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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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