path: root/kernel
diff options
authorMichal Hocko <mhocko@suse.cz>2013-09-24 15:27:30 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2013-09-24 17:00:25 -0700
commit9809b18fcf6b8d8ec4d3643677345907e6b50eca (patch)
tree0dfb9bf2f84f5a18736567d126cb9ed7478fe008 /kernel
parentwatchdog: update watchdog attributes atomically (diff)
watchdog: update watchdog_thresh properly
watchdog_tresh controls how often nmi perf event counter checks per-cpu hrtimer_interrupts counter and blows up if the counter hasn't changed since the last check. The counter is updated by per-cpu watchdog_hrtimer hrtimer which is scheduled with 2/5 watchdog_thresh period which guarantees that hrtimer is scheduled 2 times per the main period. Both hrtimer and perf event are started together when the watchdog is enabled. So far so good. But... But what happens when watchdog_thresh is updated from sysctl handler? proc_dowatchdog will set a new sampling period and hrtimer callback (watchdog_timer_fn) will use the new value in the next round. The problem, however, is that nobody tells the perf event that the sampling period has changed so it is ticking with the period configured when it has been set up. This might result in an ear ripping dissonance between perf and hrtimer parts if the watchdog_thresh is increased. And even worse it might lead to KABOOM if the watchdog is configured to panic on such a spurious lockup. This patch fixes the issue by updating both nmi perf even counter and hrtimers if the threshold value has changed. The nmi one is disabled and then reinitialized from scratch. This has an unpleasant side effect that the allocation of the new event might fail theoretically so the hard lockup detector would be disabled for such cpus. On the other hand such a memory allocation failure is very unlikely because the original event is deallocated right before. It would be much nicer if we just changed perf event period but there doesn't seem to be any API to do that right now. It is also unfortunate that perf_event_alloc uses GFP_KERNEL allocation unconditionally so we cannot use on_each_cpu() and do the same thing from the per-cpu context. The update from the current CPU should be safe because perf_event_disable removes the event atomically before it clears the per-cpu watchdog_ev so it cannot change anything under running handler feet. The hrtimer is simply restarted (thanks to Don Zickus who has pointed this out) if it is queued because we cannot rely it will fire&adopt to the new sampling period before a new nmi event triggers (when the treshold is decreased). [akpm@linux-foundation.org: the UP version of __smp_call_function_single ended up in the wrong place] Signed-off-by: Michal Hocko <mhocko@suse.cz> Acked-by: Don Zickus <dzickus@redhat.com> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@kernel.org> Cc: Fabio Estevam <festevam@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'kernel')
1 files changed, 50 insertions, 3 deletions
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index ced7d0609931..4431610f049a 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -486,7 +486,52 @@ static struct smp_hotplug_thread watchdog_threads = {
.unpark = watchdog_enable,
-static int watchdog_enable_all_cpus(void)
+static void restart_watchdog_hrtimer(void *info)
+ struct hrtimer *hrtimer = &__raw_get_cpu_var(watchdog_hrtimer);
+ int ret;
+ /*
+ * No need to cancel and restart hrtimer if it is currently executing
+ * because it will reprogram itself with the new period now.
+ * We should never see it unqueued here because we are running per-cpu
+ * with interrupts disabled.
+ */
+ ret = hrtimer_try_to_cancel(hrtimer);
+ if (ret == 1)
+ hrtimer_start(hrtimer, ns_to_ktime(sample_period),
+static void update_timers(int cpu)
+ struct call_single_data data = {.func = restart_watchdog_hrtimer};
+ /*
+ * Make sure that perf event counter will adopt to a new
+ * sampling period. Updating the sampling period directly would
+ * be much nicer but we do not have an API for that now so
+ * let's use a big hammer.
+ * Hrtimer will adopt the new period on the next tick but this
+ * might be late already so we have to restart the timer as well.
+ */
+ watchdog_nmi_disable(cpu);
+ __smp_call_function_single(cpu, &data, 1);
+ watchdog_nmi_enable(cpu);
+static void update_timers_all_cpus(void)
+ int cpu;
+ get_online_cpus();
+ preempt_disable();
+ for_each_online_cpu(cpu)
+ update_timers(cpu);
+ preempt_enable();
+ put_online_cpus();
+static int watchdog_enable_all_cpus(bool sample_period_changed)
int err = 0;
@@ -496,6 +541,8 @@ static int watchdog_enable_all_cpus(void)
pr_err("Failed to create watchdog threads, disabled\n");
watchdog_running = 1;
+ } else if (sample_period_changed) {
+ update_timers_all_cpus();
return err;
@@ -537,7 +584,7 @@ int proc_dowatchdog(struct ctl_table *table, int write,
* watchdog_*_all_cpus() function takes care of this.
if (watchdog_user_enabled && watchdog_thresh)
- err = watchdog_enable_all_cpus();
+ err = watchdog_enable_all_cpus(old_thresh != watchdog_thresh);
@@ -557,5 +604,5 @@ void __init lockup_detector_init(void)
if (watchdog_user_enabled)
- watchdog_enable_all_cpus();
+ watchdog_enable_all_cpus(false);