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

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Date:   Fri, 2 Feb 2018 12:39:20 -0500
From:   Steven Sistare <steven.sistare@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 2/2/2018 12:21 PM, Peter Zijlstra wrote:
> 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 keep it simple and accept the possibility of choosing the same candidate
more than once.

>> 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.

Righto.  Disregard this suggestion.

- Steve