linux-kernel - Re: [patch] CFS scheduler, -v19

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Message-ID: <20070717204358.GA12037@elte.hu>
Date:	Tue, 17 Jul 2007 22:43:58 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Markus <lists4me@....de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [patch] CFS scheduler, -v19


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> But why does that happen? And why would the scheduler have *anything* 
> to do with this? No idea. Maybe timing. Maybe the time.c changes. 
> Dunno.

hm, Markus indicated that he tried the v2.6.21.6-cfsv19 patch, and that 
does not include the time.c change. Markus - does your kernel include 
the code below? (if yes, please revert it via patch -p1 -R )

	Ingo

Index: linux/kernel/time.c
===================================================================
--- linux.orig/kernel/time.c
+++ linux/kernel/time.c
@@ -57,14 +57,17 @@ EXPORT_SYMBOL(sys_tz);
  */
 asmlinkage long sys_time(time_t __user * tloc)
 {
-	time_t i;
-	struct timeval tv;
+	/*
+	 * We read xtime.tv_sec atomically - it's updated
+	 * atomically by update_wall_time(), so no need to
+	 * even read-lock the xtime seqlock:
+	 */
+	time_t i = xtime.tv_sec;
 
-	do_gettimeofday(&tv);
-	i = tv.tv_sec;
+	smp_rmb(); /* sys_time() results are coherent */
 
 	if (tloc) {
-		if (put_user(i,tloc))
+		if (put_user(i, tloc))
 			i = -EFAULT;
 	}
 	return i;
@@ -373,6 +376,20 @@ void do_gettimeofday (struct timeval *tv
 
 	tv->tv_sec = sec;
 	tv->tv_usec = usec;
+
+	/*
+	 * Make sure xtime.tv_sec [returned by sys_time()] always
+	 * follows the gettimeofday() result precisely. This
+	 * condition is extremely unlikely, it can hit at most
+	 * once per second:
+	 */
+	if (unlikely(xtime.tv_sec != tv->tv_sec)) {
+		unsigned long flags;
+
+		write_seqlock_irqsave(&xtime_lock);
+		update_wall_time();
+		write_seqlock_irqrestore(&xtime_lock);
+	}
 }
 
 EXPORT_SYMBOL(do_gettimeofday);
-
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