linux-kernel - Re: [RESEND RFC PATCH V3] sched: Improve scalability of select_idle

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Date:   Fri, 2 Feb 2018 18:21:53 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Sistare <steven.sistare@...cle.com>
Cc:     subhra mazumdar <subhra.mazumdar@...cle.com>,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        dhaval.giani@...cle.com
Subject: Re: [RESEND RFC PATCH V3] sched: Improve scalability of
 select_idle_sibling using SMT balance

On Fri, Feb 02, 2018 at 11:53:40AM -0500, Steven Sistare wrote:
> It might be interesting to add a tunable for the number of random choices to
> make, and clamp it at the max nr computed from avg_cost in select_idle_cpu.

This needs a fairly complicated PRNG for it would need to visit each
possible CPU once before looping. A LFSR does that, but requires 2^n-1
elements and we have topology masks that don't match that.. The trivial
example is something with 6 cores.

> Or, choose a random starting point and then search for nr sequential 
> candidates; possibly limited by a tunable.

And this is basically what we already do. Except with the task-cpu
instead of a per-cpu rotor.