Re: [PATCH 2/2] sched/fair: Randomize equally shallow slow-path candidates

[Kayra Cizmeci]

Hello Christian :>

> Picking the first eligible idle CPU leaves a scan-order bias. Concurrent
> slow-path selectors can choose the same CPU before either task is enqueued.

> Use reservoir sampling in the tie branch, resetting the candidate count
> when a lower advertised exit latency is found. Use the per-CPU scheduler
> PRNG and reciprocal_scale() to avoid variable division or a second scan.

> This reduces deterministic convergence without reserving the chosen CPU.

OK. 

But, your test platform was 160 Cores too. I don't think randomization has the same effect on lower
CPU systems.

Let's create a scenario:

On a 80 Core System, that %50 of it's CPU's are idle the randomization's chance of choosing the same CPU
is low. Since there are 40 CPU's to choose from.

But on a 8 Core System in the same idle conditions, randomization's chance of choosing the same CPU
is really higher. Since there are only 4 CPU's to choose from.

I think this solution works better on higher CPU counted systems. 

And I don't think it fully removes the issue. Just better than the original tho.
Thinks can still go bad on this one too, just harder.

Maybe we could add a fallback. Because the real concern is the selected CPU's state
changes when we come to enqueue. If possible tho, I did not really test anything.

Thanks,
Kayra :>>>