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

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Date:   Mon, 5 Feb 2018 18:03:58 +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, tim.c.chen@...ux.intel.com
Subject: Re: [RESEND RFC PATCH V3] sched: Improve scalability of
 select_idle_sibling using SMT balance

On Mon, Feb 05, 2018 at 01:48:54PM +0100, Peter Zijlstra wrote:
> So while I see the point of tracking these numbers (for SMT>2), I don't
> think its worth doing outside of the core, and then we still need some
> powerpc (or any other architecture with abysmal atomics) tested.

FWIW Power has another 'fun' feature, their cores have asymmetric SMT.

Their cores have a static power level, based on _which_ SMT sibling is
running, not how many. A single SMT2 runs (much) slower than a single
SMT0.

So that random selection stuff really doesn't work well for them. Now
'sadly' x86 can also have ASYM_PACKING set on its SMT domain, so I'm
going to have to figure out what to do about all that.