Re: [RFC PATCH v2 1/7] sched/fair: Add related data structure for task based throttle

[Aaron Lu]

On Mon, Apr 14, 2025 at 09:28:36AM +0530, K Prateek Nayak wrote:
> Hello Aaron,
> 
> On 4/9/2025 5:37 PM, Aaron Lu wrote:
> > From: Valentin Schneider <vschneid@xxxxxxxxxx>
> > 
> > Add related data structures for this new throttle functionality.
> > 
> > Signed-off-by: Valentin Schneider <vschneid@xxxxxxxxxx>
> > Signed-off-by: Aaron Lu <ziqianlu@xxxxxxxxxxxxx>
> > ---
> >   include/linux/sched.h |  4 ++++
> >   kernel/sched/core.c   |  3 +++
> >   kernel/sched/fair.c   | 12 ++++++++++++
> >   kernel/sched/sched.h  |  2 ++
> >   4 files changed, 21 insertions(+)
> > 
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index f96ac19828934..0b55c79fee209 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -880,6 +880,10 @@ struct task_struct {
> >   #ifdef CONFIG_CGROUP_SCHED
> >   	struct task_group		*sched_task_group;
> > +#ifdef CONFIG_CFS_BANDWIDTH
> > +	struct callback_head		sched_throttle_work;
> > +	struct list_head		throttle_node;
> 
> Since throttled tasks are fully dequeued before placing on the
> "throttled_limbo_list", is it possible to reuse "p->se.group_node"?

I think it might be possible.

> Currently, it is used to track the task on "rq->cfs_tasks" and during
> load-balancing when moving a bunch of tasks between CPUs but since a
> fully throttled task is not tracked by either, it should be safe to
> reuse this bit (CONFIG_DEBUG_LIST will scream if I'm wrong) and save
> up on some space in the  task_struct.
> 
> Thoughts?

Is it that adding throttle_node would cause task_struct to just cross a
cacheline boundary? :-)

Or it's mainly a concern that system could have many tasks and any saving
in task_struct is worth to try?

I can see reusing another field would cause task_is_throttled() more
obscure to digest and implement, but I think it is doable.

Thanks,
Aaron