From: Petr Mladek pmladek@suse.com
[ Upstream commit 9bf3bc949f8aeefeacea4b1198db833b722a8e27 ]
Commit d6ad3e286d2c ("softlockup: Add sched_clock_tick() to avoid kernel warning on kgdb resume") introduced touch_softlockup_watchdog_sync().
It solved a problem when the watchdog was touched in an atomic context, the timer callback was proceed right after releasing interrupts, and the local clock has not been updated yet. In this case, sched_clock_tick() was called in watchdog_timer_fn() before updating the timer.
So far so good.
Later commit 5d1c0f4a80a6 ("watchdog: add check for suspended vm in softlockup detector") added two kvm_check_and_clear_guest_paused() calls. They touch the watchdog when the guest has been sleeping.
The code makes my head spin around.
Scenario 1:
+ guest did sleep: + PVCLOCK_GUEST_STOPPED is set
+ 1st watchdog_timer_fn() invocation: + the watchdog is not touched yet + is_softlockup() returns too big delay + kvm_check_and_clear_guest_paused(): + clear PVCLOCK_GUEST_STOPPED + call touch_softlockup_watchdog_sync() + set SOFTLOCKUP_DELAY_REPORT + set softlockup_touch_sync + return from the timer callback
+ 2nd watchdog_timer_fn() invocation:
+ call sched_clock_tick() even though it is not needed. The timer callback was invoked again only because the clock has already been updated in the meantime.
+ call kvm_check_and_clear_guest_paused() that does nothing because PVCLOCK_GUEST_STOPPED has been cleared already.
+ call update_report_ts() and return. This is fine. Except that sched_clock_tick() might allow to set it already during the 1st invocation.
Scenario 2:
+ guest did sleep
+ 1st watchdog_timer_fn() invocation + same as in 1st scenario
+ guest did sleep again: + set PVCLOCK_GUEST_STOPPED again
+ 2nd watchdog_timer_fn() invocation + SOFTLOCKUP_DELAY_REPORT is set from 1st invocation + call sched_clock_tick() + call kvm_check_and_clear_guest_paused() + clear PVCLOCK_GUEST_STOPPED + call touch_softlockup_watchdog_sync() + set SOFTLOCKUP_DELAY_REPORT + set softlockup_touch_sync + call update_report_ts() (set real timestamp immediately) + return from the timer callback
+ 3rd watchdog_timer_fn() invocation + timestamp is set from 2nd invocation + softlockup_touch_sync is set but not checked because the real timestamp is already set
Make the code more straightforward:
1. Always call kvm_check_and_clear_guest_paused() at the very beginning to handle PVCLOCK_GUEST_STOPPED. It touches the watchdog when the quest did sleep.
2. Handle the situation when the watchdog has been touched (SOFTLOCKUP_DELAY_REPORT is set).
Call sched_clock_tick() when touch_*sync() variant was used. It makes sure that the timestamp will be up to date even when it has been touched in atomic context or quest did sleep.
As a result, kvm_check_and_clear_guest_paused() is called on a single location. And the right timestamp is always set when returning from the timer callback.
Link: https://lkml.kernel.org/r/20210311122130.6788-7-pmladek@suse.com Signed-off-by: Petr Mladek pmladek@suse.com Cc: Ingo Molnar mingo@kernel.org Cc: Laurence Oberman loberman@redhat.com Cc: Michal Hocko mhocko@suse.com Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Vincent Whitchurch vincent.whitchurch@axis.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/watchdog.c | 20 ++++++++------------ 1 file changed, 8 insertions(+), 12 deletions(-)
diff --git a/kernel/watchdog.c b/kernel/watchdog.c index 8cf0678378d2..7c397907d0e9 100644 --- a/kernel/watchdog.c +++ b/kernel/watchdog.c @@ -376,7 +376,14 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer) /* .. and repeat */ hrtimer_forward_now(hrtimer, ns_to_ktime(sample_period));
- /* Reset the interval when touched externally by a known slow code. */ + /* + * If a virtual machine is stopped by the host it can look to + * the watchdog like a soft lockup. Check to see if the host + * stopped the vm before we process the timestamps. + */ + kvm_check_and_clear_guest_paused(); + + /* Reset the interval when touched by known problematic code. */ if (period_ts == SOFTLOCKUP_DELAY_REPORT) { if (unlikely(__this_cpu_read(softlockup_touch_sync))) { /* @@ -387,10 +394,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer) sched_clock_tick(); }
- /* Clear the guest paused flag on watchdog reset */ - kvm_check_and_clear_guest_paused(); update_report_ts(); - return HRTIMER_RESTART; }
@@ -402,14 +406,6 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer) */ duration = is_softlockup(touch_ts, period_ts); if (unlikely(duration)) { - /* - * If a virtual machine is stopped by the host it can look to - * the watchdog like a soft lockup, check to see if the host - * stopped the vm before we issue the warning - */ - if (kvm_check_and_clear_guest_paused()) - return HRTIMER_RESTART; - /* * Prevent multiple soft-lockup reports if one cpu is already * engaged in dumping all cpu back traces.