Re: [TERRIBLE PATCH] Re: [regression?] i915 generating wakeups even when idle

[Jesse Barnes <jbarnes@virtuousgeek.org>]

On Thu, 9 Dec 2010 12:47:52 -0500
Andrew Lutomirski <luto@mit.edu> wrote:

> On Thu, Dec 9, 2010 at 12:20 PM, Jesse Barnes <jbarnes@virtuousgeek.org> wrote:
> > On Wed, 8 Dec 2010 13:33:20 -0500
> > Andrew Lutomirski <luto@mit.edu> wrote:
> >
> >> On Wed, Dec 8, 2010 at 1:17 PM, Andrew Lutomirski <luto@mit.edu> wrote:
> >> > On Tue, Dec 7, 2010 at 5:13 PM, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> >> >> On Tue, 7 Dec 2010 16:31:24 -0500, Andrew Lutomirski <luto@mit.edu> wrote:
> >> >>> On Tue, Dec 7, 2010 at 4:03 PM, Andrew Lutomirski <luto@mit.edu> wrote:
> >> >>> > Hi all-
> >> >>> >
> >> >>> > Somewhere between Fedora 13 (with 2.6.35, I think) and Fedora 14 +
> >> >>> > 2.6.36.1, i915 started generating exactly 50 interrupts per second
> >> >>> > (suspiciously equal to my refresh rate) when X is running.  I have the
> >> >>> > Xorg driver 2.12.0 (specifically
> >> >>> > xorg-x11-drv-intel-2.12.0-6.fc14.1.x86_64).  When I switch to a text
> >> >>> > console but leave X running, the interrupts stop.
> >> >>> >
> >> >>> > Any ideas what to look at?
> >> >>>
> >> >>> Quitting compiz fixes it.  Suspending compiz also fixes it.
> >> >>
> >> >> So it is the vblank interrupt. The vblank interrupt is get alive for a few
> >> >> seconds after the last use. If it keeps going, then either the system is
> >> >> as idle as you believe or we lost track of the last user and forget to
> >> >> switch off the interrupt.
> >> >>
> >> >> drm.debug=0xf (echo 0xf > /sys/module/drm/parameters/debug) will have the
> >> >> gory details of who/when triggers the vblank interrupt.
> >> >> -Chris
> >> >
> >> > It's the seconds on the clock.  That causes activity once per second,
> >> > which looks like this:
> >> >
> >>
> >> [...]
> >>
> >> >
> >> > Maybe that "several seconds" (5, according to the source) timer is way
> >> > too long.  Is there any reason that drm_vblank_put doesn't turn off
> >> > interrupts immediately (or, at the latest, on the very next vblank
> >> > interrupt)?  After all, preventing deep sleep whenever there is
> >> > display activity every five seconds seems like a waste of power.
> >>
> >> This patch fixes it.  It's obviously not ready for prime time, but if
> >> you're OK with the idea I can fix it up and submit it.
> >>
> >> Signed-off-by: Andy Lutomirski <luto@mit.edu>
> >>
> >> Diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> >> index 9d3a503..49eca3f 100644
> >> --- a/drivers/gpu/drm/drm_irq.c
> >> +++ b/drivers/gpu/drm/drm_irq.c
> >> @@ -471,7 +471,7 @@ void drm_vblank_put(struct drm_device *dev, int crtc)
> >>
> >>       /* Last user schedules interrupt disable */
> >>       if (atomic_dec_and_test(&dev->vblank_refcount[crtc]))
> >> -             mod_timer(&dev->vblank_disable_timer, jiffies + 5*DRM_HZ);
> >> +             mod_timer(&dev->vblank_disable_timer, jiffies);
> >>  }
> >>  EXPORT_SYMBOL(drm_vblank_put);
> >
> > This will just move the problem around a bit; 5s is arguably too long
> > to wait before disabling interrupts, but having a second hand or
> > blinking : in your clock is the real issue here.  Why don't you disable
> > that instead?
> 
> I did.
> 
> But I might also scroll a webpage every second or so (or have a
> webpage loaded at some point that updates itself every second) and I
> see no reason to prevent the system from going into its maximum sleep
> state in between updates.
> 
> IOW, I think we should optimize for mostly-idle machines in addition
> to completely-idle machines.

It's probably safe to reduce the timeout quite a bit (maybe to 500ms or
so); the idea behind 5s was just to avoid whacking the interrupt
hardware too frequently and to avoid situations where we continually
enable/disable interrupts over a short period of time due to an app
that's only periodically using vblank interrupts.

So it would probably be best to make the 5*DRM_HZ into its own define,
DRM_VBLANK_IDLE_TIMEOUT or similar, and reduce it by a lot, maybe to as
low as a few frames; it could even be dynamic based on the refresh rate.

-- 
Jesse Barnes, Intel Open Source Technology Center
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