linux-kernel - Re: [PATCH v2 ] PM-runtime: fix deadlock with ktime

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Date:   Tue, 5 Feb 2019 14:40:04 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-omap <linux-omap@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Biju Das <biju.das@...renesas.com>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH v2 ] PM-runtime: fix deadlock with ktime

On Tue, 5 Feb 2019 at 14:24, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>
> Hi Ulf,
>
> On Wed, Jan 30, 2019 at 1:15 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> > On Wed, 30 Jan 2019 at 12:16, Vincent Guittot
> > <vincent.guittot@...aro.org> wrote:
> > >
> > > A deadlock has been seen when swicthing clocksources which use PM runtime.
> > > The call path is:
> > > change_clocksource
> > >     ...
> > >     write_seqcount_begin
> > >     ...
> > >     timekeeping_update
> > >         ...
> > >         sh_cmt_clocksource_enable
> > >             ...
> > >             rpm_resume
> >
> > Hmm, isn't this already a problem in genpd then?
> >
> > In genpd's ->runtime_resume() callback (genpd_runtime_resume()) we
> > call ktime_get() to measure the latency of running the callback.
> >
> > Or, perhaps irq_safe_dev_in_no_sleep_domain() returns true for the
> > genpd + device in question, which means the ktime thingy is skipped.
> >
> > Geert?
>
> Correct. After adding
>
>     dev_info(dev, "%s: pm_runtime_is_irq_safe = %d,
> genpd_is_irq_safe(%s) = %d => %s\n", __func__,
> pm_runtime_is_irq_safe(dev), genpd->name, genpd_is_irq_safe(genpd),
> ret ? "true" : "false");
>
> I see on R-Car M2-W:
>
>     sh_cmt ffca0000.timer: irq_safe_dev_in_no_sleep_domain:
> pm_runtime_is_irq_safe = 1, genpd_is_irq_safe(always-on) = 0 => true
>
> For all other devices, the result is false.

Thanks for confirming! I guess we have been lucky so far.

Anyway, if runtime PM core converts to use ktime_get_mono_fast_ns(),
we should probably consider to convert genpd to this as well. Don't
you think?

Kind regards
Uffe