[PATCH V2 1/2] sched: Fix asymmetric scheduling for POWER7

From: Preeti U Murthy
Date: Tue Oct 29 2013 - 23:15:53 EST

From: Vaidyanathan Srinivasan <svaidy@xxxxxxxxxxxxxxxxxx>

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@xxxxxxxxxxx>
Signed-off-by: Vaidyanathan Srinivasan <svaidy@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx>
Tested-by: Michael Neuling <mikey@xxxxxxxxxxx>

kernel/sched/fair.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 813dd61..e9c9549 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6781,7 +6781,7 @@ static inline int nohz_kick_needed(struct rq *rq, int cpu)
if (sd->flags & SD_SHARE_PKG_RESOURCES && nr_busy > 1)
goto need_kick_unlock;

- if (sd->flags & SD_ASYM_PACKING && nr_busy != sg->group_weight
+ 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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