Re: [RFC PATCH v7 08/23] sched: Add core wide task selection and scheduling.

[Vineeth Pillai]

Hi Joel,

On 9/1/20 1:30 PM, Joel Fernandes wrote:
> > I think we can come here when hotplug thread is scheduled during online, but
> > mask is not yet updated. Probably can add it with this comment as well.
> I don't see how that is possible. Because the cpuhp threads run during the
> CPU onlining process, the boot thread for the CPU coming online would have
> already updated the mask.
Sorry my mistake. I got confused with the online state ordering.

> > Another unrelated, but related note :-)
> > Besides this, I think we need to retain on more change from the previous
> > patch. We would need to make core_pick_seq per sibling instead of per
> > core. Having it per core might lead to unfairness. For eg: When a cpu
> > sees that its sibling's core_pick is the one which is already running, it
> > will not send IPI. but core_pick remains set and core->core_pick_seq is
> > incremented. Now if the sibling is preempted due to a high priority task
> Then don't keep the core_pick set then. If you don't send it IPI and if
> core_pick is already running, then NULL it already. I don't know why we add
> to more corner cases by making assumptions. We have enough open issues that
> are not hotplug related. Here's my suggestion :
>
> 1.  Keep the ideas consistent, forget about the exact code currently written
> and just understand the pick_seq is for siblings knowing that something was
> picked for the whole core.  So if their pick_seq != sched_seq, then they have
> to pick what was selected.
I was trying to keep the ideas consistent. The requirement of core_pick
was to let the scheduled cpu know that a pick has been made. And
initial idea was to have the counter core wide. But I found this gap
that pick is not always core wide and assuming it to be core wide can
cause fairness issues. So I was proposing the idea of changing it from
core wide to per sibling. In other words, I was trying to make sure 
core_pick,
along with task_seq and sched_seq is trying to serve its purpose of letting
a sibling know that a new task pick has been made for it. I cannot think of
a reason, why core_pick should be core wide. I might be missing something.

> 2. If core_pick should be NULL, then NULL it in some path. If you keep some
> core_pick and you increment pick_seq, then you are automatically asking the
> sibling to pick that task up then next time it enters schedule(). See if [1]
> will work?
>
> Note that, we have added logic in this patch that does a full selection if
> rq->core_pick == NULL.

I agree, setting rq->core_pick = NULL is another way to solve this 
issue, but
still I feel its semantically incorrect to think that a pick is core 
wide when it
could actually be to only a subset of siblings in the core. If there is 
a valid
reasoning for having core_pick to be core wide, I completely agree with the
fix of resetting core_pick.

> > or its time slice expired, it enters schedule. But it goes to fast path and
> > selects the running task there by starving the high priority task. Having
> > the core_pick_seq per sibling will avoid this. It might also help in some
> > hotplug corner cases as well.
> That can be a separate patch IMHO. It has nothing to do with
> stability/crashing of concurrent and rather infrequent CPU hotplug
> operations.
Agree. Sorry for the confusion, my intention was to not have the logic in
this patch.

> Also, Peter said pick_seq is for core-wide picking. If you want to add
> another semantic, then maybe add another counter which has a separate
> meaning and justify why you are adding it.
I think just one counter is enough. Unless, there is a need to keep the 
counter
to track core wide pick, I feel it is worth to change the design and 
make the
counter serve its purpose. Will think through this and send it as a separate
patch if needed.

Thanks,
Vineeth