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

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Message-ID: <1304272264.7417.20.camel@marge.simson.net>
Date:	Sun, 01 May 2011 19:51:04 +0200
From:	Mike Galbraith <efault@....de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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, 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?

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

> 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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