linux-kernel - [patch] fix the softlockup watchdog to actually work

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Date:	Tue, 17 Jul 2007 17:49:34 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	stable@...nel.org, Greg KH <greg@...ah.com>,
	Chris Wright <chrisw@...s-sol.org>
Subject: [patch] fix the softlockup watchdog to actually work


* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> Ingo Molnar wrote:
> > Subject: softlockup: fix Xen bogosity
> > From: Ingo Molnar <mingo@...e.hu>
> >
> > this Xen related commit:
> >   
> 
> Well, not just Xen.  It relates to any virtual environment: kvm, 
> lguest, vmi, xen...  (Not that they all implement a measure of 
> unstolen time.)
> 
> How about a more descriptive patch title, along the lines of 
> "softlockup watchdog: fix rate limiting"?

uhm, the problem was that it did not work _at all_, not something about 
'rate limiting'. Yes, i got quite a bit grumpy when i found this, 
because you completely broke the softlockup watchdog via a pretty 
intrusive commit and you apparently didnt even do a minimal check 
whether its functionality was preserved! Updated patch for Andrew/Linus 
and for -stable attached.

	Ingo

----------------------------->
Subject: fix the softlockup watchdog to actually work
From: Ingo Molnar <mingo@...e.hu>

this Xen related commit:

   commit 966812dc98e6a7fcdf759cbfa0efab77500a8868
   Author: Jeremy Fitzhardinge <jeremy@...p.org>
   Date:   Tue May 8 00:28:02 2007 -0700

       Ignore stolen time in the softlockup watchdog

broke the softlockup watchdog to never report any lockups. (!)

print_timestamp defaults to 0, this makes the following condition
always true:

	if (print_timestamp < (touch_timestamp + 1) ||

and we'll in essence never report soft lockups.

apparently the functionality of the soft lockup watchdog was never
actually tested with that patch applied ...

[this is -stable material too.]

Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 kernel/softlockup.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

Index: linux/kernel/softlockup.c
===================================================================
--- linux.orig/kernel/softlockup.c
+++ linux/kernel/softlockup.c
@@ -79,10 +79,11 @@ void softlockup_tick(void)
 	print_timestamp = per_cpu(print_timestamp, this_cpu);
 
 	/* report at most once a second */
-	if (print_timestamp < (touch_timestamp + 1) ||
-		did_panic ||
-			!per_cpu(watchdog_task, this_cpu))
+	if ((print_timestamp >= touch_timestamp &&
+			print_timestamp < (touch_timestamp + 1)) ||
+			did_panic || !per_cpu(watchdog_task, this_cpu)) {
 		return;
+	}
 
 	/* do not print during early bootup: */
 	if (unlikely(system_state != SYSTEM_RUNNING)) {
-
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