Re: [PATCH 2/5] softlockup: make detector be aware of task switch of processes hogging cpu

[Don Zickus <dzickus@redhat.com>]

On Mon, Aug 18, 2014 at 11:03:19AM +0200, Ingo Molnar wrote:
> * 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 pid of the hogging process and use that to reset
> > soft_watchdog_warn too.
> > 
> > Signed-off-by: chai wen <chaiw.fnst@cn.fujitsu.com>
> > [modified the comment and changelog to be more specific]
> > Signed-off-by: Don Zickus <dzickus@redhat.com>
> > ---
> >  kernel/watchdog.c |   20 ++++++++++++++++++--
> >  1 files changed, 18 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> > index 4c2e11c..6d0a891 100644
> > --- a/kernel/watchdog.c
> > +++ b/kernel/watchdog.c
> > @@ -42,6 +42,7 @@ 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(pid_t, softlockup_warn_pid_saved);
> >  #ifdef CONFIG_HARDLOCKUP_DETECTOR
> >  static DEFINE_PER_CPU(bool, hard_watchdog_warn);
> >  static DEFINE_PER_CPU(bool, watchdog_nmi_touch);
> > @@ -317,6 +318,8 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
> >  	 */
> >  	duration = is_softlockup(touch_ts);
> >  	if (unlikely(duration)) {
> > +		pid_t pid = task_pid_nr(current);
> > +
> >  		/*
> >  		 * 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
> > @@ -326,8 +329,20 @@ 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 pids to detect this.
> > +			 */
> > +			if (__this_cpu_read(softlockup_warn_pid_saved) != pid) {
> 
> So I agree with the motivation of this improvement, but is this 
> implementation namespace-safe?

What namespace are you worried about colliding with?  I thought
softlockup_ would provide the safety??  Maybe I am missing something
obvious. :-(

Cheers,
Don