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From:   Vineeth Remanan Pillai <vpillai@digitalocean.com>
Date:   Sun, 13 Oct 2019 08:44:32 -0400
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Subject: Re: [RFC PATCH v3 00/16] Core scheduling v3
To:     Aaron Lu <aaron.lu@linux.alibaba.com>
Cc:     Tim Chen <tim.c.chen@linux.intel.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
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On Fri, Oct 11, 2019 at 11:55 PM Aaron Lu <aaron.lu@linux.alibaba.com> wrote:

>
> I don't think we need do the normalization afterwrads and it appears
> we are on the same page regarding core wide vruntime.
>
> The intent of my patch is to treat all the root level sched entities of
> the two siblings as if they are in a single cfs_rq of the core. With a
> core wide min_vruntime, the core scheduler can decide which sched entity
> to run next. And the individual sched entity's vruntime shouldn't be
> changed based on the change of core wide min_vruntime, or faireness can
> hurt(if we add or reduce vruntime of a sched entity, its credit will
> change).
>
Ok, I think I get it now. I see that your first patch actually wraps
all the places
where min_vruntime is accessed. So yes, the tree vruntime updation is needed
only one time. From then on, since we use the wrapper cfs_rq_min_vruntime(),
both the runqueues would self adjust from then on based on the code wide
min_vruntime. Also by the virtue that min_vruntime stays min from there on, the
tree updation logic will not be called more than once. So I think the
changes are safe.
I will do some profiling to make sure that it is actually called once only.

> The weird thing about my patch is, the min_vruntime is often increased,
> it doesn't point to the smallest value as in a traditional cfs_rq. This
> probabaly can be changed to follow the tradition, I don't quite remember
> why I did this, will need to check this some time later.

Yeah, I noticed this. In my patch, I had already accounted for this and changed
to min() instead of max() which is more logical that min_vruntime should be the
minimum of both the run queue.

> All those sub cfs_rq's sched entities are not interesting. Because once
> we decided which sched entity in the root level cfs_rq should run next,
> we can then pick the final next task from there(using the usual way). In
> other words, to make scheduler choose the correct candidate for the core,
> we only need worry about sched entities on both CPU's root level cfs_rqs.
>
Understood. The only reason I did the normalize is to get both the runqueues
under one min_vruntime always. And as long as we use the cfs_rq_min_vruntime
from then on, we wouldn't be calling the balancing logic any more.

> Does this make sense?

Sure, thanks for the clarification.

Thanks,
Vineeth