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

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Date:	Mon, 18 Aug 2014 14:43:43 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	akpm@...ux-foundation.org, kvm@...r.kernel.org,
	pbonzini@...hat.com, mingo@...hat.com,
	LKML <linux-kernel@...r.kernel.org>,
	chai wen <chaiw.fnst@...fujitsu.com>
Subject: Re: [PATCH 2/5] softlockup: make detector be aware of task switch of
 processes hogging cpu

On Mon, Aug 18, 2014 at 08:01:58PM +0200, Ingo Molnar wrote:
> > > >  	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. :-(
> 
> I meant PID namespaces - a PID in itself isn't guaranteed to be 
> unique across the system.

Ah,  I don't think we thought about that.  Is there a better way to do
this?  Is there a domain id or something that can be OR'd with the pid?

Cheers,
Don
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