linux-kernel - [tip:sched/core] sched: Fix asymmetric scheduling for POWER7

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Message-ID: <tip-2042abe7977222ef606306faa2dce8fd51e98e65@git.kernel.org>
Date:	Wed, 6 Nov 2013 05:20:09 -0800
From:	tip-bot for Vaidyanathan Srinivasan <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
	peterz@...radead.org, michael.neuling@....ibm.com,
	preeti@...ux.vnet.ibm.com, tglx@...utronix.de,
	svaidy@...ux.vnet.ibm.com, mikey@...ling.org
Subject: [tip:sched/core] sched: Fix asymmetric scheduling for POWER7

Commit-ID:  2042abe7977222ef606306faa2dce8fd51e98e65
Gitweb:     http://git.kernel.org/tip/2042abe7977222ef606306faa2dce8fd51e98e65
Author:     Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
AuthorDate: Wed, 30 Oct 2013 08:42:42 +0530
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 6 Nov 2013 12:37:54 +0100

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>
Tested-by: Michael Neuling <mikey@...ling.org>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Cc: vincent.guittot@...aro.org
Cc: bitbucket@...ine.de
Cc: benh@...nel.crashing.org
Cc: anton@...ba.org
Cc: Morten.Rasmussen@....com
Cc: pjt@...gle.com
Link: http://lkml.kernel.org/r/20131030031242.23426.13019.stgit@preeti.in.ibm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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 41c02b6..074551a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6800,7 +6800,7 @@ 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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