linux-kernel - Re: [PATCH 20/35] autonuma: avoid CFS select_task_rq

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Date:	Tue, 29 May 2012 16:02:53 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Hillf Danton <dhillf@...il.com>, Dan Smith <danms@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Bharata B Rao <bharata.rao@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH 20/35] autonuma: avoid CFS select_task_rq_fair to return
 -1

On Fri, 2012-05-25 at 19:02 +0200, Andrea Arcangeli wrote:
> Fix to avoid -1 retval.
> 
> Includes fixes from Hillf Danton <dhillf@...il.com>.

This changelog is very much insufficient. It fails to mention why your
solution is the right one or if there's something else wrong with that
code.

> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> ---
>  kernel/sched/fair.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 940e6d1..137119f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2789,6 +2789,9 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
>  		if (new_cpu == -1 || new_cpu == cpu) {
>  			/* Now try balancing at a lower domain level of cpu */
>  			sd = sd->child;
> +			if (new_cpu < 0)
> +				/* Return prev_cpu is find_idlest_cpu failed */
> +				new_cpu = prev_cpu;
>  			continue;
>  		}
>  
> @@ -2807,6 +2810,7 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
>  unlock:
>  	rcu_read_unlock();
>  
> +	BUG_ON(new_cpu < 0);
>  	return new_cpu;
>  }
>  #endif /* CONFIG_SMP */

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