linux-kernel - Re: [PATCH 7/8] ACPI: acpi_idle: touch TS

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Date:	Fri, 28 May 2010 14:24:11 -0700
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	Len Brown <lenb@...nel.org>
Cc:	linux-pm@...ts.linux-foundation.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/8] ACPI: acpi_idle: touch TS_POLLING only in the 
	non-MWAIT case

On Fri, May 28, 2010 at 10:59 AM, Len Brown <lenb@...nel.org> wrote:
> On Fri, 28 May 2010, Venkatesh Pallipadi wrote:
>
>> On Thu, May 27, 2010 at 11:02 PM, Len Brown <lenb@...nel.org> wrote:
>> > From: Len Brown <len.brown@...el.com>
>> >
>> > commit d306ebc28649b89877a22158fe0076f06cc46f60
>> > (ACPI: Be in TS_POLLING state during mwait based C-state entry)
>> > fixed an important power & performance issue where ACPI c2 and c3 C-states
>> > were clearing TS_POLLING even when using MWAIT (ACPI_STATE_FFH).
>> > That bug had been causing us to receive redundant scheduling interrups
>> > when we had already been woken up by MONITOR/MWAIT.
>> >
>> > Following up on that...
>> >
>> > In the MWAIT case, we don't have to subsequently
>> > check need_resched(), as that c heck was there
>> > for the TS_POLLING-clearing case.
>> >
>> > Note that not only does the cpuidle calling function
>> > already check need_resched() before calling us, the
>> > low-level entry into monitor/mwait calls it twice --
>> > guaranteeing that a write to the trigger address
>> > can not go un-noticed.
>>
>> Ack this part of the change.
>>
>> > Also, in this case, we don't have to set TS_POLLING
>> > when we wake, because we never cleared it.
>>
>> I thought about this part of the change while working on the original
>> patch. But decided to leave the set to be done unconditionally as in
>> this case we are replacing "one write" by either "one read and one
>> jump" or "one read and one write" and I am not sure we gain much by
>> that.
>
> I don't know if the write is more expensive than the read and branch.
> In the scheme of things, these things are free and it is the IO
> accesses and locks in the idle path that take time.
>

Acked-by: Venkatesh Pallipadi <venki@...gle.com>
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