linux-kernel - Re: [PATCH 0/7] Cpuidle: Minor fixes

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Date:	Tue, 11 Mar 2014 15:35:06 -0400
From:	Rik van Riel <riel@...hat.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Tuukka Tikkanen <tuukka.tikkanen@...aro.org>
CC:	linux-pm@...r.kernel.org, daniel.lezcano@...aro.org,
	linux-kernel@...r.kernel.org, Len Brown <lenb@...nel.org>
Subject: Re: [PATCH 0/7] Cpuidle: Minor fixes

On 03/07/2014 07:12 AM, Rafael J. Wysocki wrote:
> On Wednesday, February 26, 2014 01:46:25 AM Rafael J. Wysocki wrote:
>> On Monday, February 24, 2014 08:29:30 AM Tuukka Tikkanen wrote:
>>> This set of patches makes some minor changes to menu governor and the poll
>>> idle state.
>>>
>>> Patch 1 is simply a rename of a variable to make the name better represent
>>> the contained information.
>>>
>>> Patch 2 fixes calculating actual residency in cases where the entered state
>>> is different from the state decided by the menu governor.
>>>
>>> Patch 3 makes sure the menu governor timer coefficients are not updated
>>> with values that might cause a coefficient to reach a value greater than
>>> unity.
>>>
>>> Patch 4 fixes calculation actual residency in cases where the entered state
>>> does not support measuring residency. In such cases the residency time
>>> is taken from time remaining until next timer expiry. The timer is expected
>>> to go off at the start of exit latency, not after it. Therefore the exit
>>> latency must not be substracted from the assumed value.
>>>
>>> Patch 5 moves the performance multiplier based comparison out of the state
>>> selection loop by changing it into a latency requirement. This allows
>>> using a generic state selection function accepting only (duration, latency)
>>> tuple as input. The change is possible by noting that performance multiplier
>>> is used only to check for a minimum predicted idle duration to exit latency
>>> ratio. As predicted idle duration is a constant for the loop, the maximum
>>> allowed latency can be calculated outside of the loop.
>>>
>>> Patch 6 prevents using negative values from tick_nohz_get_sleep_length()
>>> in the menu governor. If unchecked, the negative values are used as huge
>>> unsigned values. Negative values occur fairly often (e.g. on x86_64 I've
>>> seen this happen several times per minute) on a busy system, allowing
>>> the deepest state to win the selection while the shallowest should be picked.
>>>
>>> Patch 7 adds CPUIDLE_FLAG_TIME_VALID to poll_idle. I do not know of any
>>> platfrom where cpu_relax() would break ktime_get() and in fact poll_idle
>>> uses ktime_get itself.
>>> (Note: poll_idle updates dev->last_residency for some reason. Does it ever
>>> get called without going through cpuidle_enter_state, which will overwrite
>>> the value? Even if some state redirects to this state, the call will
>>> eventually return to the framework. The redundant time measurement could
>>> be removed, unless there is some obscure way of getting called on some
>>> platform that I am unable to figure out.)
>>>
>>> Tuukka Tikkanen (7):
>>>    Cpuidle: rename expected_us to next_timer_us in menu governor
>>>    Cpuidle: Use actual state latency in menu governor
>>>    Cpuidle: Ensure menu coefficients stay within domain
>>>    Cpuidle: Do not substract exit latency from assumed sleep length
>>>    Cpuidle: Move perf multiplier calculation out of the selection loop
>>>    Cpuidle: Deal with timer expiring in the past
>>>    Cpuidle: poll state can measure residency
>>>
>>>   drivers/cpuidle/driver.c         |    2 +-
>>>   drivers/cpuidle/governors/menu.c |   85 ++++++++++++++++++++++++--------------
>>>   2 files changed, 54 insertions(+), 33 deletions(-)
>>
>> I'm queuing this series up for 3.15.
>
> Patch [6/7] dropped, because of the Len's objection.

Did a replacement for patch 6/7 (fix the timer code so it
does not return a negative expiry time) get written and
merged somewhere? :)

I have been looking at the menu governor too recently,
but at a different aspect. I'll send out an RFC patch
soon (which will probably be shot down, and start
a discussion).

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