linux-kernel - Re: [PATCH 01/70] cpufreq: interactive: New 'interactive' governor

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Message-ID: <4534884.dsz2GVUDrG@vostro.rjw.lan>
Date:	Wed, 28 Oct 2015 01:59:01 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Bálint Czobor <czoborbalint@...il.com>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	Mike Chan <mike@...roid.com>,
	Todd Poynor <toddpoynor@...gle.com>
Subject: Re: [PATCH 01/70] cpufreq: interactive: New 'interactive' governor

Hi,

On Tuesday, October 27, 2015 06:29:49 PM Bálint Czobor wrote:
> From: Mike Chan <mike@...roid.com>
> 
> This governor is designed for latency-sensitive workloads, such as
> interactive user interfaces.  The interactive governor aims to be
> significantly more responsive to ramp CPU quickly up when CPU-intensive
> activity begins.
> 
> Existing governors sample CPU load at a particular rate, typically
> every X ms.  This can lead to under-powering UI threads for the period of
> time during which the user begins interacting with a previously-idle system
> until the next sample period happens.
> 
> The 'interactive' governor uses a different approach. Instead of sampling
> the CPU at a specified rate, the governor will check whether to scale the
> CPU frequency up soon after coming out of idle.  When the CPU comes out of
> idle, a timer is configured to fire within 1-2 ticks.  If the CPU is very
> busy from exiting idle to when the timer fires then we assume the CPU is
> underpowered and ramp to MAX speed.
> 
> If the CPU was not sufficiently busy to immediately ramp to MAX speed, then
> the governor evaluates the CPU load since the last speed adjustment,
> choosing the highest value between that longer-term load or the short-term
> load since idle exit to determine the CPU speed to ramp to.
> 
> A realtime thread is used for scaling up, giving the remaining tasks the
> CPU performance benefit, unlike existing governors which are more likely to
> schedule rampup work to occur after your performance starved tasks have
> completed.
> 
> The tuneables for this governor are:
> /sys/devices/system/cpu/cpufreq/interactive/min_sample_time:
> 	The minimum amount of time to spend at the current frequency before
> 	ramping down. This is to ensure that the governor has seen enough
> 	historic CPU load data to determine the appropriate workload.
> 	Default is 80000 uS.
> /sys/devices/system/cpu/cpufreq/interactive/go_maxspeed_load
> 	The CPU load at which to ramp to max speed.  Default is 85.
> 
> Change-Id: Ib2b362607c62f7c56d35f44a9ef3280f98c17585
> Signed-off-by: Mike Chan <mike@...roid.com>
> Signed-off-by: Todd Poynor <toddpoynor@...gle.com>
> Bug: 3152864
> Signed-off-by: Bálint Czobor <czoborbalint@...il.com>

It's good to see that submitted, but it'll have to go through a detailed
review which is going to take some time.

One my observation after a cursory look at it is that at least some later
patches of the series modify drivers/cpufreq/cpufreq_interactive.c which is
a new file added by the first patch.  Is there any particular reason to
avoid folding all of those patches into the first one?

Thanks,
Rafael

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