linux-kernel - [PATCH 4/9] select: Micro-optimise __estimate

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Date:   Mon,  9 Sep 2019 11:23:35 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@...il.com>,
        Dmitry Safonov <dima@...sta.com>,
        Adrian Reber <adrian@...as.de>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrei Vagin <avagin@...nvz.org>,
        Andy Lutomirski <luto@...nel.org>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Ingo Molnar <mingo@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        containers@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org
Subject: [PATCH 4/9] select: Micro-optimise __estimate_accuracy()

Shift on s64 is faster than division, use it instead.

As the result of the patch there is a hardly user-visible effect:
poll(), select(), etc syscalls will be a bit more precise on ~2.3%
than before because 1000 != 1024 :)

Signed-off-by: Dmitry Safonov <dima@...sta.com>
---
 fs/select.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/fs/select.c b/fs/select.c
index 12cdefd3be2d..2477c202631e 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -51,15 +51,14 @@
 
 static long __estimate_accuracy(ktime_t slack)
 {
-	int divfactor = 1000;
-
 	if (slack < 0)
 		return 0;
 
-	if (task_nice(current) > 0)
-		divfactor = divfactor / 5;
+	/* A bit more precise than 0.1% */
+	slack = slack >> 10;
 
-	slack = ktime_divns(slack, divfactor);
+	if (task_nice(current) > 0)
+		slack = slack * 5;
 
 	if (slack > MAX_SLACK)
 		return MAX_SLACK;
-- 
2.23.0