linux-kernel - [PATCH 1/3] sched: Enable wake balancing for the SMT/HT domain

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Date:	Sat, 24 Oct 2009 12:58:53 -0700
From:	Arjan van de Ven <arjan@...radead.org> (by way of Arjan van de
	Ven <arjan@...radead.org>)
To:	Peter Zijlstra <peterz@...radead.org>, mingo@...e.hu
Subject: [PATCH 1/3] sched: Enable wake balancing for the SMT/HT domain

Subject: sched: Enable wake balancing for the SMT/HT domain
From: Arjan van de Ven <arjan@...ux.intel.com>

Logical CPUs that are part of a hyperthreading/SMT set are equivalent
in terms of where to execute a task; after all they share pretty much
all resources including the L1 cache.

This means that if task A wakes up task B, we should really consider
all logical CPUs in the SMT/HT set to run task B, not just the CPU that
task A is running on; in case task A keeps running, task B now gets to 
execute with no latency. In the case where task A then immediately goes
to wait for a response from task B, nothing is lost due to the aforementioned
equivalency.

This patch turns on the "balance on wakup" and turns of "affine wakeups"
for the SMT/HT scheduler domain to get this lower latency behavior.

Signed-off-by: Arjan van de Ven <arjan@...ux.intel.com>

diff --git a/include/linux/topology.h b/include/linux/topology.h
index fc0bf3e..3665dc2 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -95,8 +95,8 @@ int arch_update_cpu_topology(void);
 				| 1*SD_BALANCE_NEWIDLE			\
 				| 1*SD_BALANCE_EXEC			\
 				| 1*SD_BALANCE_FORK			\
-				| 0*SD_BALANCE_WAKE			\
-				| 1*SD_WAKE_AFFINE			\
+				| 1*SD_BALANCE_WAKE			\
+				| 0*SD_WAKE_AFFINE			\
 				| 1*SD_SHARE_CPUPOWER			\
 				| 0*SD_POWERSAVINGS_BALANCE		\
 				| 0*SD_SHARE_PKG_RESOURCES		\

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
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