linux-kernel - Re: [PATCH V2 4/5] cpufreq: Register notifiers with the PM QoS framework

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Message-ID: <20190226100028.rfvth7r4j33qljrv@e107158-lin.cambridge.arm.com>
Date:   Tue, 26 Feb 2019 10:00:28 +0000
From:   Qais Yousef <qais.yousef@....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>, mka@...omium.org,
        juri.lelli@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 4/5] cpufreq: Register notifiers with the PM QoS
 framework

On 02/26/19 08:00, Viresh Kumar wrote:
> On 25-02-19, 12:14, Qais Yousef wrote:
> > On 02/25/19 14:39, Viresh Kumar wrote:
> > > On 25-02-19, 08:58, Qais Yousef wrote:
> > > > On 02/25/19 10:01, Viresh Kumar wrote:
> > > > > > > +	min = dev_pm_qos_read_value(cpu_dev, DEV_PM_QOS_MIN_FREQUENCY);
> > > > > > > +	max = dev_pm_qos_read_value(cpu_dev, DEV_PM_QOS_MAX_FREQUENCY);
> > > > > > > +
> > > > > > > +	if (min > new_policy->min)
> > > > > > > +		new_policy->min = min;
> > > > > > > +	if (max < new_policy->max)
> > > > > > > +		new_policy->max = max;
> > > 
> > > > And this is why we need to check here if the PM QoS value doesn't conflict with
> > > > the current min/max, right? Until the current notifier code is removed they
> > > > could trip over each others.
> > > 
> > > No. The above if/else block is already removed as part of patch 5/5. It was
> > > required because of conflict between userspace specific min/max and qos min/max,
> > > which are migrated to use qos by patc 5/5.
> > > 
> > > The cpufreq notifier mechanism already lets users play with min/max and that is
> > > already safe from conflicts.
> > > 
> > > 
> > > > It would be nice to add a comment here about PM QoS managing and remembering
> > > > values
> > > 
> > > I am not sure if that would add any value. Some documentation update may be
> > > useful for people looking for details though, that I shall do after all the
> > > changes get in and things become a bit stable.
> > > 
> > 
> > Up to you. But not everyone is familiar with the code and a one line comment
> > that points to where aggregation is happening would be helpful for someone
> > scanning this code IMHO.
> 
> Okay, will add something then.
> 
> > > > and that we need to be careful that both mechanisms don't trip over
> > > > each others until this transient period is over.
> > > 
> > > The second mechanism will die very very soon once this is merged, migrating them
> > > shouldn't be a big challenge AFAICT. I didn't attempt that because I didn't
> > > wanted to waste time updating things in case this version also doesn't make
> > > sense to others.
> > > 
> > > > I have a nit too. It would be nice to explicitly state this is
> > > > CPU_{MIN,MAX}_FREQUENCY. I can see someone else adding {MIN,MAX}_FREQUENCY for
> > > > something elsee (memory maybe?)
> > > 
> > > This is not CPU specific, but any device. The same interface shall be used by
> > > devfreq as well, who wanted to use freq-constraints initially.
> > > 
> > 
> > I don't get that to be honest. I probably have to read more.
> > 
> > Is what you're saying that when applying a MIN_FREQUENCY constraint the same
> > value will be applied to both cpufreq and devfreq? Isn't this too coarse?
> 
> Oh no. A constraint with QoS is added like this:
> 
>         dev_pm_qos_add_request(dev, req, DEV_PM_QOS_MIN_FREQUENCY, min);
> 
> Now dev here can be any device struct, CPU's or GPU's or anything else. All the
> MIN freq requests are stored/processed per device and for a CPU in cpufreq all
> we will see is MIN requests for the CPUs. And so the macro is required to be a
> bit generic and shouldn't have CPU word within it.
> 
> Hope I was able to clarify your doubt a bit. Thanks.

Ah I see yes it all makes sense now.

Thanks!

--
Qais Yousef