linux-kernel - [PATCH] tsacct: optimize acct_update

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Message-Id: <1372841481-23899-1-git-send-email-schwidefsky@de.ibm.com>
Date:	Wed,  3 Jul 2013 10:51:21 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Jonathan Lim <jlim@....com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: [PATCH] tsacct: optimize acct_update_integrals

The conversion of a cputime to micro seconds can be done without the
detour via jiffies. This avoids unnecessary and costly calculations,
e.g. on s390 a 64-bit division and a multiplication can be replaced
with a simple shift.

Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
---
 kernel/tsacct.c |    5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index a1dd9a1..bf09be1 100644
--- a/kernel/tsacct.c
+++ b/kernel/tsacct.c
@@ -126,16 +126,13 @@ static void __acct_update_integrals(struct task_struct *tsk,
 {
 	if (likely(tsk->mm)) {
 		cputime_t time, dtime;
-		struct timeval value;
 		unsigned long flags;
 		u64 delta;
 
 		local_irq_save(flags);
 		time = stime + utime;
 		dtime = time - tsk->acct_timexpd;
-		jiffies_to_timeval(cputime_to_jiffies(dtime), &value);
-		delta = value.tv_sec;
-		delta = delta * USEC_PER_SEC + value.tv_usec;
+		delta = cputime_to_usecs(dtime);
 
 		if (delta == 0)
 			goto out;
-- 
1.7.9.5

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