linux-kernel - [PATCH] hrtimers: Special-case zero length sleeps

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Date:	Thu, 29 Sep 2011 10:59:32 -0400
From:	Matthew Garrett <mjg@...hat.com>
To:	tglx@...utronix.de
Cc:	linux-kernel@...r.kernel.org, arjan@...radead.org,
	Matthew Garrett <mjg@...hat.com>
Subject: [PATCH] hrtimers: Special-case zero length sleeps

sleep(0) is a common construct used by applications that want to trigger
the scheduler. sched_yield() might make more sense, but only appeared in
POSIX.1-2001 and so plenty of example code still uses the sleep(0) form.
This wouldn't normally be a problem, but it means that event-driven
applications that are merely trying to avoid starving other processes may
actually end up sleeping due to having large timer_slack values. Special-
casing this seems reasonable.

Signed-off-by: Matthew Garrett <mjg@...hat.com>
---
 kernel/hrtimer.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index a9205e3..0bb70a7 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -1566,6 +1566,14 @@ long hrtimer_nanosleep(struct timespec *rqtp, struct timespec __user *rmtp,
 	if (rt_task(current))
 		slack = 0;
 
+	/*
+	 * Applications will often sleep(0) to indicate that they wish to
+	 * be scheduled. Special case that to avoid actually putting them
+	 * to sleep for the duration of the slack.
+	 */
+	if (rqtp->tv_sec == 0 && rqtp->tv_nsec == 0)
+		slack = 0;
+
 	hrtimer_init_on_stack(&t.timer, clockid, mode);
 	hrtimer_set_expires_range_ns(&t.timer, timespec_to_ktime(*rqtp), slack);
 	if (do_nanosleep(&t, mode))
-- 
1.7.6.4

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