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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Mon, 10 Sep 2018 10:55:43 +0200
Message-ID: <CAJZ5v0jSE2u7DpT6KBiR=zEN0RSBq4cO1Ve=e+pju6cwj8toFQ@mail.gmail.com>
Subject: Re: [PATCH v6 13/14] sched/topology: Make Energy Aware Scheduling
 depend on schedutil
To:     Quentin Perret <quentin.perret@arm.com>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Ingo Molnar <mingo@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Chris Redpath <chris.redpath@arm.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Thara Gopinath <thara.gopinath@linaro.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Todd Kjos <tkjos@google.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Steve Muckle <smuckle@google.com>, adharmap@codeaurora.org,
        Saravana Kannan <skannan@codeaurora.org>,
        Pavan Kondeti <pkondeti@codeaurora.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Eduardo Valentin <edubezval@gmail.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        currojerez@riseup.net, Javi Merino <javi.merino@kernel.org>
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On Mon, Sep 10, 2018 at 10:24 AM Quentin Perret <quentin.perret@arm.com> wrote:
>
> Hi Rafael,
>
> On Sunday 09 Sep 2018 at 22:13:52 (+0200), Rafael J. Wysocki wrote:
> > On Fri, Sep 7, 2018 at 5:29 PM Quentin Perret <quentin.perret@arm.com> wrote:
> > > On Friday 07 Sep 2018 at 10:52:01 (+0200), Rafael J. Wysocki wrote:
> > > > Well, why don't you implement it as something like "if the governor changes
> > > > from sugov to something else (or the other way around), call this function
> > > > from the scheduler"?
> > >
> > > I just gave it a try and ended up with the diff below. It's basically
> > > the exact same patch with a direct function call instead of a notifier.
> > > (I also tried the sugov_start/stop thing I keep mentioning but it is
> > > more complex, so let's see if the simplest solution could work first).
> > >
> > > What do you think ?
> >
> > This generally works for me from the cpufreq perspective, but I would
> > add "cpufreq" to the name of the new function, that is call it
> > something like sched_cpufreq_governor_change().
>
> Ok, no problem.
>
> > Also do you really need the extra work item?  Governor changes are
> > carried out in process context anyway.
>
> Ah, good point, I can remove that. I just tried and got the following
> lock warning on boot, though:
>
> [    2.518684] ============================================
> [    2.523942] WARNING: possible recursive locking detected
> [    2.529200] 4.18.0-rc6-00086-g940e7a9fd5ec #10 Not tainted
> [    2.534630] --------------------------------------------
> [    2.539888] kworker/2:3/1349 is trying to acquire lock:
> [    2.545059] (____ptrval____) (cpu_hotplug_lock.rw_sem){++++}, at: rebuild_sched_domains_locked+0x2c/0x598
> [    2.554559]
> [    2.554559] but task is already holding lock:
> [    2.560332] (____ptrval____) (cpu_hotplug_lock.rw_sem){++++}, at: cpufreq_register_driver+0x80/0x1d0
> [    2.569396]
> [    2.569396] other info that might help us debug this:
> [    2.575858]  Possible unsafe locking scenario:
> [    2.575858]
> [    2.581717]        CPU0
> [    2.584135]        ----
> [    2.586553]   lock(cpu_hotplug_lock.rw_sem);
> [    2.590785]   lock(cpu_hotplug_lock.rw_sem);
> [    2.595017]
> [    2.595017]  *** DEADLOCK ***
> [    2.595017]
> [    2.600877]  May be due to missing lock nesting notation
>
> That seems to happen because cpufreq_register_driver() calls
> cpus_read_lock(), which is then called again by rebuild_sched_domains()
> down the line. So it might just be a missing lock nesting notation as
> the warning suggests ?
>
> I'll have a look.

It only is nested in the _register_driver() code path, otherwise it may not be.

Using the work item may be the most straightforward way to deal with
that, but then I would add a comment to explain what's up.

Thanks,
Rafael