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SubjectRe: [PATCH 2/3] sched: Fix asymmetric scheduling for POWER7
Date
Vaidyanathan Srinivasan <svaidy@linux.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@au1.ibm.com>

Tested-by: Michael Neuling <mikey@neuling.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@linux.vnet.ibm.com>
> Signed-off-by: Preeti U Murthy <preeti@linux.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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