linux-kernel - Re: [tip:sched/core] sched: Fix load avg vs cpu-hotplug

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Message-ID: <20120905233951.GX3308@linux.vnet.ibm.com>
Date:	Wed, 5 Sep 2012 16:39:51 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>, hpa@...or.com,
	linux-kernel@...r.kernel.org, rakib.mullick@...il.com,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched: Fix load avg vs cpu-hotplug

On Thu, Sep 06, 2012 at 12:03:50AM +0200, Peter Zijlstra wrote:
> On Wed, 2012-09-05 at 19:01 +0200, Peter Zijlstra wrote:
> > > Please do a delta.
> 
> OK, so I suppose something like the below ought to do. Paul its slightly
> different than the one in your tree, given the changelog below, do you
> see anything wrong with it?
> 
> Rakib, again, sorry for getting your name wrong, and this time for
> getting it merged :/
> 
> ---
> Subject: sched: Fix load avg vs cpu-hotplug mk-II
> 
> Commit f319da0c68 ("sched: Fix load avg vs cpu-hotplug") was a known
> broken version that got in by accident.
> 
> In particular, the problem is that at the point it calls
> calc_load_migrate() nr_running := 1 (the stopper thread), so move the
> call to CPU_DEAD where we're sure that nr_running := 0.
> 
> Also note that we can call calc_load_migrate() without serialization, we
> know the state of rq is stable since its cpu is dead, and we modify the
> global state using appropriate atomic ops. 
> 
> Suggested-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>

Given your point about atomic ops, my version was indeed overkill.

Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> ---
>  kernel/sched/core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c46a011..8c089cb 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5086,7 +5086,9 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
>  		migrate_tasks(cpu);
>  		BUG_ON(rq->nr_running != 1); /* the migration thread */
>  		raw_spin_unlock_irqrestore(&rq->lock, flags);
> +		break;
>  
> +	case CPU_DEAD:
>  		calc_load_migrate(rq);
>  		break;
>  #endif
> 

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