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

[Abel Wu <wuyun.abel@bytedance.com>]

On 11/20/23 6:56 PM, Peter Zijlstra Wrote:
> 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

Ah, right. I misunderstood (again) the comment which says:

	vl_i = (W + w_i)*vl'_i / W

So the current implementation is:

	v' = V - vl'

and what I was proposing is:

	v' = V' - vl

and they are equal in fact.

> 
>>
>> 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.

Yes. You did the math in an iterative fashion and mine is facing the
final state:

	v' = V' - vlag
	V' = (WV + wv') / (W + w)

which gives:

	V' = V - w * vlag / W

> 
> 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

What I 'm proposing is:

	V' = V - w * vlag / W

so we have:

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

which is exactly the same as current implementation.

> 
> ?
> 
> 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?

Yes, I think I won't misunderstand again for the 3rd time :)

Thanks!
	Abel