| Date | Wed, 11 Oct 2023 20:00:22 +0800 | Subject | Re: [PATCH 03/15] sched/fair: Add lag based placement | From | Abel Wu <> |
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On 5/31/23 7:58 PM, Peter Zijlstra Wrote: > /* > - * Halve their sleep time's effect, to allow > - * for a gentler effect of sleepers: > + * If we want to place a task and preserve lag, we have to > + * consider the effect of the new entity on the weighted > + * average and compensate for this, otherwise lag can quickly > + * evaporate. > + * > + * Lag is defined as: > + * > + * lag_i = S - s_i = w_i * (V - v_i) > + * > + * To avoid the 'w_i' term all over the place, we only track > + * the virtual lag: > + * > + * vl_i = V - v_i <=> v_i = V - vl_i > + * > + * And we take V to be the weighted average of all v: > + * > + * V = (\Sum w_j*v_j) / W > + * > + * Where W is: \Sum w_j > + * > + * Then, the weighted average after adding an entity with lag > + * 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
Maybe a stupid question, but why vl'_i < vl_i? Since vl_i can be negative.
> + * 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. > + * > + * 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 > */ > - if (sched_feat(GENTLE_FAIR_SLEEPERS)) > - thresh >>= 1; > + load = cfs_rq->avg_load; > + if (curr && curr->on_rq) > + load += curr->load.weight; > > - vruntime -= thresh; > + lag *= load + se->load.weight; > + if (WARN_ON_ONCE(!load)) > + load = 1; > + lag = div_s64(lag, load); > + > + vruntime -= lag; > }
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