linux-kernel - Re: [Question]: about 'cpuinfo_cur_freq' shown in sysfs when the CPU is in idle state

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Message-ID: <20200604125822.GB12397@bogus>
Date:   Thu, 4 Jun 2020 13:58:22 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Xiongfeng Wang <wangxiongfeng2@...wei.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Hanjun Guo <guohanjun@...wei.com>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Question]: about 'cpuinfo_cur_freq' shown in sysfs when the CPU
 is in idle state

On Thu, Jun 04, 2020 at 12:42:06PM +0200, Rafael J. Wysocki wrote:
> On Thu, Jun 4, 2020 at 6:41 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> >
> > On 04-06-20, 09:32, Xiongfeng Wang wrote:
> > > On 2020/6/3 21:39, Rafael J. Wysocki wrote:
> > > > The frequency value obtained by kicking the CPU out of idle
> > > > artificially is bogus, though.  You may as well return a random number
> > > > instead.
> > >
> > > Yes, it may return a randowm number as well.
> > >
> > > >
> > > > The frequency of a CPU in an idle state is in fact unknown in the case
> > > > at hand, so returning 0 looks like the cleanest option to me.
> > >
> > > I am not sure about how the user will use 'cpuinfo_cur_freq' in sysfs. If I
> > > return 0 when the CPU is idle, when I run a light load on the CPU, I will get a
> > > zero value for 'cpuinfo_cur_freq' when the CPU is idle. When the CPU is not
> > > idle, I will get a non-zero value. The user may feel odd about
> > > 'cpuinfo_cur_frreq' switching between a zero value and a non-zero value. They
> > > may hope it can return the frequency when the CPU execute instructions, namely
> > > in C0 state. I am not so sure about the user will look at 'cpuinfo_cur_freq'.
> >
> > This is what I was worried about as well. The interface to sysfs needs
> > to be robust. Returning frequency on some readings and 0 on others
> > doesn't look right to me as well. This will break scripts (I am not
> > sure if some scripts are there to look for these values) with the
> > randomness of values returned by it.
> 
> The only thing the scripts need to do is to skip zeros (or anything
> less than the minimum hw frequency for that matter) coming from that
> attribute.
> 
> > On reading values locally from the CPU, I thought about the case where
> > userspace can prevent a CPU going into idle just by reading its
> > frequency from sysfs (and so waste power), but the same can be done by
> > userspace to run arbitrary load on the CPUs.
> >
> > Can we do some sort of caching of the last frequency the CPU was
> > running at before going into idle ? Then we can just check if cpu is
> > idle and so return cached value.
> 
> That is an option, but it looks like in this case the cpuinfo_cur_freq
> attribute should not be present at all, as per the documentation.
> 

+1 for dropping the attribute.

-- 
Regards,
Sudeep