linux-kernel - Re: [RFC][PATCH] Introduce CLOCK_REALTIME

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Message-ID: <13a12eea0907182000v654e38a5l265ae5bdadb1a175@mail.gmail.com>
Date:	Sat, 18 Jul 2009 23:00:55 -0400
From:	Chris Snook <chris.snook@...il.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...x.de>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <ak@...e.de>, nikolag@...ibm.com,
	Darren Hart <dvhltc@...ibm.com>
Subject: Re: [RFC][PATCH] Introduce CLOCK_REALTIME_COARSE

On Fri, Jul 17, 2009 at 7:39 PM, john stultz<johnstul@...ibm.com> wrote:
> -       if (likely(gtod->sysctl_enabled && gtod->clock.vread))
> +       if (likely(gtod->sysctl_enabled))

This irks me.  If the sysctl is enabled and the codepath is getting
used often enough that we care about performance, branch prediction
should do the right thing without compiler hints.  On the other hand,
if the sysctl is disabled, and the compiler is telling the cpu to
ignore its branch predictor, it'll hurt.  I don't think we should be
wrapping (un)likely annotations around configuration options, unless
we're biasing against debug conditions where we definitely don't care
about performance.  The patch is certainly no worse than the existing
code, but while we have the hood up, it might be nice to remove the
annotation, unless we're sure that it does no harm, and does some
good.

-- Chris
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