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

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Date:   Tue, 5 Feb 2019 14:24:33 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Ulf Hansson <ulf.hansson@...aro.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

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.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds