linux-kernel - Re: [PATCH v4 4/4] cpufreq: Use arch specific feedback for cpuinfo_cur

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Date: Mon, 20 May 2024 14:48:53 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: Beata Michalska <beata.michalska@....com>
Cc: Vanshidhar Konda <vanshikonda@...amperecomputing.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	ionela.voinescu@....com, sudeep.holla@....com, will@...nel.org,
	catalin.marinas@....com, vincent.guittot@...aro.org,
	sumitg@...dia.com, yang@...amperecomputing.com,
	lihuisong@...wei.com
Subject: Re: [PATCH v4 4/4] cpufreq: Use arch specific feedback for
 cpuinfo_cur_freq

On 07-05-24, 12:02, Beata Michalska wrote:
> On Tue, May 07, 2024 at 10:31:52AM +0200, Beata Michalska wrote:
> > On Mon, Apr 29, 2024 at 02:55:15PM +0530, Viresh Kumar wrote:
> > > Lets forget for once what X86 and ARM may have done and think about it
> > > once again. I also had a chat with Vincent today about this.
> > > 
> > > The documentation says it clearly, cpuinfo_cur_freq is the one
> > > received from hardware and scaling_cur_freq is the one requested from
> > > software.
> > > 
> > > Now, I know that X86 has made both of them quite similar and I
> > > suggested to make them all aligned (and never received a reply on my
> > > previous message).
> > > 
> > > There are few reasons why it may be worth keeping the definition (and
> > > behavior) of the sysfs files as is, at least for ARM:
> > > - First is that the documentation says so.
> > > - There is no point providing the same information via both the
> > >   interfaces, there are two interfaces here for a reason.
> > > - There maybe tools around which depend on the documented behavior.
> > > - From userspace, currently there is only one way to know the exact
> > >   frequency that the cpufreq governors have requested from a platform,
> > >   i.e. the value from scaling_cur_freq. If we make it similar to
> > >   cpuinfo_cur_freq, then userspace will never know about the requested
> > >   frequency and the eventual one and if they are same or different.
> > > 
> > > Lets keep the behavior as is and update only cpuinfo_cur_freq with
> > > arch_freq_get_on_cpu().
> > > 
> > > Makes sense ?
> > >
> > First of all - apologies for late reply.
> > 
> > It all makes sense, though to clarify things up, for my own benefit, and to
> > avoid any potential confusion ....
> > 
> > Adding arch_freq_get_on_cpu to cpuinfo_cur_freq does seem to be the right
> > approach - no argue on this one. Doing that though means we need a way to
> > skip calling arch_freq_get_on_cpu() from show_scaling_cur_freq(), to avoid
> > having both providing the same information when that should not be the case.
> > In the initial approach [1], that was handled by checking whether the cpufreq
> > driver supports 'get' callback (and thus cpuinfo_cur_freq). In case it didn't,
> > things remained unchanged for scaling_cur_freq. That does not seem to be a viable
> > option though, as there are at least few drivers, that will support both:
> > cpuinfo_cur_freq alongside scaling_cur_freq (+ APERF/MPERF) and for those,
> > we would hit the initial problem of both relying on arch_freq_get_on_cpu.
> > So I guess we need another way of avoiding calling arch_freq_get_on_cpu
> > for show_scaling_cur_freq (and most probably avoid calling that one for
> > cpuinfo_cur_freq). Quick idea on how to not bring arch specificity into
> > cpufreq generic code would be to introduce a new flag for cpufreq drivers though
> > that seems a bit stretched. Will ponder a bit about that but in the meantime
> > suggestions are more than welcomed.
> Alternatively we could add a parameter to arch_freq_get_on_cpu specifying type
> of feedback required: hw vs sw. Then the arch specific implementation could
> decide which to provide when. It will get slightly counter-intuitive, especially
> for cases when sw feedback provides hw one, as it is the case for current
> arch_freq_get_on_cpu() and scaling_cur_freq but at least the changes would be
> minimal and it will contain handling the tricky bits inside arch specific
> implementation - hiding those messy bits.

I think we should just move the call to arch_freq_get_on_cpu() from
show_scaling_cur_freq() to show_cpuinfo_cur_freq() and post it.

Lets see what X86 guys say to that. You can provide all the reasoning
we discussed here, which makes perfect sense.

-- 
viresh