Subject: Re: [PATCH v3] PM / QoS: Introduce new classes: DMA-Throughput and DVFS-Latency

From: "Rafael J. Wysocki" <[email-masked]>
Date: Sat, 10 Mar 2012 23:25:57 +0100

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 <[email-masked]>
> > Signed-off-by: Kyungmin Park <[email-masked]>
> > 
> > --
> > 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