linux-kernel - Re: [PATCH V3 2/4] cpufreq: cppc: Pass structure instance by reference

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Message-ID: <20210625103058.GC15540@arm.com>
Date:   Fri, 25 Jun 2021 11:30:58 +0100
From:   Ionela Voinescu <ionela.voinescu@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Qian Cai <quic_qiancai@...cinc.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 2/4] cpufreq: cppc: Pass structure instance by
 reference

Hey,

On Thursday 24 Jun 2021 at 07:52:52 (+0530), Viresh Kumar wrote:
> On 23-06-21, 14:45, Ionela Voinescu wrote:
> > On Monday 21 Jun 2021 at 14:49:35 (+0530), Viresh Kumar wrote:
> > > Don't pass structure instance by value, pass it by reference instead.
> > >
> > 
> > Might be best to justify the change a bit :)
> 
> I had it and removed later as I thought it would be obvious :)
> 
> > For me this is a judgement call, and I don't really see the reasons for
> > changing it: we don't care if the structure is modified or not, as we're
> > not reusing the data after the call to cppc_get_rate_from_fbctrs().
> > More so, in this scenario we might not even want for the called function
> > to modify the counter values. Also there is no further call to a function
> > in cppc_get_rate_from_fbctrs(), that might require references to the
> > fb_ctrs.
> > 
> > So what is the reason behind this change?
> 
> How about this commit log then:
> 
> Theoretically speaking, call by reference is cheaper/faster than call by value
> for structures as the later requires the compiler to make a new copy of the
> whole structure (which has four u64 values here), to be used by the called
> function, while with call by reference we just need to pass a single pointer
> (u64 on 64-bit architectures) to the existing structure.
> 
> Yes, on modern architectures, the compilers will likely end up using the
> processor registers for passing this structure as it isn't doesn't have lot of
> fields and it shouldn't be bad eventually, but nevertheless the code should do
> the right thing without assuming about the compiler's or architecture's
> optimizations.
> 

Yes, that's why "judgement call", which I'll let you make. The code is
sane and I like the longer commit message.

Reviewed-by: Ionela Voinescu <ionela.voinescu@....com>

> Don't pass structure instance by value, pass it by reference instead.
> 
> -- 
> viresh