linux-kernel - Re: [PATCH tip/core/rcu 2/8] rcutorture: Dump stack when GP kthread stalls

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Date:	Sat, 5 Dec 2015 17:45:31 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 2/8] rcutorture:  Dump stack when GP kthread
 stalls

On Fri, Dec 04, 2015 at 04:59:48PM -0800, Josh Triplett wrote:
> On Fri, Dec 04, 2015 at 04:23:46PM -0800, Paul E. McKenney wrote:
> > This commit increases debug information in the case where the grace-period
> > kthread is being prevented from running by dumping that kthread's stack.
> 
> That's not everything this patch does; see below.
> 
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> >  kernel/rcu/tree.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 81aa1cdc6bc9..7b78c88e19a3 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1196,12 +1196,15 @@ static void rcu_check_gp_kthread_starvation(struct rcu_state *rsp)
> >  
> >  	j = jiffies;
> >  	gpa = READ_ONCE(rsp->gp_activity);
> > -	if (j - gpa > 2 * HZ)
> > +	if (j - gpa > 2 * HZ) {
> >  		pr_err("%s kthread starved for %ld jiffies! g%lu c%lu f%#x s%d ->state=%#lx\n",
> >  		       rsp->name, j - gpa,
> >  		       rsp->gpnum, rsp->completed,
> >  		       rsp->gp_flags, rsp->gp_state,
> > -		       rsp->gp_kthread ? rsp->gp_kthread->state : 0);
> > +		       rsp->gp_kthread ? rsp->gp_kthread->state : ~0);
> 
> The commit message doesn't explain this change.  Could you please 1)
> document this change in the commit message, and 2) split it into a
> separate patch unless it directly relates to the other change (which
> doesn't seem like the case)?

Good point, especially given that it took me some time to remember why
that change was necessary.  It is now in a separate commit, with a commit
log stating that this disambiguates the case where there is no RCU GP
kthread from the case where there is a TASK_RUNNING RCU GP kthread.

							Thanx, Paul

> > +		if (rsp->gp_kthread)
> > +			sched_show_task(rsp->gp_kthread);
> > +	}
> >  }
> >  
> >  /*
> > -- 
> > 2.5.2
> > 
> 

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