[PATCH] Fix posix-cpu-timer breakage caused by stale p->last_ran value

[PATCH] Fix posix-cpu-timer breakage caused by stale p->last_ran value

[Thomas Gleixner]

Problem description at:
http://bugzilla.kernel.org/show_bug.cgi?id=8048

Commit b18ec80396834497933d77b81ec0918519f4e2a7 
    [PATCH] sched: improve migration accuracy
optimized the scheduler time calculations, but broke posix-cpu-timers.

The problem is that the p->last_ran value is not updated after a context
switch. So a subsequent call to current_sched_time() calculates with a
stale p->last_ran value, i.e. accounts the full time, which the task was
scheduled away.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -3566,7 +3566,7 @@ switch_tasks:
 
 	sched_info_switch(prev, next);
 	if (likely(prev != next)) {
-		next->timestamp = now;
+		next->timestamp = next->last_ran = now;
 		rq->nr_switches++;
 		rq->curr = next;
 		++*switch_count;

Re: [PATCH] Fix posix-cpu-timer breakage caused by stale p->last_ran value

[Ingo Molnar]

* Thomas Gleixner <tglx@linutronix.de> wrote:

> The problem is that the p->last_ran value is not updated after a 
> context switch. So a subsequent call to current_sched_time() 
> calculates with a stale p->last_ran value, i.e. accounts the full 
> time, which the task was scheduled away.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

>  	sched_info_switch(prev, next);
>  	if (likely(prev != next)) {
> -		next->timestamp = now;
> +		next->timestamp = next->last_ran = now;

ouch! nice catch. Also for v2.6.20.2 i think. 2.6.19 should be 
unaffected.

Acked-by: Ingo Molnar <mingo@elte.hu>

	Ingo

Re: [PATCH] Fix posix-cpu-timer breakage caused by stale p->last_ran value

[Thomas Gleixner]

On Thu, 2007-02-22 at 08:46 +0100, Ingo Molnar wrote:
> * Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> > The problem is that the p->last_ran value is not updated after a 
> > context switch. So a subsequent call to current_sched_time() 
> > calculates with a stale p->last_ran value, i.e. accounts the full 
> > time, which the task was scheduled away.
> > 
> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> 
> >  	sched_info_switch(prev, next);
> >  	if (likely(prev != next)) {
> > -		next->timestamp = now;
> > +		next->timestamp = next->last_ran = now;
> 
> ouch! nice catch. Also for v2.6.20.2 i think. 2.6.19 should be 
> unaffected.

Yes, was introduced in 2.6.20 and definitely should hit the stable tree.

	tglx

Re: [PATCH] Fix posix-cpu-timer breakage caused by stale p->last_ran value

[Mike Galbraith]

On Thu, 2007-02-22 at 01:33 +0100, Thomas Gleixner wrote:
> Problem description at:
> http://bugzilla.kernel.org/show_bug.cgi?id=8048
> 
> Commit b18ec80396834497933d77b81ec0918519f4e2a7 
>     [PATCH] sched: improve migration accuracy
> optimized the scheduler time calculations, but broke posix-cpu-timers.
> 
> The problem is that the p->last_ran value is not updated after a context
> switch. So a subsequent call to current_sched_time() calculates with a
> stale p->last_ran value, i.e. accounts the full time, which the task was
> scheduled away.

Oops, missed that.  You could also remove the prev->last_ran assignment
just above your addition, and turn this into a negative cost bugfix :)

	-Mike

Re: [PATCH] Fix posix-cpu-timer breakage caused by stale p->last_ran value

[Chuck Ebbert]

Thomas Gleixner wrote:
> Problem description at:
> http://bugzilla.kernel.org/show_bug.cgi?id=8048
> 
> Commit b18ec80396834497933d77b81ec0918519f4e2a7 
>     [PATCH] sched: improve migration accuracy
> optimized the scheduler time calculations, but broke posix-cpu-timers.
> 
> The problem is that the p->last_ran value is not updated after a context
> switch. So a subsequent call to current_sched_time() calculates with a
> stale p->last_ran value, i.e. accounts the full time, which the task was
> scheduled away.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> 
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -3566,7 +3566,7 @@ switch_tasks:
>  
>  	sched_info_switch(prev, next);
>  	if (likely(prev != next)) {
> -		next->timestamp = now;
> +		next->timestamp = next->last_ran = now;
>  		rq->nr_switches++;
>  		rq->curr = next;
>  		++*switch_count;
> 

Is this going to be merged or not??

Re: [PATCH] Fix posix-cpu-timer breakage caused by stale p->last_ran value

[Andrew Morton]

On Thu, 01 Mar 2007 18:45:14 -0500
Chuck Ebbert <cebbert@redhat.com> wrote:

> Thomas Gleixner wrote:
> > Problem description at:
> > http://bugzilla.kernel.org/show_bug.cgi?id=8048
> > 
> > Commit b18ec80396834497933d77b81ec0918519f4e2a7 
> >     [PATCH] sched: improve migration accuracy
> > optimized the scheduler time calculations, but broke posix-cpu-timers.
> > 
> > The problem is that the p->last_ran value is not updated after a context
> > switch. So a subsequent call to current_sched_time() calculates with a
> > stale p->last_ran value, i.e. accounts the full time, which the task was
> > scheduled away.
> > 
> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> > 
> > Index: linux-2.6/kernel/sched.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/sched.c
> > +++ linux-2.6/kernel/sched.c
> > @@ -3566,7 +3566,7 @@ switch_tasks:
> >  
> >  	sched_info_switch(prev, next);
> >  	if (likely(prev != next)) {
> > -		next->timestamp = now;
> > +		next->timestamp = next->last_ran = now;
> >  		rq->nr_switches++;
> >  		rq->curr = next;
> >  		++*switch_count;
> > 
> 
> Is this going to be merged or not??

It just hit mainline.

Re: [PATCH] Fix posix-cpu-timer breakage caused by stale p->last_ran value

[John]

Thomas Gleixner wrote:

> Problem description at:
> http://bugzilla.kernel.org/show_bug.cgi?id=8048
> 
> Commit b18ec80396834497933d77b81ec0918519f4e2a7 
>     [PATCH] sched: improve migration accuracy
> optimized the scheduler time calculations, but broke posix-cpu-timers.
> 
> The problem is that the p->last_ran value is not updated after a context
> switch. So a subsequent call to current_sched_time() calculates with a
> stale p->last_ran value, i.e. accounts the full time, which the task was
> scheduled away.

Could you expand on the impact of this bug for non-kernel hackers? :-)

I'm currently testing 2.6.20-rt5. My app runs in SCHED_RR and just 
blocks waiting for several signals. Since my app is the only SCHED_RR 
process running on the system, I get the impression that I'm not 
affected by this bug. Is that correct?

> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> 
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -3566,7 +3566,7 @@ switch_tasks:
>  
>  	sched_info_switch(prev, next);
>  	if (likely(prev != next)) {
> -		next->timestamp = now;
> +		next->timestamp = next->last_ran = now;
>  		rq->nr_switches++;
>  		rq->curr = next;
>  		++*switch_count;

Regards,

John