linux-kernel - [tip:sched/core] sched: Call tick_check_idle before __irq

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Date:	Mon, 18 Oct 2010 19:27:13 GMT
From:	tip-bot for Venkatesh Pallipadi <venki@...gle.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...hat.com,
	a.p.zijlstra@...llo.nl, tglx@...utronix.de, yong.zhang0@...il.com,
	venki@...gle.com, mingo@...e.hu
Subject: [tip:sched/core] sched: Call tick_check_idle before __irq_enter

Commit-ID:  d267f87fb8179c6dba03d08b91952e81bc3723c7
Gitweb:     http://git.kernel.org/tip/d267f87fb8179c6dba03d08b91952e81bc3723c7
Author:     Venkatesh Pallipadi <venki@...gle.com>
AuthorDate: Mon, 4 Oct 2010 17:03:23 -0700
Committer:  Ingo Molnar <mingo@...e.hu>
CommitDate: Mon, 18 Oct 2010 20:52:29 +0200

sched: Call tick_check_idle before __irq_enter

When CPU is idle and on first interrupt, irq_enter calls tick_check_idle()
to notify interruption from idle. But, there is a problem if this call
is done after __irq_enter, as all routines in __irq_enter may find
stale time due to yet to be done tick_check_idle.

Specifically, trace calls in __irq_enter when they use global clock and also
account_system_vtime change in this patch as it wants to use sched_clock_cpu()
to do proper irq timing.

But, tick_check_idle was moved after __irq_enter intentionally to
prevent problem of unneeded ksoftirqd wakeups by the commit ee5f80a:

    irq: call __irq_enter() before calling the tick_idle_check
    Impact: avoid spurious ksoftirqd wakeups

Moving tick_check_idle() before __irq_enter and wrapping it with
local_bh_enable/disable would solve both the problems.

Fixed-by: Yong Zhang <yong.zhang0@...il.com>
Signed-off-by: Venkatesh Pallipadi <venki@...gle.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
LKML-Reference: <1286237003-12406-9-git-send-email-venki@...gle.com>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 kernel/sched.c   |    2 +-
 kernel/softirq.c |   12 +++++++++---
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index bff9ef5..567f5cb 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1974,8 +1974,8 @@ void account_system_vtime(struct task_struct *curr)
 
 	local_irq_save(flags);
 
-	now = sched_clock();
 	cpu = smp_processor_id();
+	now = sched_clock_cpu(cpu);
 	delta = now - per_cpu(irq_start_time, cpu);
 	per_cpu(irq_start_time, cpu) = now;
 	/*
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 267f7b7..79ee8f1 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -296,10 +296,16 @@ void irq_enter(void)
 
 	rcu_irq_enter();
 	if (idle_cpu(cpu) && !in_interrupt()) {
-		__irq_enter();
+		/*
+		 * Prevent raise_softirq from needlessly waking up ksoftirqd
+		 * here, as softirq will be serviced on return from interrupt.
+		 */
+		local_bh_disable();
 		tick_check_idle(cpu);
-	} else
-		__irq_enter();
+		_local_bh_enable();
+	}
+
+	__irq_enter();
 }
 
 #ifdef __ARCH_IRQ_EXIT_IRQS_DISABLED
--
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