linux-kernel - Re: [PATCH tip/core/rcu 31/86] rcu: further lower priority in rcu

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Date:	Mon, 2 May 2011 01:11:21 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mike Galbraith <efault@....de>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 31/86] rcu: further lower priority in
 rcu_yield()

On Sun, May 01, 2011 at 07:51:04PM +0200, Mike Galbraith wrote:
> On Sun, 2011-05-01 at 06:21 -0700, Paul E. McKenney wrote:
> > From: Paul E. McKenney <paul.mckenney@...aro.org>
> > 
> > Although rcu_yield() dropped from real-time to normal priority, there
> > is always the possibility that the competing tasks have been niced.
> > So nice to 19 in rcu_yield() to help ensure that other tasks have a
> > better chance of running.
> 
> But.. that just prolongs the pain of overhead you _have_ to eat, no?  In
> a brief surge, fine, you can spread the cost out.. but how do you know
> when it's ok to yield?

I modeled this code on the existing code in ksoftirqd.  But yes, this is
a heuristic.  I do believe that it is quite robust, but time will tell.

> (When maintenance threads worrying about their CPU usage is worrisome.)

Indeed.  But I am not introducing this, just moving the existing checking
from ksoftirqd.

So I believe that I am OK here.

							Thanx, Paul

> > Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> >  kernel/rcutree.c |    1 +
> >  1 files changed, 1 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 3295c7b..963b4b1 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -1561,6 +1561,7 @@ static void rcu_yield(void (*f)(unsigned long), unsigned long arg)
> >  	mod_timer(&yield_timer, jiffies + 2);
> >  	sp.sched_priority = 0;
> >  	sched_setscheduler_nocheck(current, SCHED_NORMAL, &sp);
> > +	set_user_nice(current, 19);
> >  	schedule();
> >  	sp.sched_priority = RCU_KTHREAD_PRIO;
> >  	sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
> 
> 
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