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

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Date:	Mon, 21 Oct 2013 17:14:52 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <bitbucket@...ine.de>,
	Paul Turner <pjt@...gle.com>, Ingo Molnar <mingo@...nel.org>
Cc:	Michael Neuling <mikey@...ling.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: [PATCH 2/3] sched: Fix asymmetric scheduling for POWER7

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>
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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