Re: [RFC PATCH 09/11] sched/rt: Fix proxy/current (push,pull)ability

[Juri Lelli]

On 19/10/22 18:05, Valentin Schneider wrote:
> On 14/10/22 15:32, Connor O'Brien wrote:
> > On Mon, Oct 10, 2022 at 4:40 AM Valentin Schneider <vschneid@xxxxxxxxxx> wrote:
> >> Consider:
> >>
> >>    p0 (FIFO42)
> >>     |
> >>     | blocked_on
> >>     v
> >>    p1 (FIFO41)
> >>     |
> >>     | blocked_on
> >>     v
> >>    p2 (FIFO40)
> >>
> >>   Add on top p3 an unrelated FIFO1 task, and p4 an unrelated CFS task.
> >>
> >>   CPU0
> >>   current:  p0
> >>   proxy:    p2
> >>   enqueued: p0, p1, p2, p3
> >>
> >>   CPU1
> >>   current:  p4
> >>   proxy:    p4
> >>   enqueued: p4
> >>
> >>
> >> pick_next_pushable_task() on CPU0 would pick p1 as the next highest
> >> priority task to push away to e.g. CPU1, but that would be undone as soon
> >> as proxy() happens on CPU1: we'd notice the CPU boundary and punt it back
> >> to CPU0. What we would want here is to pick p3 instead to have it run on
> >> CPU1.
> >
> > Given this point, is there any reason that blocked tasks should ever
> > be pushable, even if they are not part of the blocked chain for the
> > currently running task? If we could just check task_is_blocked()
> > rather than needing to know whether the task is in the middle of the
> > "active" chain, that would seem to simplify things greatly. I think
> > that approach might also require another dequeue/enqueue, in
> > ttwu_runnable(), to catch non-proxy blocked tasks becoming unblocked
> > (and therefore pushable), but that *seems* OK...though I could
> > certainly be missing something.
> >
> 
> So for an active chain yes we probably don't want any task in the chain to
> be visible to load-balance - proxy and curr because they both make up the
> currently-executed task (but there are active load balances in
> e.g. CFS...), and the rest of the chain because of the above issues.
> 
> As for blocked tasks in a separate chain, ideally we would want them to be
> picked up by load-balance. Consider:
> 
>       blocked_on       owner
>   p0 ------------> m0 -------> p1
> FIFO4                         FIFO3
> 
>       blocked_on       owner
>   p2 ------------> m1 -------> p3
> FIFO2                         FIFO1
> 
> 
> If all those tasks end up on a single CPU for whatever reason, we'll pick
> p0, go through proxy(), and run p1.
> 
> If p2 isn't made visible to load-balance, we'll try to move p3 away -
> unfortunately nothing will make it bring p2 with it. So if all other CPUs
> are running FIFO1 tasks, load-balance will do nothing.
> 
> If p2 is made visible to load-balance, we'll try to move it away, but
> if/when we try to pick it we'll move it back to where p3 is...
> One possible change here is to make the blocked chain migrate towards the
> proxy rather than the owner - this makes scheduling priority considerations
> a bit saner, but is bad towards the owner (migrating blocked tasks is
> "cheap", migrating running tasks isn't).

Plus we need to consider owner's affinity, maybe it can't really migrate
towards proxy's CPU.

It looks like in general we would like to perform load balancing
decisions considering potential proxies attributes? Guess it might soon
turn into a mess to implement, though.