linux-kernel - Re: Problem? intel_iommu=off; perf top shows acpi_os_read

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Date:	Sat, 28 Nov 2009 09:45:30 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Michael Breuer <mbreuer@...jas.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Len Brown <lenb@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Problem? intel_iommu=off; perf top shows acpi_os_read_port as
 extremely busy

On Sat, 28 Nov 2009 08:18:08 +0100
Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Michael Breuer <mbreuer@...jas.com> wrote:
> 
> > Having given up for now on VT-D, I rebooted 2.6.38 rc8 with 
> > intel_iommu=off. Whilst my myriad of broken bios issues cleared, I
> > now see in perf top acpi_os_read_port as continually the busiest
> > function. With intel_iommu enabled, _spin_lock was always on top,
> > and nothing else was notable.
> > 
> > This seems odd to me, perhaps this will make sense to someone else.
> > 
> > FWIW, I'm running on an Asus p6t deluxe v2; ht enabled; no errors
> > or oddities in dmesg or /var/log/messages.
> 
> Could you post the perf top output please?
> 
> Also, could you also post the output of:
> 
> 	perf stat -a --repeat 10 sleep 1
> 
> this will show us how idle the system is. (My guess is that your
> system is idle and perf top shows acpi_os_read_port because the
> system goes to idle via ACPI methods and PIO is slow. In that case
> all is nominal and your system is fine. But it's hard to tell without
> more details.)
> 

yeah the os_read_port is part of the idle loop, so if your system is
idle it'll show up big.... not much we can optimize there though...
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