Re: Async suspend-resume patch w/ completions (was: Re: Async suspend-resume patch w/ rwsems)

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Fri, Dec 18, 2009 at 11:43:29PM +0100, Rafael J. Wysocki wrote:
> On Wednesday 16 December 2009, Dmitry Torokhov wrote:
> > On Wed, Dec 16, 2009 at 03:11:05AM +0100, Rafael J. Wysocki wrote:
> > > On Tuesday 15 December 2009, Linus Torvalds wrote:
> > > > 
> > > > On Tue, 15 Dec 2009, Rafael J. Wysocki wrote:
> > > > > > 
> > > > > > Give a real example that matters.
> > > > > 
> > > > > I'll try.  Let -> denote child-parent relationships and assume dpm_list looks
> > > > > like this:
> > > > 
> > > > No. 
> > > > 
> > > > I mean something real - something like
> > > > 
> > > >  - if you run on a non-PC with two USB buses behind non-PCI controllers.
> > > > 
> > > >  - device xyz.
> > > > 
> > > > > If this applies to _resume_ only, then I agree, but the Arjan's data clearly
> > > > > show that serio devices take much more time to suspend than USB.
> > > > 
> > > > I mean in general - something where you actually have hard data that some 
> > > > device really needs anythign more than my one-liner, and really _needs_ 
> > > > some complex infrastructure.
> > > > 
> > > > Not "let's imagine a case like xyz".
> > > 
> > > As I said I would, I made some measurements.
> > > 
> > > I measured the total time of suspending and resuming devices as shown by the
> > > code added by this patch:
> > > http://git.kernel.org/?p=linux/kernel/git/rafael/suspend-2.6.git;a=commitdiff_plain;h=c1b8fc0a8bff7707c10f31f3d26bfa88e18ccd94;hp=087dbf5f079f1b55cbd3964c9ce71268473d5b67
> > > on two boxes, HP nx6325 and MSI Wind U100 (hardware-wise they are quite
> > > different and the HP was running 64-bit kernel and user space).
> > > 
> > > I took four cases into consideration:
> > > (1) synchronous suspend and resume (/sys/power/pm_async = 0)
> > > (2) asynchronous suspend and resume as introduced by the async branch at:
> > >     http://git.kernel.org/?p=linux/kernel/git/rafael/suspend-2.6.git;a=shortlog;h=refs/heads/async
> > > (3) asynchronous suspend and resume like in (2), but with your one-liner setting
> > >     the power.async_suspend flag for PCI bridges on top
> > > (4) asynchronous suspend and resume like in (2), but with an extra patch that
> > >     is appended on top
> > > 
> > > For those tests I set power.async_suspend for all USB devices, all serio input
> > > devices, the ACPI battery and the USB PCI controllers (to see the impact of the
> > > one-liner, if any).
> > > 
> > > I carried out 5 consecutive suspend-resume cycles (started from under X) on
> > > each box in each case, and the raw data are here (all times in milliseconds):
> > > http://www.sisk.pl/kernel/data/async-suspend.pdf
> > > 
> > > The summarized data are below (the "big" numbers are averages and the +/-
> > > numbers are standard deviations, all in milliseconds):
> > > 
> > > 			HP nx6325		MSI Wind U100
> > > 
> > > sync suspend		1482 (+/- 40)	1180 (+/- 24)
> > > sync resume		2955 (+/- 2)	3597 (+/- 25)
> > > 
> > > async suspend		1553 (+/- 49)	1177 (+/- 32)
> > > async resume		2692 (+/- 326)	3556  (+/- 33)
> > > 
> > > async+one-liner suspend	1600 (+/- 39)	1212 (+/- 41)
> > > async+one-liner resume	2692 (+/- 324)	3579 (+/- 24)
> > > 
> > > async+extra suspend	1496 (+/- 37)	1217 (+/- 38)
> > > async+extra resume	1859 (+/- 114)	1923 (+/- 35)
> > > 
> > > So, in my opinion, with the above set of "async" devices, it doesn't
> > > make sense to do async suspend at all, because the sync suspend is actually
> > > the fastest on both machines.
> > 
> > I think the async suspend is not asynchronous enough then - what kind of
> > time do you get if you simply comment out call to psmouse_reset() in
> > drivers/input/mouse/psmouse-base.c:psmouse_cleanup()?  (Just for testing
> > purposes only, I don't think we want to do that by default.)
> 
> The problem apparently is that the i8042 suspend/resume is synchronous.
> 
> Do you think it's safe to mark it as asynchronous?
> 

Umm.. there lie dragons. There is an implicit relationship between i8042
and PNP/ACPI devices representing keyboard and mouse ports, and I am not
sure how happy i8042 (and most importantly the BIOS) will be if they get
shut down before i8042. Also there is EC which is in theory independent
but in practice not so much.

-- 
Dmitry