linux-kernel - [PATCH 3/4] time,acct: drop irq save & restore from __acct_update

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Message-Id: <1454292751-23472-4-git-send-email-riel@redhat.com>
Date:	Sun, 31 Jan 2016 21:12:30 -0500
From:	riel@...hat.com
To:	linux-kernel@...r.kernel.org
Cc:	fweisbec@...il.com, tglx@...utronix.de, mingo@...nel.org,
	luto@...capital.net, peterz@...radead.org, clark@...hat.com
Subject: [PATCH 3/4] time,acct: drop irq save & restore from __acct_update_integrals

From: Rik van Riel <riel@...hat.com>

It looks like all the call paths that lead to __acct_update_integrals
already have irqs disabled, and __acct_update_integrals does not need
to disable irqs itself.

This is very convenient since about half the CPU time left in this
function was spent in local_irq_save alone.

Performance of a microbenchmark that calls an invalid syscall
ten million times in a row on a nohz_full CPU improves 21% vs.
4.5-rc1 with both the removal of divisions from __acct_update_integrals
and this patch, with runtime dropping from 3.7 to 2.9 seconds.

With these patches applied, the highest remaining cpu user in
the trace is native_sched_clock, which is addressed in the next
patch.

Suggested-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Rik van Riel <riel@...hat.com>
---
 kernel/tsacct.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index 9c23584c76c4..31fb6c9746d4 100644
--- a/kernel/tsacct.c
+++ b/kernel/tsacct.c
@@ -124,20 +124,18 @@ static void __acct_update_integrals(struct task_struct *tsk,
 				    cputime_t utime, cputime_t stime)
 {
 	cputime_t time, dtime;
-	unsigned long flags;
 	u64 delta;

 	if (!likely(tsk->mm))
 		return;

-	local_irq_save(flags);
 	time = stime + utime;
 	dtime = time - tsk->acct_timexpd;
 	/* Avoid division: cputime_t is often in nanoseconds already. */
 	delta = cputime_to_nsecs(dtime);

 	if (delta < TICK_NSEC)
-		goto out;
+		return;

 	tsk->acct_timexpd = time;
 	/*
@@ -147,8 +145,6 @@ static void __acct_update_integrals(struct task_struct *tsk,
 	 */
 	tsk->acct_rss_mem1 += delta * get_mm_rss(tsk->mm) >> 10;
 	tsk->acct_vm_mem1 += delta * tsk->mm->total_vm >> 10;
-out:
-	local_irq_restore(flags);
 }

 /**
-- 
2.5.0