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

[Mike Galbraith]

On Sun, 2011-05-01 at 06:21 -0700, Paul E. McKenney wrote:
> From: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>  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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