linux-kernel - [PATCH 8/8] Call tick_check_idle before __irq

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Date:	Mon,  4 Oct 2010 17:03:23 -0700
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, Paul Turner <pjt@...gle.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Venkatesh Pallipadi <venki@...gle.com>
Subject: [PATCH 8/8] 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.

Signed-off-by: Venkatesh Pallipadi <venki@...gle.com>
---
 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 6487da4..442e151 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1983,8 +1983,8 @@ 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..6bb7e19 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
-- 
1.7.1

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