Re: Problem? intel_iommu=off; perf top shows acpi_os_read_port asextremely busy

[Michael Breuer]

Ok - one more rather odd (to me) data point...
I started playing around with various settings, and traced the calls to 
acpi_os_read_port.

To summarize:
With intel_iommu=off, I see a large percentage of calls to 
acpi_os_read_port resulting from user apps (portsentry is #1).
With intel_iommu=on, NONE of trace points to any user apps - all derive 
from the idle loop.
To make things more interesting, when I enable intel_iommu and disable 
vt-d in bios, the system performs much better (20% improvement in 
glxgears, for example), perf top looks like this:

------------------------------------------------------------------------------
   PerfTop:    4863 irqs/sec  kernel:62.7% [100000 cycles],  (all, 8 CPUs)
------------------------------------------------------------------------------

             samples    pcnt   kernel function
             _______   _____   _______________

             2213.00 -  5.5% : acpi_idle_enter_bm
             2001.00 -  5.0% : acpi_os_read_port
             1544.00 -  3.9% : _spin_lock_irqsave
             1075.00 -  2.7% : ioread32
              928.00 -  2.3% : find_busiest_group
              851.00 -  2.1% : _spin_unlock_irqrestore
              823.00 -  2.1% : hpet_next_event
              810.00 -  2.0% : tg_shares_up
              655.00 -  1.6% : fget_light
              641.00 -  1.6% : schedule
              639.00 -  1.6% : tick_nohz_stop_sched_tick
              638.00 -  1.6% : sub_preempt_count
              634.00 -  1.6% : add_preempt_count
              548.00 -  1.4% : do_sys_poll
              446.00 -  1.1% : trace_hardirqs_off

And additionally, one recurring boot warning I've seen since I first 
booted this box has disappeared - first boot message of IRQ16 disabled.

I'm thinking that bad VT-D bios is causing trouble even when intel_iommu 
is disabled.

On 11/29/2009 03:47 PM, Arjan van de Ven wrote:
> On Sat, 28 Nov 2009 13:10:21 -0500
> Michael Breuer<mbreuer@xxxxxxxxxx>  wrote:
>
>    
> > Ok - my only question then is why things appear so different with
> > intel_iommu enabled.
> >      
> something else is even more expensive then :0
>
>
>    

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