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workqueue/watchdog: Make unbound workqueues aware of touch_softlockup_watchdog()

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There are two workqueue-specific watchdog timestamps:

    + @wq_watchdog_touched_cpu (per-CPU) updated by
      touch_softlockup_watchdog()

    + @wq_watchdog_touched (global) updated by
      touch_all_softlockup_watchdogs()

watchdog_timer_fn() checks only the global @wq_watchdog_touched for
unbound workqueues. As a result, unbound workqueues are not aware
of touch_softlockup_watchdog(). The watchdog might report a stall
even when the unbound workqueues are blocked by a known slow code.

Solution:
touch_softlockup_watchdog() must touch also the global @wq_watchdog_touched
timestamp.

The global timestamp can no longer be used for bound workqueues because
it is now updated from all CPUs. Instead, bound workqueues have to check
only @wq_watchdog_touched_cpu and these timestamps have to be updated for
all CPUs in touch_all_softlockup_watchdogs().

Beware:
The change might cause the opposite problem. An unbound workqueue
might get blocked on CPU A because of a real softlockup. The workqueue
watchdog would miss it when the timestamp got touched on CPU B.

It is acceptable because softlockups are detected by softlockup
watchdog. The workqueue watchdog is there to detect stalls where
a work never finishes, for example, because of dependencies of works
queued into the same workqueue.

V3:
- Modify the commit message clearly according to Petr's suggestion.

Signed-off-by: Wang Qing <wangqing@vivo.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Wang Qing 2021-03-24 19:40:29 +08:00 committed by Tejun Heo
parent 0687c66b5f
commit 89e28ce60c
2 changed files with 9 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
kernel/watchdog.c
View File

@ -278,9 +278,10 @@ void touch_all_softlockup_watchdogs(void)
* update as well, the only side effect might be a cycle delay for
* the softlockup check.
*/
for_each_cpu(cpu, &watchdog_allowed_mask)
for_each_cpu(cpu, &watchdog_allowed_mask) {
per_cpu(watchdog_touch_ts, cpu) = SOFTLOCKUP_RESET;
wq_watchdog_touch(-1);
wq_watchdog_touch(cpu);
}
}
void touch_softlockup_watchdog_sync(void)

17
kernel/workqueue.c
View File

@ -5787,22 +5787,17 @@ static void wq_watchdog_timer_fn(struct timer_list *unused)
continue;
/* get the latest of pool and touched timestamps */
if (pool->cpu >= 0)
touched = READ_ONCE(per_cpu(wq_watchdog_touched_cpu, pool->cpu));
else
touched = READ_ONCE(wq_watchdog_touched);
pool_ts = READ_ONCE(pool->watchdog_ts);
touched = READ_ONCE(wq_watchdog_touched);
if (time_after(pool_ts, touched))
ts = pool_ts;
else
ts = touched;
if (pool->cpu >= 0) {
unsigned long cpu_touched =
READ_ONCE(per_cpu(wq_watchdog_touched_cpu,
pool->cpu));
if (time_after(cpu_touched, ts))
ts = cpu_touched;
}
/* did we stall? */
if (time_after(jiffies, ts + thresh)) {
lockup_detected = true;
@ -5826,8 +5821,8 @@ notrace void wq_watchdog_touch(int cpu)
{
if (cpu >= 0)
per_cpu(wq_watchdog_touched_cpu, cpu) = jiffies;
else
wq_watchdog_touched = jiffies;
wq_watchdog_touched = jiffies;
}
static void wq_watchdog_set_thresh(unsigned long thresh)