[PATCH v5 10/18] watchdog/hardlockup: Add a "cpu" param to watchdog_hardlockup_check()

Douglas Anderson dianders at chromium.org
Sat May 20 03:18:34 AEST 2023

In preparation for the buddy hardlockup detector where the CPU
checking for lockup might not be the currently running CPU, add a
"cpu" parameter to watchdog_hardlockup_check().

As part of this change, make hrtimer_interrupts an atomic_t since now
the CPU incrementing the value and the CPU reading the value might be
different. Technially this could also be done with just READ_ONCE and
WRITE_ONCE, but atomic_t feels a little cleaner in this case.

While hrtimer_interrupts is made atomic_t, we change
hrtimer_interrupts_saved from "unsigned long" to "int". The "int" is
needed to match the data type backing atomic_t for hrtimer_interrupts.
Even if this changes us from 64-bits to 32-bits (which I don't think
is true for most compilers), it doesn't really matter. All we ever do
is increment it every few seconds and compare it to an old value so
32-bits is fine (even 16-bits would be). The "signed" vs "unsigned"
also doesn't matter for simple equality comparisons.

hrtimer_interrupts_saved is _not_ switched to atomic_t nor even
accessed with READ_ONCE / WRITE_ONCE. The hrtimer_interrupts_saved is
always consistently accessed with the same CPU. NOTE: with the
upcoming "buddy" detector there is one special case. When a CPU goes
offline/online then we can change which CPU is the one to consistently
access a given instance of hrtimer_interrupts_saved. We still can't
end up with a partially updated hrtimer_interrupts_saved, however,
because we end up petting all affected CPUs to make sure the new and
old CPU can't end up somehow read/write hrtimer_interrupts_saved at
the same time.

Signed-off-by: Douglas Anderson <dianders at chromium.org>

Changes in v5:
- Don't dump stack on the buddy CPU if we fail to backtrace the hung CPU.
- Use atomic_t for hrtimer_interrupts.

Changes in v4:
- ("Add a "cpu" param to watchdog_hardlockup_check()") new for v4.

 include/linux/nmi.h    |  2 +-
 kernel/watchdog.c      | 52 ++++++++++++++++++++++++++----------------
 kernel/watchdog_perf.c |  2 +-
 3 files changed, 34 insertions(+), 22 deletions(-)

diff --git a/include/linux/nmi.h b/include/linux/nmi.h
index 0c62c1bf0a71..92aa568c0c42 100644
--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h
@@ -88,7 +88,7 @@ static inline void hardlockup_detector_disable(void) {}
-void watchdog_hardlockup_check(struct pt_regs *regs);
+void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs);
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 169e5dffbc00..2552e224f76a 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -87,29 +87,34 @@ __setup("nmi_watchdog=", hardlockup_panic_setup);
-static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts);
-static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts_saved);
+static DEFINE_PER_CPU(atomic_t, hrtimer_interrupts);
+static DEFINE_PER_CPU(int, hrtimer_interrupts_saved);
 static DEFINE_PER_CPU(bool, watchdog_hardlockup_warned);
 static unsigned long watchdog_hardlockup_all_cpu_dumped;
-static bool is_hardlockup(void)
+static bool is_hardlockup(unsigned int cpu)
-	unsigned long hrint = __this_cpu_read(hrtimer_interrupts);
+	int hrint = atomic_read(&per_cpu(hrtimer_interrupts, cpu));
-	if (__this_cpu_read(hrtimer_interrupts_saved) == hrint)
+	if (per_cpu(hrtimer_interrupts_saved, cpu) == hrint)
 		return true;
-	__this_cpu_write(hrtimer_interrupts_saved, hrint);
+	/*
+	 * NOTE: we don't need any fancy atomic_t or READ_ONCE/WRITE_ONCE
+	 * for hrtimer_interrupts_saved. hrtimer_interrupts_saved is
+	 * written/read by a single CPU.
+	 */
+	per_cpu(hrtimer_interrupts_saved, cpu) = hrint;
 	return false;
 static void watchdog_hardlockup_kick(void)
-	__this_cpu_inc(hrtimer_interrupts);
+	atomic_inc(raw_cpu_ptr(&hrtimer_interrupts));
-void watchdog_hardlockup_check(struct pt_regs *regs)
+void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
 	 * Check for a hardlockup by making sure the CPU's timer
@@ -117,35 +122,42 @@ void watchdog_hardlockup_check(struct pt_regs *regs)
 	 * fired multiple times before we overflow'd. If it hasn't
 	 * then this is a good indication the cpu is stuck
-	if (is_hardlockup()) {
+	if (is_hardlockup(cpu)) {
 		unsigned int this_cpu = smp_processor_id();
+		struct cpumask backtrace_mask = *cpu_online_mask;
 		/* Only print hardlockups once. */
-		if (__this_cpu_read(watchdog_hardlockup_warned))
+		if (per_cpu(watchdog_hardlockup_warned, cpu))
-		pr_emerg("Watchdog detected hard LOCKUP on cpu %d\n", this_cpu);
+		pr_emerg("Watchdog detected hard LOCKUP on cpu %d\n", cpu);
-		if (regs)
-			show_regs(regs);
-		else
-			dump_stack();
+		if (cpu == this_cpu) {
+			if (regs)
+				show_regs(regs);
+			else
+				dump_stack();
+			cpumask_clear_cpu(cpu, &backtrace_mask);
+		} else {
+			if (trigger_single_cpu_backtrace(cpu))
+				cpumask_clear_cpu(cpu, &backtrace_mask);
+		}
-		 * Perform all-CPU dump only once to avoid multiple hardlockups
-		 * generating interleaving traces
+		 * Perform multi-CPU dump only once to avoid multiple
+		 * hardlockups generating interleaving traces
 		if (sysctl_hardlockup_all_cpu_backtrace &&
 		    !test_and_set_bit(0, &watchdog_hardlockup_all_cpu_dumped))
-			trigger_allbutself_cpu_backtrace();
+			trigger_cpumask_backtrace(&backtrace_mask);
 		if (hardlockup_panic)
 			nmi_panic(regs, "Hard LOCKUP");
-		__this_cpu_write(watchdog_hardlockup_warned, true);
+		per_cpu(watchdog_hardlockup_warned, cpu) = true;
 	} else {
-		__this_cpu_write(watchdog_hardlockup_warned, false);
+		per_cpu(watchdog_hardlockup_warned, cpu) = false;
diff --git a/kernel/watchdog_perf.c b/kernel/watchdog_perf.c
index 04415812d079..4e60e8023515 100644
--- a/kernel/watchdog_perf.c
+++ b/kernel/watchdog_perf.c
@@ -120,7 +120,7 @@ static void watchdog_overflow_callback(struct perf_event *event,
-	watchdog_hardlockup_check(regs);
+	watchdog_hardlockup_check(smp_processor_id(), regs);
 static int hardlockup_detector_event_create(void)

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