linux-kernel - Re: [PATCH 2/2] PM / Domains: Avoid infinite loops in attach/detach code

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Message-ID: <CAMuHMdU8sHuAKrRvVF1pV7FOCLYrdi1xfAiqb50tfJWhKK3jDQ@mail.gmail.com>
Date:	Tue, 23 Jun 2015 15:45:43 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	Ulf Hansson <ulf.hansson@...aro.org>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Kevin Hilman <khilman@...nel.org>,
	Magnus Damm <damm@...nsource.se>,
	Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux-sh list <linux-sh@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] PM / Domains: Avoid infinite loops in attach/detach code

Hi Rafael,

On Tue, Jun 23, 2015 at 3:38 PM, Rafael J. Wysocki <rafael@...nel.org> wrote:
>>>> @@ -2218,10 +2227,13 @@ int genpd_dev_pm_attach(struct device *dev)
>>>>
>>>>         dev_dbg(dev, "adding to PM domain %s\n", pd->name);
>>>>
>>>> -       while (1) {
>>>> +       for (i = 0; i < GENPD_RETRIES; i++) {
>>>>                 ret = pm_genpd_add_device(pd, dev);
>>>>                 if (ret != -EAGAIN)
>>>>                         break;
>>>> +
>>>> +               if (i > GENPD_RETRIES / 2)
>>>> +                       udelay(GENPD_DELAY_US);
>>>
>>> In this execution path, we retry when getting -EAGAIN while believing
>>> the reason to the error are only *temporary* as we are soon waiting
>>> for all devices in the genpd to be system PM resumed. At least that's
>>> my understanding to why we want to deal with -EAGAIN here, but I might
>>> be wrong.
>>>
>>> In this regards, I wonder whether it could be better to re-try only a
>>> few times but with a far longer interval time than a couple us. What
>>> do you think?
>>
>> That's indeed viable. I have no idea for how long this temporary state can
>> extend.
>
> A usual approach to this kind of thing is to use exponential fallback
> where you increase the delay twice with respect to the previous one
> every time.

Right, but when do you give up?

Note that udelay() is a busy loop. Should it fall back to msleep() after
a while?

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
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