Re: [PATCH tip/core/rcu 31/86] rcu: further lower priority in rcu_yield()

Paul E. McKenney
2011-05-02T08:11:31Z

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@linaro.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@linaro.org>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.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);
>
>