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

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Message-id: <4B11678D.8020601@majjas.com>
Date:	Sat, 28 Nov 2009 13:10:21 -0500
From:	Michael Breuer <mbreuer@...jas.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	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

Ok - my only question then is why things appear so different with 
intel_iommu enabled.
Arjan van de Ven wrote:
> 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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