linux-kernel - Re: [2.6.24.x] UML select()/poll() oversleeping reproducibly (was Re: [uml-devel] g_timeout

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Message-ID: <20080317170328.GB9536@c2.user-mode-linux.org>
Date:	Mon, 17 Mar 2008 13:03:28 -0400
From:	Jeff Dike <jdike@...toit.com>
To:	Nix <nix@...eri.org.uk>
Cc:	clowncoder <vincent-perrier@...b-internet.fr>,
	user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...esys.com>
Subject: Re: [2.6.24.x] UML select()/poll() oversleeping reproducibly (was
	Re: [uml-devel] g_timeout_add)

On Fri, Mar 14, 2008 at 11:01:35PM +0000, Nix wrote:
> bash-3.2# ./select-sleep 10
> Slept for 13 seconds.

See what kind of difference the patch below makes - it reduces the 30%
oversleeping down to 10% for me.  That's still way too much, but it's
better.

The problem being fixed here is that setitimer consistently returns a
remaining time greater than what was originally requested, by ~20%:

1205773032.413780 setitimer(ITIMER_VIRTUAL, {it_interval={0, 10000}, it_value={0, 10000}}, NULL) = 0
1205773032.413814 setitimer(ITIMER_VIRTUAL, {it_interval={0, 0},
it_value={0, 0}}, {it_interval={0, 10998}, it_value={0, 11998}}) = 0

This is setting a 100 HZ timer, immediately followed by a cancellation
which also requests the amount left on the timer.  The interval is
rounded up by 10%, and the first tick by 20%.

	       	       	    Jeff

-- 
Work email - jdike at linux dot intel dot com

Index: linux-2.6.22/arch/um/os-Linux/time.c
===================================================================
--- linux-2.6.22.orig/arch/um/os-Linux/time.c	2008-02-18 11:53:51.000000000 -0500
+++ linux-2.6.22/arch/um/os-Linux/time.c	2008-03-17 12:55:41.000000000 -0400
@@ -58,12 +58,17 @@ static inline long long timeval_to_ns(co
 long long disable_timer(void)
 {
 	struct itimerval time = ((struct itimerval) { { 0, 0 }, { 0, 0 } });
+	int remain, max = UM_NSEC_PER_SEC / UM_HZ;
 
 	if (setitimer(ITIMER_VIRTUAL, &time, &time) < 0)
 		printk(UM_KERN_ERR "disable_timer - setitimer failed, "
 		       "errno = %d\n", errno);
 
-	return timeval_to_ns(&time.it_value);
+	remain = timeval_to_ns(&time.it_value);
+	if (remain > max)
+		remain = max;
+
+	return remain;
 }
 
 long long os_nsecs(void)
--
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