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Date: Wed, 7 Mar 2012 19:47:22 -0800
From: mark gross <markgross@...gnar.org>
To: MyungJoo Ham <myungjoo.ham@...sung.com>
Cc: "Rafael J. Wysocki" <rjw@...k.pl>,
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>,
markgross <markgross@...gnar.org>, 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 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,
> --
> 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
--
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