Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]

Ingo Molnar
2007-04-15T19:21:12Z

* Willy Tarreau <w@1wt.eu> wrote:

> > to debug this, could you try to apply this add-on as well:
> >
> > http://redhat.com/~mingo/cfs-scheduler/sched-fair-print.patch
> >
> > with this patch applied you should have a /proc/sched_debug file
> > that prints all runnable tasks and other interesting info from the
> > runqueue.
>
> I don't know if you have seen my mail from yesterday evening (here). I
> found that changing keventd prio fixed the problem. You may be
> interested in the description. I sent it at 21:01 (+200).

ah, indeed i missed that mail - the response to the patches was quite
overwhelming (and i naively thought people dont do Linux hacking over
the weekends anymore ;).

so Linus was right: this was caused by scheduler starvation. I can see
one immediate problem already: the 'nice offset' is not divided by
nr_running as it should. The patch below should fix this but i have yet
to test it accurately, this change might as well render nice levels
unacceptably ineffective under high loads.

Ingo

--------->
---
kernel/sched_fair.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

Index: linux/kernel/sched_fair.c
===================================================================
--- linux.orig/kernel/sched_fair.c
+++ linux/kernel/sched_fair.c
@@ -31,7 +31,9 @@ static void __enqueue_task_fair(struct r
int leftmost = 1;
long long key;

- key = rq->fair_clock - p->wait_runtime + p->nice_offset;
+ key = rq->fair_clock - p->wait_runtime;
+ if (unlikely(p->nice_offset))
+ key += p->nice_offset / rq->nr_running;

p->fair_key = key;