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

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.

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(-)

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?

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

> 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);

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);
> 
>

On Mon, 2011-05-02 at 01:11 -0700, Paul E. McKenney wrote:
> 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.

(It probably is fine, but when I see 'yield', alarms and sirens go off)