linux-kernel - Re: [RFC PATCH] PM / QoS: Introduce new classes: DMA-Throughput and DVFS-Latency

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Date:	Sun, 26 Feb 2012 00:43:50 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	MyungJoo Ham <myungjoo.ham@...il.com>
Cc:	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Kevin Hilman <khilman@...com>, Jean Pihet <j-pihet@...com>,
	markgross <markgross@...gnar.org>, kyungmin.park@...sung.com
Subject: Re: [RFC PATCH] PM / QoS: Introduce new classes: DMA-Throughput and DVFS-Latency

On Tuesday, February 14, 2012, MyungJoo Ham wrote:
> On Tue, Feb 14, 2012 at 11:26 AM, MyungJoo Ham <myungjoo.ham@...sung.com> wrote:
> > 1. CPU_DMA_THROUGHPUT
> >
> > This might look simliar to CPU_DMA_LATENCY. However, there are H/W
> > blocks that creates QoS requirement based on DMA throughput, not
> > latency, while their (those QoS requester H/W blocks) services are
> > short-term bursts that cannot be effectively responsed by DVFS
> > mechanisms (CPUFreq and Devfreq).
> >
> > In the Exynos4412 systems that are being tested, such H/W blocks include
> > MFC (multi-function codec)'s decoding and enconding features, TV-out
> > (including HDMI), and Cameras. When the display is operated at 60Hz,
> > each chunk of task should be done within 16ms and the workload on DMA is
> > not well spread and fluctuates between frames; some frame requires more
> > and some do not and within a frame, the workload also fluctuates
> > heavily and the tasks within a frame are usually not parallelized; they
> > are processed through specific H/W blocks, not CPU cores. They often
> > have PPMU capabilities; however, they need to be polled very frequently
> > in order to let DVFS mechanisms react properly. (less than 5ms).
> >
> > For such specific tasks, allowing them to request QoS requirements seems
> > adequete because DVFS mechanisms (as long as the polling rate is 5ms or
> > longer) cannot follow up with them. Besides, the device drivers know
> > when to request and cancel QoS exactly.
> >
> > 2. DVFS_LATENCY
> >
> > Both CPUFreq and Devfreq have response latency to a sudden workload
> > increase. With near-100% (e.g., 95%) up-threshold, the average response
> > latency is approximately 1.5 x polling-rate.
> >
> > A specific polling rate (e.g., 100ms) may generally fit for its system;
> > however, there could be exceptions for that. For example,
> > - When a user input suddenly starts: typing, clicking, moving cursors, and
> >  such, the user might need the full performance immediately. However,
> >  we do not know whether the full performance is actually needed or not
> >  until we calculate the utilization; thus, we need to calculate it
> >  faster with user inputs or any similar events. Specifying QoS on CPU
> >  processing power or Memory bandwidth at every user input is an
> >  overkill because there are many cases where such speed-up isn't
> >  necessary.
> > - When a device driver needs a faster performance response from DVFS
> >  mechanism. This could be addressed by simply putting QoS requests.
> >  However, such QoS requests may keep the system running fast
> >  unnecessary in some cases, especially if a) the device's resource
> >  usage bursts with some duration (e.g., 100ms-long bursts) and
> >  b) the driver doesn't know when such burst come. MMC/WiFi often had
> >  such behaviors although there are possibilities that part (b) might
> >  be addressed with further efforts.
> >
> > The cases shown above can be tackled with putting QoS requests on the
> > response time or latency of DVFS mechanism, which is directly related to
> > its polling interval (if the DVFS mechanism is polling based).
> >
> > Signed-off-by: MyungJoo Ham <myungjoo.ham@...sung.com>
> > Signed-off-by: Kyungmin Park <kyungmin.park@...sung.com>
> 
> In this PM-QoS patch, register_pm_qos_misc() for the new classes in
> pm_qos_power_init() is missing.
> 
> Those will be included in the next version of the patch.

Has the new version been posted already?  I seem to have missed it if so.

Thanks,
Rafael
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