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

From: John
Date: Thu Feb 22 2007 - 11:49:08 EST

Thomas Gleixner wrote:

Problem description at:

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@xxxxxxxxxxxxx>

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->curr = next;


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at