linux-kernel - [RFC PATCH] sched/cputime: drop local_irq_safe() in vtime_init

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Message-Id: <1460636954-20389-1-git-send-email-bigeasy@linutronix.de>
Date:	Thu, 14 Apr 2016 14:29:14 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [RFC PATCH] sched/cputime: drop local_irq_safe() in vtime_init_idle()

A while ago vtime_init_idle() used to invoke sched_clock_cpu() and had
write_seqlock_irqsave() because sched_clock_cpu() required to be called
with interrupts off.
This requirement is written before the body of the function and was
introduced in 2010 via c676329abb2b ("sched_clock: Add local_clock() API
and improve documentation"). This requirement has been dropped in 2013
via ef08f0fff876 ("sched/clock: Remove local_irq_disable() from the clocks")
but the body of the function still has the comment.

Now, vtime_init_idle() was converted from seqlock to seqcount via
b7ce2277f087 ("sched/cputime: Convert vtime_seqlock to seqcount") and
while doing so the IRQ-off region around sched_clock_cpu() was preserved
while not strictly required (given the first part of this commit).
A little later, sched_clock_cpu() was replaced with jiffies via
ff9a9b4c4334 ("sched, time: Switch VIRT_CPU_ACCOUNTING_GEN to jiffy
granularity").
Based on this events I assume it is safe to drop the local_irq_safe()
section.

Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Rik van Riel <riel@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/sched/cputime.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 75f98c5498d5..816cd4ef5873 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -785,14 +785,10 @@ void arch_vtime_task_switch(struct task_struct *prev)

 void vtime_init_idle(struct task_struct *t, int cpu)
 {
-	unsigned long flags;
-
-	local_irq_save(flags);
 	write_seqcount_begin(&t->vtime_seqcount);
 	t->vtime_snap_whence = VTIME_SYS;
 	t->vtime_snap = jiffies;
 	write_seqcount_end(&t->vtime_seqcount);
-	local_irq_restore(flags);
 }

 cputime_t task_gtime(struct task_struct *t)
-- 
2.8.0.rc3