linux-kernel - [PATCH] nohz: Fix ondemand governor not always ramping up on 100% load.

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Date:	Fri, 29 May 2009 22:00:19 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org
Cc:	swetland@...gle.com,
	Arve Hjønnevåg <arve@...roid.com>
Subject: [PATCH] nohz: Fix ondemand governor not always ramping up on 100% load.

On systems that perform the context switch with interrupts enabled
(e.g. ARM) get_cpu_idle_time_us could get stuck returning the last
idle time when the cpu is busy. If an interrupt occurs while
switching to the idle thread, and this interrupt wakes up a thread,
need-resched will be set on the idle thread, but since we have not
finished switching to the idle thread tick_nohz_stop_sched_tick will
be called. (idle_cpu is true since the head of the runqueue is the
idle thread, but need_resched returns false since we are still
running in the context of the old thread)

Fix this by not calling tick_nohz_start_idle when returning from an
interrupt while the idle thread is in its schedule phase.
(!ts->inidle)

Signed-off-by: Arve Hjønnevåg <arve@...roid.com>
---
 kernel/time/tick-sched.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index d3f1ef4..4012da6 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -222,7 +222,6 @@ void tick_nohz_stop_sched_tick(int inidle)

 	cpu = smp_processor_id();
 	ts = &per_cpu(tick_cpu_sched, cpu);
-	now = tick_nohz_start_idle(ts);

 	/*
 	 * If this cpu is offline and it is the one which updates
@@ -242,6 +241,7 @@ void tick_nohz_stop_sched_tick(int inidle)
 	if (!inidle && !ts->inidle)
 		goto end;

+	now = tick_nohz_start_idle(ts);
 	ts->inidle = 1;

 	if (need_resched())
-- 
1.6.1

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