linux-kernel - Re: [PATCH v1 07/10] sched/fair: Remove useless 64-bit to 32-bit variable conversion

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Message-ID: <20160829010059.GG3273@intel.com>
Date:   Mon, 29 Aug 2016 09:00:59 +0800
From:   Yuyang Du <yuyang.du@...el.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Paul Turner <pjt@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [PATCH v1 07/10] sched/fair: Remove useless 64-bit to 32-bit
 variable conversion

On Thu, Aug 25, 2016 at 09:11:56AM +0200, Vincent Guittot wrote:
> On 10 August 2016 at 02:14, Yuyang Du <yuyang.du@...el.com> wrote:
> > In __decay_sum(), the 64-bit to 32-bit variable conversion makes no
> > performance nor correctness use.
> 
> Are you sure that there is no impact on 32bits system ?
> Even this will be mainly shift manipulation
> 
 
Honestly, I didn't consider 32-bit system at all, :(

And after all, it's 64-bit shift 32-bit vs. 64-bit shift 64-bit operations.
I'm not an expert, but I intend to believe they are about the same, because
the arithmetic overhead seems to lie in the left part of the shift operation,
both of which are 64-bit (correct me if I'm wrong).