linux-kernel - Re: [PATCH] rcu: Avoid unnecessary contention of rcu node lock

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Date:   Wed, 16 Nov 2016 08:59:30 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     paulmck@...ux.vnet.ibm.com, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Avoid unnecessary contention of rcu node lock

On Wed, 16 Nov 2016 13:49:31 +0900
Byungchul Park <byungchul.park@....com> wrote:

> On Wed, Nov 09, 2016 at 05:57:13PM +0900, Byungchul Park wrote:
> > It's unnecessary to try to print stacks of blocked tasks in the case
> > that ndetected == 0. Furthermore, calling rcu_print_detail_task_stall()
> > causes to acquire rnp locks as many times as the number of leaf nodes
> > plus one for root node. It's unnecessary at all in the case.  
> 
> Hello,
> 
> I have two questions. Could you answer them?
> 
> 1. What do you think about this patch?
> 2. Is there a tree where patches about rcu are pulled into, before
>    being pulled into mainline tree?
>    For example, tip tree in case of scheduler patches.

I believe Paul sends to the tip tree too.

> 
> It would be appriciated if you answer them.
> 
> Thank you in advance,
> Byungchul
> 
> > 
> > Signed-off-by: Byungchul Park <byungchul.park@....com>
> > ---
> >  kernel/rcu/tree.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 287f468..ab2f743 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1374,6 +1374,9 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
> >  	       (long)rsp->gpnum, (long)rsp->completed, totqlen);
> >  	if (ndetected) {
> >  		rcu_dump_cpu_stacks(rsp);
> > +
> > +		/* Complain about tasks blocking the grace period. */
> > +		rcu_print_detail_task_stall(rsp);
> >  	} else {
> >  		if (READ_ONCE(rsp->gpnum) != gpnum ||
> >  		    READ_ONCE(rsp->completed) == gpnum) {
> > @@ -1390,9 +1393,6 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
> >  		}

What about showing it in else statement of the above if as well?

	sched_show_task(current);
	rcu_print_detail_task_stall(rsp);

-- Steve

> >  	}
> >  
> > -	/* Complain about tasks blocking the grace period. */
> > -	rcu_print_detail_task_stall(rsp);
> > -
> >  	rcu_check_gp_kthread_starvation(rsp);
> >  
> >  	panic_on_rcu_stall();
> > -- 
> > 1.9.1