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 20:01:58 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Don Zickus <dzickus@...hat.com>
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


* Don Zickus <dzickus@...hat.com> wrote:

> On Mon, Aug 18, 2014 at 11:03:19AM +0200, Ingo Molnar wrote:
> > * Don Zickus <dzickus@...hat.com> wrote:
> > 
> > > From: chai wen <chaiw.fnst@...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@...fujitsu.com>
> > > [modified the comment and changelog to be more specific]
> > > Signed-off-by: Don Zickus <dzickus@...hat.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. :-(

I meant PID namespaces - a PID in itself isn't guaranteed to be 
unique across the system.

Thanks,

	Ingo
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