On Thu, Aug 28, 2014 at 04:07:23PM -0700, Andrew Morton wrote:
> On Thu, 28 Aug 2014 00:52:24 -0400 Don Zickus <dzic...@redhat.com> wrote:
>
> > From: chai wen <chaiw.f...@cn.fujitsu.com>
> >
> > For now, soft lockup detector warns once for each case of process
> > softlockup.
> > But the thread 'watchdog/n' may not always get the cpu at the time slot
> > between
> > the task switch of two processes hogging that cpu to reset
> > soft_watchdog_warn.
> >
> > An example would be two processes hogging the cpu. Process A causes the
> > softlockup warning and is killed manually by a user. Process B immediately
> > becomes the new process hogging the cpu preventing the softlockup code from
> > resetting the soft_watchdog_warn variable.
> >
> > This case is a false negative of "warn only once for a process", as there
> > may
> > be a different process that is going to hog the cpu. Resolve this by
> > saving/checking the task pointer of the hogging process and use that to
> > reset
> > soft_watchdog_warn too.
> >
>
> OK, this should address the PID uniqueness issue which Ingo identified.
>
> > --- a/kernel/watchdog.c
> > +++ b/kernel/watchdog.c
> > @@ -47,6 +47,7 @@ static DEFINE_PER_CPU(bool, softlockup_touch_sync);
> > static DEFINE_PER_CPU(bool, soft_watchdog_warn);
> > static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts);
> > static DEFINE_PER_CPU(unsigned long, soft_lockup_hrtimer_cnt);
> > +static DEFINE_PER_CPU(struct task_struct *, softlockup_task_ptr_saved);
> > #ifdef CONFIG_HARDLOCKUP_DETECTOR
> > static DEFINE_PER_CPU(bool, hard_watchdog_warn);
> > static DEFINE_PER_CPU(bool, watchdog_nmi_touch);
> > @@ -331,8 +332,20 @@ static enum hrtimer_restart watchdog_timer_fn(struct
> > hrtimer *hrtimer)
> > return HRTIMER_RESTART;
> >
> > /* only warn once */
> > - if (__this_cpu_read(soft_watchdog_warn) == true)
> > + if (__this_cpu_read(soft_watchdog_warn) == true) {
> > + /*
> > + * Handle the case where multiple processes are
> > + * causing softlockups but the duration is small
> > + * enough, the softlockup detector can not reset
> > + * itself in time. Use task pointers to detect this.
> > + */
>
> This comment is rather hard to follow ("the duration" of what?). Can
> you think of some words which are a bit more complete/clear?
Agreed. Does this work better?
"
/*
* When multiple processes are causing softlockups
* the softlockup detector only warns on the first
* one because the code relies on a full quiet cycle
* to re-arm. The second process prevents the
* quiet cycle and never gets reported. Use task
* pointers to detect this.
*/
Cheers,
Don
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