Thunderbolt, Apple Monitor & MBA, and excess kworker load

Thunderbolt, Apple Monitor & MBA, and excess kworker load

[Dominik Brodowski]

Hi,

At work, I wish to attach a 27" Apple Display using thunderbolt to my Mac
Book Air 4,2 13". Using OS X, it works fine, but as I prefer using Linux, I
tested out Ubuntu's stock kernel and (among other) 3.5.0-rc3+ as of f40759e.
Hotplug doesn't work (known issue), but also with coldplug the display
reacts _very_ slow, way too slow for real use: The screen updates only every
half second or so. Is this a known bug? Might something go afoul with
kworker -- they need about 75% of the CPU with the large display attached,
and less than 1% _without_ the large display.

Any hints are most welcome, also on where I may dig further to determine the
root causes...

Thanks & best,

	Dominik

Re: Thunderbolt, Apple Monitor & MBA, and excess kworker load

[Greg KH]

On Wed, Jun 20, 2012 at 05:48:42PM +0200, Dominik Brodowski wrote:
> Hi,
> 
> At work, I wish to attach a 27" Apple Display using thunderbolt to my Mac
> Book Air 4,2 13". Using OS X, it works fine, but as I prefer using Linux, I
> tested out Ubuntu's stock kernel and (among other) 3.5.0-rc3+ as of f40759e.
> Hotplug doesn't work (known issue), but also with coldplug the display
> reacts _very_ slow, way too slow for real use: The screen updates only every
> half second or so. Is this a known bug? Might something go afoul with
> kworker -- they need about 75% of the CPU with the large display attached,
> and less than 1% _without_ the large display.

It's not really a known bug, but it is known that Thunderbolt doesn't
work all that well, if at all, in Linux due to the BIOS issues that you
have run into (the hotplug stuff.)

Once the device is seen, it should just work like any other PCI device,
and so, that might be a DRM driver issue somehow.

greg k-h

Re: Thunderbolt, Apple Monitor & MBA, and excess kworker load

[Daniel Vetter]

On Wed, Jun 20, 2012 at 08:56:26AM -0700, Greg KH wrote:
> On Wed, Jun 20, 2012 at 05:48:42PM +0200, Dominik Brodowski wrote:
> > Hi,
> > 
> > At work, I wish to attach a 27" Apple Display using thunderbolt to my Mac
> > Book Air 4,2 13". Using OS X, it works fine, but as I prefer using Linux, I
> > tested out Ubuntu's stock kernel and (among other) 3.5.0-rc3+ as of f40759e.
> > Hotplug doesn't work (known issue), but also with coldplug the display
> > reacts _very_ slow, way too slow for real use: The screen updates only every
> > half second or so. Is this a known bug? Might something go afoul with
> > kworker -- they need about 75% of the CPU with the large display attached,
> > and less than 1% _without_ the large display.
> 
> It's not really a known bug, but it is known that Thunderbolt doesn't
> work all that well, if at all, in Linux due to the BIOS issues that you
> have run into (the hotplug stuff.)
> 
> Once the device is seen, it should just work like any other PCI device,
> and so, that might be a DRM driver issue somehow.

A quick full-system profile would be a good start to tell where we're
burning through all these cycles exaclty. Also, anything special going on
with interrupts (if this excessive load is due to supurious display
hotplug events, drm/i915 should get _tons_ of interrupts).
-Daniel
-- 
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48

Re: Thunderbolt, Apple Monitor & MBA, and excess kworker load

[Dominik Brodowski]

On Wed, Jun 20, 2012 at 06:10:12PM +0200, Daniel Vetter wrote:
> On Wed, Jun 20, 2012 at 08:56:26AM -0700, Greg KH wrote:
> > On Wed, Jun 20, 2012 at 05:48:42PM +0200, Dominik Brodowski wrote:
> > > Hi,
> > > 
> > > At work, I wish to attach a 27" Apple Display using thunderbolt to my Mac
> > > Book Air 4,2 13". Using OS X, it works fine, but as I prefer using Linux, I
> > > tested out Ubuntu's stock kernel and (among other) 3.5.0-rc3+ as of f40759e.
> > > Hotplug doesn't work (known issue), but also with coldplug the display
> > > reacts _very_ slow, way too slow for real use: The screen updates only every
> > > half second or so. Is this a known bug? Might something go afoul with
> > > kworker -- they need about 75% of the CPU with the large display attached,
> > > and less than 1% _without_ the large display.
> > 
> > It's not really a known bug, but it is known that Thunderbolt doesn't
> > work all that well, if at all, in Linux due to the BIOS issues that you
> > have run into (the hotplug stuff.)

Is there anything where I could help?

> > Once the device is seen, it should just work like any other PCI device,
> > and so, that might be a DRM driver issue somehow.
> 
> A quick full-system profile would be a good start to tell where we're
> burning through all these cycles exaclty. Also, anything special going on
> with interrupts (if this excessive load is due to supurious display
> hotplug events, drm/i915 should get _tons_ of interrupts).

Did a $ perf record -a -g sleep 10 , and that shows:

-  11,67%      kworker/1:1  [kernel.kallsyms]
   + native_read_tsc
   + delay_tsc

-   9,56%      kworker/3:2  [kernel.kallsyms]
   + native_read_tsc
   + delay_tsc

and other uninteresting things. 

   $ echo workqueue:workqueue_queue_work > /sys/kernel/debug/tracing/set_event
   $ cat /sys/kernel/debug/tracing/trace_pipe

shows these top functions:

    305 i915_hotplug_work_func
    504 flush_to_ldisc
  14759 do_dbs_timer

so nothing of interest either. Interrupts: pretty normal.

Any ideas on how to debug this further?

Thanks & best,

	Dominik

Re: Thunderbolt, Apple Monitor & MBA, and excess kworker load

[Greg KH]

On Thu, Jun 21, 2012 at 12:59:00PM +0200, Dominik Brodowski wrote:
> 
> On Wed, Jun 20, 2012 at 06:10:12PM +0200, Daniel Vetter wrote:
> > On Wed, Jun 20, 2012 at 08:56:26AM -0700, Greg KH wrote:
> > > On Wed, Jun 20, 2012 at 05:48:42PM +0200, Dominik Brodowski wrote:
> > > > Hi,
> > > > 
> > > > At work, I wish to attach a 27" Apple Display using thunderbolt to my Mac
> > > > Book Air 4,2 13". Using OS X, it works fine, but as I prefer using Linux, I
> > > > tested out Ubuntu's stock kernel and (among other) 3.5.0-rc3+ as of f40759e.
> > > > Hotplug doesn't work (known issue), but also with coldplug the display
> > > > reacts _very_ slow, way too slow for real use: The screen updates only every
> > > > half second or so. Is this a known bug? Might something go afoul with
> > > > kworker -- they need about 75% of the CPU with the large display attached,
> > > > and less than 1% _without_ the large display.
> > > 
> > > It's not really a known bug, but it is known that Thunderbolt doesn't
> > > work all that well, if at all, in Linux due to the BIOS issues that you
> > > have run into (the hotplug stuff.)
> 
> Is there anything where I could help?

If you can figure out how Apple implemented this with their custom ACPI
interface, that would be most helpful.  I have a Thunderbolt machine on
order and will start to work on this in a month or so when I receive it.

thanks,

greg k-h