linux-kernel - Re: sched: Avoid SMT siblings in select_idle

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Message-ID: <1332750583.16159.79.camel@twins>
Date:	Mon, 26 Mar 2012 10:29:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Mike Galbraith <efault@....de>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>
Subject: Re: sched: Avoid SMT siblings in select_idle_sibling() if possible

On Thu, 2012-03-22 at 21:02 +0530, Srivatsa Vaddagiri wrote:
> Anyway, there are degradations as well, considering which I see
> several 
> possibilities:
> 
> 1. Do balance-on-wake for vcpu threads only.

Hell no ;-) we're not going to special case some threads over others.

> 2. Document tuning possibility to improve performance in virtualized
>    environment:
>         - Either via sched_domain flags (disable SD_WAKE_AFFINE 
>           at all levels and enable SD_BALANCE_WAKE at SMT/MC levels)

But domain flags are not exported -- except under SCHED_DEBUG and that
sysctl mess.. also SD_flags are not stable.

>         - Or via a new sched_feat(BALANCE_WAKE) tunable 

sched_feat() is not a stable ABI and shouldn't ever be used for anything
but debugging (hence it lives in debugfs and goes away if you disable
SCHED_DEBUG).


I would very much like more information on why things are a loss. Is it
really related to what cpu you pick, or is it the cost of doing the
balance thing?
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