linux-kernel - Re: [RFC/RFT][PATCH v0.1] ACPI: OSL: Use usleep_range() in acpi_os

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Message-ID: <CAJZ5v0jTxBt8+bc+EgUZmE84N+Vok_aM16D8HyLQUv=BSoqRCw@mail.gmail.com>
Date: Fri, 22 Nov 2024 20:27:32 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Mario Limonciello <mario.limonciello@....com>
Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>, Linux ACPI <linux-acpi@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Linux PM <linux-pm@...r.kernel.org>, 
	Len Brown <len.brown@...el.com>, Arjan van de Ven <arjan@...ux.intel.com>, 
	Pierre Gondois <pierre.gondois@....com>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Hans de Goede <hdegoede@...hat.com>, "Gautham R. Shenoy" <gautham.shenoy@....com>
Subject: Re: [RFC/RFT][PATCH v0.1] ACPI: OSL: Use usleep_range() in acpi_os_sleep()

On Thu, Nov 21, 2024 at 11:27 PM Mario Limonciello
<mario.limonciello@....com> wrote:
>
> On 11/21/2024 07:15, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > As stated by Len in [1], the extra delay added by msleep() to the
> > sleep time value passed to it can be significant, roughly between
> > 1.5 ns on systems with HZ = 1000 and as much as 15 ms on systems with
> > HZ = 100, which is hardly acceptable, at least for small sleep time
> > values.
> >
> > Address this by using usleep_range() in acpi_os_sleep() instead of
> > msleep().  For short sleep times this is a no-brainer, but even for
> > long sleeps usleep_range() should be preferred because timer wheel
> > timers are optimized for cancellation before they expire and this
> > particular timer is not going to be canceled.
> >
> > Add at least 50 us on top of the requested sleep time in case the
> > timer can be subject to coalescing, which is consistent with what's
> > done in user space in this context [2], but for sleeps longer than 5 ms
> > use 1% of the requested sleep time for this purpose.
> >
> > The rationale here is that longer sleeps don't need that much of a timer
> > precision as a rule and making the timer a more likely candidate for
> > coalescing in these cases is generally desirable.  It starts at 5 ms so
> > that the delta between the requested sleep time and the effective
> > deadline is a contiuous function of the former.
> >
> > Link: https://lore.kernel.org/linux-pm/c7db7e804c453629c116d508558eaf46477a2d73.1731708405.git.len.brown@intel.com/ [1]
> > Link: https://lore.kernel.org/linux-pm/CAJvTdK=Q1kwWA6Wxn8Zcf0OicDEk6cHYFAvQVizgA47mXu63+g@mail.gmail.com/ [2]
> > Reported-by: Len Brown <lenb@...nel.org>
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> You probably should also pick up this tag from the earlier version.
>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=216263

Good point.

> > ---
> >
> > This is a follow-up to the discussion started by [1] above and since
> > the beginning of it I have changed my mind a bit, as you can see.
> >
> > Given Arjan's feedback, I've concluded that using usleep_range() for
> > all sleep values is the right choice and that some slack should be
> > used there.  I've taken 50 us as the minimum value of it because that's
> > what is used in user space FWICT and I'm not convinced that shorter
> > values would be suitable here.
> >
> > The other part, using 1% of the sleep time as the slack for longer
> > sleeps, is likely more controversial.  It is roughly based on the
> > observation that if one timer interrupt is sufficient for something,
> > then using two of them will be wasteful even if this is just somewhat.
> >
> > Anyway, please let me know what you think.  I'd rather do whatever
> > the majority of you are comfortable with.
>
> Generally I'm fine with this.
>
> I'm about to head on US holiday, but I will forward this to folks that
> aren't and get some testing input on it to bring back later when I'm back.

Thanks!