linux-kernel - Re: [PATCH v3 8/9] KVM-GST: adjust scheduler cpu power

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Date:	Thu, 30 Jun 2011 23:54:27 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Glauber Costa <glommer@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Avi Kivity <avi@...hat.com>,
	Anthony Liguori <aliguori@...ibm.com>,
	Eric B Munson <emunson@...bm.net>
Subject: Re: [PATCH v3 8/9] KVM-GST: adjust scheduler cpu power

On Wed, 2011-06-29 at 11:29 -0400, Glauber Costa wrote:
> +       return __touch_steal_time(is_idle, UINT_MAX, NULL);

That wants to be ULLONG_MAX, because max_steal is a u64, with UINT_MAX
the comparison:

+               if (steal > max_steal)

Isn't true per-se and the compiler cannot optimize the branch away.
Also, make sure it does indeed optimize that branch away, otherwise
simply duplicate the code. This is on the very hottest scheduler paths,
branches do count.

Same for:

+               if (ticks)

with ticks == NULL, make sure it never emits code for the above call.

Ah, also, all that requires you strip that noinline crap you put on it,
if it cannot inline, it cannot assume anything on the input and will
this have to emit all this nonsense.

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