fast_ns());
 }
+#else
+static inline bool watchdog_check_timestamp(void) { return true; }
+static inline void watchdog_init_timestamp(void) { }
 #endif
 
 static struct perf_event_attr wd_hw_attr = {
@@ -161,6 +165,7 @@ void watchdog_hardlockup_enable(unsigned int cpu)
 	if (!atomic_fetch_inc(&watchdog_cpus))
 		pr_info("Enabled. Permanently consumes one hw-PMU counter.\n");
 
+	watchdog_init_timestamp();
 	perf_event_enable(this_cpu_read(watchdog_ev));
 }