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

From: mark gross <[email-masked]>
Date: Wed, 7 Mar 2012 19:47:22 -0800

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,
> -- 
> 1.7.4.1
>

The cpu_dma_throughput looks ok to me.  I do however; wonder about the
dvfs_lat_pm_qos.  Should that knob be exposed to user mode?  Does that
matter so much?  why can't dvfs_lat use the cpu_dma_lat?

BTW I'll be out of town for the next 10 days and probably will not get
to this email account until I get home.

--mark