Re: [git] CFS-devel, latest code

Srivatsa Vaddagiri
2007-09-25T14:50:18Z

On Tue, Sep 25, 2007 at 01:33:06PM +0200, Ingo Molnar wrote:
> > hm. perhaps this fixup in kernel/sched.c:set_task_cpu():
> >
> > p->se.vruntime -= old_rq->cfs.min_vruntime - new_rq->cfs.min_vruntime;
> >
> > needs to become properly group-hierarchy aware?

You seem to have hit the nerve for this problem. The two patches I sent:

http://lkml.org/lkml/2007/9/25/117
http://lkml.org/lkml/2007/9/25/168

partly help, but we can do better.

> ===================================================================
> --- linux.orig/kernel/sched.c
> +++ linux/kernel/sched.c
> @@ -1039,7 +1039,8 @@ void set_task_cpu(struct task_struct *p,
> {
> int old_cpu = task_cpu(p);
> struct rq *old_rq = cpu_rq(old_cpu), *new_rq = cpu_rq(new_cpu);
> - u64 clock_offset;
> + struct sched_entity *se;
> + u64 clock_offset, voffset;
>
> clock_offset = old_rq->clock - new_rq->clock;
>
> @@ -1051,7 +1052,11 @@ void set_task_cpu(struct task_struct *p,
> if (p->se.block_start)
> p->se.block_start -= clock_offset;
> #endif
> - p->se.vruntime -= old_rq->cfs.min_vruntime - new_rq->cfs.min_vruntime;
> +
> + se = &p->se;
> + voffset = old_rq->cfs.min_vruntime - new_rq->cfs.min_vruntime;

This one feels wrong, although I can't express my reaction correctly ..

> + for_each_sched_entity(se)
> + se->vruntime -= voffset;

Note that parent entities for a task is per-cpu. So if a task A
belonging to userid guest hops from CPU0 to CPU1, then it gets a new parent
entity as well, which is different from its parent entity on CPU0.

Before:
taskA->se.parent = guest's tg->se[0]

After:
taskA->se.parent = guest's tg->se[1]

So walking up the entity hierarchy and fixing up (parent)se->vruntime will do
little good after the task has moved to a new cpu.

IMO, we need to be doing this :

- For dequeue of higher level sched entities, simulate as if
they are going to "sleep"
- For enqueue of higher level entities, simulate as if they are
"waking up". This will cause enqueue_entity() to reset their
vruntime (to existing value for cfs_rq->min_vruntime) when they
"wakeup".

If we don't do this, then lets say a group had only one task (A) and it
moves from CPU0 to CPU1. Then on CPU1, when group level entity for task
A is enqueued, it will have a very low vruntime (since it was never
running) and this will give task A unlimited cpu time, until its group
entity catches up with all the "sleep" time.

Let me try a fix for this next ..

--
Regards,
vatsa