Re: [PATCH 07/16] sched_ext: Add a DL server for sched_ext tasks

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Sep 03, 2025 at 10:41:21AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Wed, Sep 03, 2025 at 10:08:22PM +0200, Peter Zijlstra wrote:
> > > I'm a bit confused. This series doesn't have prep patches to add @rf to
> > > dl_server_pick_f. Is this the right patch?
> > 
> > Patch 14 seems to be the proposed alternative, and I'm not liking that
> > at all.
> > 
> > That rf passing was very much also needed for that other issue; I'm not
> > sure why that's gone away.
> 
> Using balance() was my suggestion to stay within the current framework. If
> we want to add @rf to pick_task(), that's more fundamental change. We
> dropped the discussion in the other thread but I found it odd to add @rf to
> pick_task() while disallowing the use of @rf in non-dl-server pick path and
> if we want to allow that, we gotta solve the race between pick_task()
> dropping rq lock and the ttwu inserting high pri task.

I thought the idea was to add rf unconditionally, dl-server or not, it
is needed in both cases.

Yes, that race needs dealing with. We have this existing pattern that
checks if a higher class has runnable tasks and restarting the pick.
This is currently only done for pick_next_task_fair() but that can
easily be extended.

You suggested maybe moving this to the ttwu side -- but up to this point
I thought we were in agreement. I'm not sure moving it to the ttwu side
makes things better; it would need ttwu to know a pick is in progress
and for which class. The existing restart pick is simpler, I think.

Yes, the restart is somewhat more complicated if we want to deal with
the dl-server, but not terribly so. It could just store a snapshot of
rq->dl.dl_nr_running from before the pick and only restart if that went
up.