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Message-ID: <20150326175126.GN18994@e105550-lin.cambridge.arm.com>
Date: Thu, 26 Mar 2015 17:51:26 +0000
From: Morten Rasmussen <morten.rasmussen@....com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Vincent Guittot <vincent.guittot@...aro.org>,
Ingo Molnar <mingo@...nel.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
Rik van Riel <riel@...hat.com>,
Mike Galbraith <efault@....de>,
"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
Dietmar Eggemann <Dietmar.Eggemann@....com>,
Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>
Subject: Re: [PATCH v10 04/11] sched: Make sched entity usage tracking
scale-invariant
On Thu, Mar 26, 2015 at 05:47:00PM +0000, Peter Zijlstra wrote:
> On Thu, Mar 26, 2015 at 05:38:45PM +0000, Morten Rasmussen wrote:
> > Another potential solution is to stay with weak functions but move the
> > multiplication and shift into the arch_scale_*() functions by passing
> > the value we want to scale into the arch_scale_*() function. That way we
> > can completely avoid multiplication and shift in the default case (no
> > arch_scale*() implementations, which is better than what we have today.
> >
> > The only downside is that for frequency invariance we need three
> > arch_scale_freq_capacity() calls instead of two.
>
> That would still result in unconditional function calls, which on some
> archs are _more_ expensive than 64bit mults.
Right. Then it can only be preprocessor magic I think.
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