Re: [PATCH] softlockup: Make detector be aware of task switch of processes hogging cpu

[Don Zickus <dzickus@redhat.com>]

On Thu, Aug 28, 2014 at 04:07:23PM -0700, Andrew Morton wrote:
> On Thu, 28 Aug 2014 00:52:24 -0400 Don Zickus <dzickus@redhat.com> wrote:
> 
> > From: chai wen <chaiw.fnst@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