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

[John <linux.kernel@free.fr>]

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