The patch titled Subject: watchdog/perf: Properly initialize the turbo mode timestamp and rearm counter has been added to the -mm mm-nonmm-unstable branch. Its filename is watchdog-perf-properly-initialize-the-turbo-mode-timestamp-and-rearm-counter.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-nonmm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Thomas Gleixner tglx@linutronix.de Subject: watchdog/perf: Properly initialize the turbo mode timestamp and rearm counter Date: Thu, 11 Jul 2024 22:25:21 +0200

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: 7edaeb6841df ("kernel/watchdog: Prevent false positives with turbo modes") Signed-off-by: Thomas Gleixner tglx@linutronix.de Cc: Arjan van de Ven arjan@linux.intel.com Cc: Peter Zijlstra peterz@infradead.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

kernel/watchdog_perf.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-)

--- a/kernel/watchdog_perf.c~watchdog-perf-properly-initialize-the-turbo-mode-timestamp-and-rearm-counter +++ a/kernel/watchdog_perf.c @@ -75,11 +75,15 @@ static bool watchdog_check_timestamp(voi __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 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)); }

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Patches currently in -mm which might be from tglx@linutronix.de are

watchdog-perf-properly-initialize-the-turbo-mode-timestamp-and-rearm-counter.patch