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@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index 422e567eecf63636a7c349b6319e6d541c6f3818..5e30317b7c68d7134783b8e4b0204d5a1a7729c1 100644 (file)
--- 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))