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

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Message-ID: <CAJZ5v0gLtbixTDEeKrEb5m6VjEn6oeXJJDSOTTrcvFe5EEf-gA@mail.gmail.com>
Date: Mon, 2 Dec 2024 22:57:32 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Mario Limonciello <mario.limonciello@....com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, "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 Mon, Dec 2, 2024 at 10:54 PM Mario Limonciello
<mario.limonciello@....com> wrote:
>
> On 11/22/2024 13:27, Rafael J. Wysocki wrote:
> > 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!
>
> Hi Rafael,
>
> I loaded this onto my personal laptop before the holiday and also got
> others in AMD to do testing on a wider variety of client hardware.
> No concerns were raised with this patch.
>
> Feel free to include:
>
> Reviewed-by: Mario Limonciello <mario.limonciello@....com>
> Tested-by: Mario Limonciello <mario.limonciello@....com>

Thank you!