[tip:core/urgent] watchdog/hardlockup/perf: Use atomics to track in-use cpu counter

tip-bot for Don Zickus
2017-11-01T20:31:33Z

Commit-ID: 42f930da7f00c0ab23df4c7aed36137f35988980
Gitweb: https://git.kernel.org/tip/42f930da7f00c0ab23df4c7aed36137f35988980
Author: Don Zickus <dzickus@redhat.com>
AuthorDate: Wed, 1 Nov 2017 14:11:27 -0400
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Wed, 1 Nov 2017 21:18:40 +0100

watchdog/hardlockup/perf: Use atomics to track in-use cpu counter

Guenter reported:
There is still a problem. When running
echo 6 > /proc/sys/kernel/watchdog_thresh
echo 5 > /proc/sys/kernel/watchdog_thresh
repeatedly, the message

NMI watchdog: Enabled. Permanently consumes one hw-PMU counter.

stops after a while (after ~10-30 iterations, with fluctuations).
Maybe watchdog_cpus needs to be atomic ?

That's correct as this again is affected by the asynchronous nature of the
smpboot thread unpark mechanism.

CPU 0 CPU1 CPU2
write(watchdog_thresh, 6)
stop()
park()
update()
start()
unpark()
thread->unpark()
cnt++;
write(watchdog_thresh, 5) thread->unpark()
stop()
park() thread->park()
cnt--; cnt++;
update()
start()
unpark()

That's not a functional problem, it just affects the informational message.

Convert watchdog_cpus to atomic_t to prevent the problem

Reported-and-tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20171101181126.j727fqjmdthjz4xk@redhat.com

---
kernel/watchdog_hld.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c
index a7f137c..a84b205 100644
--- a/kernel/watchdog_hld.c
+++ b/kernel/watchdog_hld.c
@@ -12,6 +12,7 @@
#define pr_fmt(fmt) "NMI watchdog: " fmt

#include <linux/nmi.h>
+#include <linux/atomic.h>
#include <linux/module.h>
#include <linux/sched/debug.h>

@@ -25,7 +26,7 @@ static DEFINE_PER_CPU(struct perf_event *, dead_event);
static struct cpumask dead_events_mask;

static unsigned long hardlockup_allcpu_dumped;
-static unsigned int watchdog_cpus;
+static atomic_t watchdog_cpus = ATOMIC_INIT(0);

void arch_touch_nmi_watchdog(void)
{
@@ -189,7 +190,8 @@ void hardlockup_detector_perf_enable(void)
if (hardlockup_detector_event_create())
return;

- if (!watchdog_cpus++)
+ /* use original value for check */
+ if (!atomic_fetch_inc(&watchdog_cpus))
pr_info("Enabled. Permanently consumes one hw-PMU counter.\n");

perf_event_enable(this_cpu_read(watchdog_ev));
@@ -207,7 +209,7 @@ void hardlockup_detector_perf_disable(void)
this_cpu_write(watchdog_ev, NULL);
this_cpu_write(dead_event, event);
cpumask_set_cpu(smp_processor_id(), &dead_events_mask);
- watchdog_cpus--;
+ atomic_dec(&watchdog_cpus);
}
}