linux-kernel - Re: [PATCH][RFC] cpufreq: Bring CPUs up even if cpufreq

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Date:   Tue, 28 Mar 2017 18:40:01 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Chen Yu <yu.c.chen@...el.com>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH][RFC] cpufreq: Bring CPUs up even if cpufreq_online failed

On Tuesday, March 28, 2017 06:23:17 PM Sebastian Andrzej Siewior wrote:
> On 2017-03-25 12:20:11 [+0800], Chen Yu wrote:
> > There is a report that after
> > commit 27622b061eb4 ("cpufreq: Convert to hotplug state machine"),
> > the normal CPU offline/online cycle failed on some platforms.
> > According to the ftrace result, this problem was triggered on
> > platforms using acpi-freq as the default cpufreq driver,
> > and due to the lack of some ACPI freq method(_PCT eg), the
> > cpufreq_online failed and returned a negative value, thus the cpu
> > hotplug statemachine rollbacked the CPU online process. Actually
> > the failure of cpufreq_online should not impact the whole CPU
> > online process according to the original semantics before above patch.
> 
> Well, an error during bring up of CPU should not keep the system going
> like nothing happend and cpufreq was ignoring return values without a
> comment _why_ it is a good iea to do so.
> 
> > BTW, during system bootup the cpufreq_online is not invoked via
> > cpuhotplug statemachine but by the cpufreq device creation process,
> > thus the APs can be brought up although cpufreq_online failed in that
> > stage.
> > 
> > This patch ignores the return value of cpufreq_online/offline and
> > prints a warning if there is a failure.
> 
> What about dealing with this known error instead printing? If something
> like "cpufreq_policy_alloc()" fails I will definitely a rollback and not
> just a print. 

cpufreq_online() will do a proper rollback in that case.  It may even log an
error by itself. :-)

> So what happens if we miss this "method(_PCT eg)"? We still want the
> hotplug event right? So I would suggest a pr_once() that this _PCT
> thingy is missing and continue without an error. I think pr_err_once()
> is enough because I doubt the situation changes without an BIOS update
> and a pr_err() will be visible also during suspend/resume, right?

Right.

That's why I wouldn't print anything here and let cpufreq_online()
and cpufreq_offline() deal with that.

Thanks,
Rafael