linux-kernel - Re: [PATCH 2/3] sched: Fix asymmetric scheduling for POWER7

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Date:	Tue, 22 Oct 2013 09:55:10 +1100
From:	Michael Neuling <michael.neuling@....ibm.com>
To:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <bitbucket@...ine.de>,
	Paul Turner <pjt@...gle.com>, Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-kernel@...r.kernel.org, Anton Blanchard <anton@...ba.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 2/3] sched: Fix asymmetric scheduling for POWER7

Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com> wrote:

> Asymmetric scheduling within a core is a scheduler loadbalancing
> feature that is triggered when SD_ASYM_PACKING flag is set.  The goal
> for the load balancer is to move tasks to lower order idle SMT threads
> within a core on a POWER7 system.
> 
> In nohz_kick_needed(), we intend to check if our sched domain (core)
> is completely busy or we have idle cpu.
> 
> The following check for SD_ASYM_PACKING:
> 
>     (cpumask_first_and(nohz.idle_cpus_mask, sched_domain_span(sd)) < cpu)
> 
> already covers the case of checking if the domain has an idle cpu,
> because cpumask_first_and() will not yield any set bits if this domain
> has no idle cpu.
> 
> Hence, nr_busy check against group weight can be removed.
> 
> Reported-by: Michael Neuling <michael.neuling@....ibm.com>

Tested-by: Michael Neuling <mikey@...ling.org>

Peter, I tested this only a brief while back but it turned out my test
wasn't stringent enough and it was actually broken (in v3.9).  This
fixes it.

Mikey

> Signed-off-by: Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
> Signed-off-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
> ---
>  kernel/sched/fair.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 12f0eab..828ed97 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5821,8 +5821,8 @@ static inline int nohz_kick_needed(struct rq *rq, int cpu)
>  				goto need_kick_unlock;
>  		}
>  
> -		if (sd->flags & SD_ASYM_PACKING && nr_busy != sg->group_weight
> -		    && (cpumask_first_and(nohz.idle_cpus_mask,
> +		if (sd->flags & SD_ASYM_PACKING &&
> +			(cpumask_first_and(nohz.idle_cpus_mask,
>  					  sched_domain_span(sd)) < cpu))
>  			goto need_kick_unlock;
>  
> 
--
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