Re: Re: EEVDF/vhost regression (bisected to 86bfbb7ce4f6 sched/fair: Add lag based placement)

Peter Zijlstra
2023-11-20T10:57:19Z

On Sat, Nov 18, 2023 at 01:14:32PM +0800, Abel Wu wrote:

> Hi Peter, I'm a little confused here. As we adopt placement strategy #1
> when PLACE_LAG is enabled, the lag of that entity needs to be preserved.
> Given that the weight doesn't change, we have:
>
> vl' = vl
>
> But in fact it is scaled on placement:
>
> vl' = vl * W/(W + w)

(W+w)/W

>
> Does this intended?

The scaling, yes that's intended and the comment explains why. So now
you have me confused too :-)

Specifically, I want the lag after placement to be equal to the lag we
come in with. Since placement will affect avg_vruntime (adding one
element to the average changes the average etc..) the placement also
affects the lag as measured after placement.

Or rather, if you enqueue and dequeue, I want the lag to be preserved.
If you do not take placement into consideration, lag will dissipate real
quick.

> And to illustrate my understanding of strategy #1:

> @@ -5162,41 +5165,17 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> * vl_i is given by:
> *
> * V' = (\Sum w_j*v_j + w_i*v_i) / (W + w_i)
> - * = (W*V + w_i*(V - vl_i)) / (W + w_i)
> - * = (W*V + w_i*V - w_i*vl_i) / (W + w_i)
> - * = (V*(W + w_i) - w_i*l) / (W + w_i)
> - * = V - w_i*vl_i / (W + w_i)
> - *
> - * And the actual lag after adding an entity with vl_i is:
> - *
> - * vl'_i = V' - v_i
> - * = V - w_i*vl_i / (W + w_i) - (V - vl_i)
> - * = vl_i - w_i*vl_i / (W + w_i)
> - *
> - * Which is strictly less than vl_i. So in order to preserve lag
> - * we should inflate the lag before placement such that the
> - * effective lag after placement comes out right.
> - *
> - * As such, invert the above relation for vl'_i to get the vl_i
> - * we need to use such that the lag after placement is the lag
> - * we computed before dequeue.
> + * = (W*V + w_i*(V' - vl_i)) / (W + w_i)
> + * = V - w_i*vl_i / W
> *
> - * vl'_i = vl_i - w_i*vl_i / (W + w_i)
> - * = ((W + w_i)*vl_i - w_i*vl_i) / (W + w_i)
> - *
> - * (W + w_i)*vl'_i = (W + w_i)*vl_i - w_i*vl_i
> - * = W*vl_i
> - *
> - * vl_i = (W + w_i)*vl'_i / W
> */
> load = cfs_rq->avg_load;
> if (curr && curr->on_rq)
> load += scale_load_down(curr->load.weight);
> -
> - lag *= load + scale_load_down(se->load.weight);
> if (WARN_ON_ONCE(!load))
> load = 1;
> - lag = div_s64(lag, load);
> +
> + vruntime -= div_s64(lag * scale_load_down(se->load.weight), load);
> }
> se->vruntime = vruntime - lag;

So you're proposing we do:

v = V - (lag * w) / (W + w) - lag

?

That can be written like:

v = V - (lag * w) / (W+w) - (lag * (W+w)) / (W+w)
= V - (lag * (W+w) + lag * w) / (W+w)
= V - (lag * (W+2w)) / (W+w)

And that turns into a mess AFAICT.

Let me repeat my earlier argument. Suppose v,w,l are the new element.
V,W are the old avg_vruntime and sum-weight.

Then: V = V*W / W, and by extention: V' = (V*W + v*w) / (W + w).

The new lag, after placement:

l' = V' - v = (V*W + v*w) / (W+w) - v
= (V*W + v*w) / (W+w) - v * (W+w) / (W+v)
= (V*W + v*w -v*W - v*w) / (W+w)
= (V*W - v*W) / (W+w)
= W*(V-v) / (W+w)
= W/(W+w) * (V-v)

Substitute: v = V - (W+w)/W * l, my scaling thing, to obtain:

l' = W/(W+w) * (V - (V - (W+w)/W * l))
= W/(W+w) * (V - V + (W+w)/W * l)
= W/(W+w) * (W+w)/W * l
= l

So by scaling, we've preserved lag across placement.

That make sense?