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Message-Id: <201203102325.57743.rjw@sisk.pl>
Date: Sat, 10 Mar 2012 23:25:57 +0100
From: "Rafael J. Wysocki" <rjw@...k.pl>
To: markgross@...gnar.org, MyungJoo Ham <myungjoo.ham@...sung.com>
Cc: Stephen Rothwell <sfr@...b.auug.org.au>,
Dave Jones <mavej@...hat.com>, linux-pm@...r.kernel.org,
"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
Kevin Hilman <khilman@...com>, Jean Pihet <j-pihet@...com>,
kyungmin.park@...sung.com, myungjoo.ham@...il.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] PM / QoS: Introduce new classes: DMA-Throughput and DVFS-Latency
On Thursday, March 08, 2012, mark gross wrote:
> On Wed, Mar 07, 2012 at 02:02:01PM +0900, MyungJoo Ham 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>
> >
> > --
> > Changes from v2
> > - Rebased on the recent PM QoS patches, resolving the merge conflict.
> >
> > Changes from RFC(v1)
> > - Added omitted part (registering new classes)
> > ---
> > include/linux/pm_qos.h | 4 ++++
> > kernel/power/qos.c | 31 ++++++++++++++++++++++++++++++-
> > 2 files changed, 34 insertions(+), 1 deletions(-)
> >
> > diff --git a/include/linux/pm_qos.h b/include/linux/pm_qos.h
> > index c8a541e..0ee7caa 100644
> > --- a/include/linux/pm_qos.h
> > +++ b/include/linux/pm_qos.h
> > @@ -14,6 +14,8 @@ enum {
> > PM_QOS_CPU_DMA_LATENCY,
> > PM_QOS_NETWORK_LATENCY,
> > PM_QOS_NETWORK_THROUGHPUT,
> > + PM_QOS_CPU_DMA_THROUGHPUT,
> > + PM_QOS_DVFS_RESPONSE_LATENCY,
> >
> > /* insert new class ID */
> > PM_QOS_NUM_CLASSES,
> > @@ -24,6 +26,8 @@ enum {
> > #define PM_QOS_CPU_DMA_LAT_DEFAULT_VALUE (2000 * USEC_PER_SEC)
> > #define PM_QOS_NETWORK_LAT_DEFAULT_VALUE (2000 * USEC_PER_SEC)
> > #define PM_QOS_NETWORK_THROUGHPUT_DEFAULT_VALUE 0
> > +#define PM_QOS_CPU_DMA_THROUGHPUT_DEFAULT_VALUE 0
> > +#define PM_QOS_DVFS_LAT_DEFAULT_VALUE (2000 * USEC_PER_SEC)
> > #define PM_QOS_DEV_LAT_DEFAULT_VALUE 0
> >
> > struct pm_qos_request {
> > diff --git a/kernel/power/qos.c b/kernel/power/qos.c
> > index d6d6dbd..3e122db 100644
> > --- a/kernel/power/qos.c
> > +++ b/kernel/power/qos.c
> > @@ -101,11 +101,40 @@ static struct pm_qos_object network_throughput_pm_qos = {
> > };
> >
> >
> > +static BLOCKING_NOTIFIER_HEAD(cpu_dma_throughput_notifier);
> > +static struct pm_qos_constraints cpu_dma_tput_constraints = {
> > + .list = PLIST_HEAD_INIT(cpu_dma_tput_constraints.list),
> > + .target_value = PM_QOS_CPU_DMA_THROUGHPUT_DEFAULT_VALUE,
> > + .default_value = PM_QOS_CPU_DMA_THROUGHPUT_DEFAULT_VALUE,
> > + .type = PM_QOS_MAX,
> > + .notifiers = &cpu_dma_throughput_notifier,
> > +};
> > +static struct pm_qos_object cpu_dma_throughput_pm_qos = {
> > + .constraints = &cpu_dma_tput_constraints,
> > + .name = "cpu_dma_throughput",
> > +};
> > +
> > +
> > +static BLOCKING_NOTIFIER_HEAD(dvfs_lat_notifier);
> > +static struct pm_qos_constraints dvfs_lat_constraints = {
> > + .list = PLIST_HEAD_INIT(dvfs_lat_constraints.list),
> > + .target_value = PM_QOS_DVFS_LAT_DEFAULT_VALUE,
> > + .default_value = PM_QOS_DVFS_LAT_DEFAULT_VALUE,
> > + .type = PM_QOS_MIN,
> > + .notifiers = &dvfs_lat_notifier,
> > +};
> > +static struct pm_qos_object dvfs_lat_pm_qos = {
> > + .constraints = &dvfs_lat_constraints,
> > + .name = "dvfs_latency",
> > +};
> > +
> > static struct pm_qos_object *pm_qos_array[] = {
> > &null_pm_qos,
> > &cpu_dma_pm_qos,
> > &network_lat_pm_qos,
> > - &network_throughput_pm_qos
> > + &network_throughput_pm_qos,
> > + &cpu_dma_throughput_pm_qos,
> > + &dvfs_lat_pm_qos,
> > };
> >
> > static ssize_t pm_qos_power_write(struct file *filp, const char __user *buf,
> >
>
> The cpu_dma_throughput looks ok to me.
I agree with Mark, but I'm not sure about the name. Specifically, I'm not sure
what the CPU has to do with that?
Rafael
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