Re: [RFC] [PATCH 0/3] IRQ affinity notifier and per-cpu PM QoS

[Daniel Lezcano <daniel.lezcano@linaro.org>]

On 07/25/2014 06:55 PM, Lina Iyer wrote:
> This series of patches adds a new feature to allow per-cpu PM QoS.

Hi Lina,

this description lacks in the reason of why allowing per cpu PM QoS is 
needed. Could you give the context of the proposed feature ?

> The first of the patch, modifies the irq manager to allow multiple clients for
> IRQ SMP affinity change notification. Today, only one client can register a
> notification per IRQ. The PM QoS framework is also now interested in knowing
> when the SMP affinity changes for an IRQ.

Why ?

> With the current implementation, a
> second registration on the change notification releases the current
> notification callbacks and registers the new one. Modify the notification
> mechanism to use a list for notification instead of single data structure.

May be this is not necessary if the notification is always needed. Even 
if that breaks the code encapsulation, invoking directly a pm_qos 
function when the irq is changed, in addition of the notification, 
should suffice instead of adding a list of notifications with all the 
races that implies.

> Also, a client that wants to de-register from the notification will now need to
> call a separate API instead of the overloaded function call with a NULL
> argument.
>
> The next two patches re-organize PM QoS framework to allow QoS and the Dev PM
> QoS frameworks to specify a request type. Most requestors of PM QoS do not know
> or care about the CPU(s) the QoS needs to be effected. In many cases, it is
> still desirable to have the QoS apply on all available cpus. However, in
> conjunction with an IRQ balancer or a driver that has specific cpu(s)
> requirement for its use, can specify a QoS request only for that set of cpus.
> For example in a case, where a certain IRQ might need a performance QoS, but
> does not want to affect the general power consumption of all the cpus in the
> system, can specify an QoS request thats affine only to that cpu(s), where the
> IRQ can be triggered.
>
> The change adds ability to specify the PM QoS request types and two new PM QoS
> request types in addition to the default that applies to all cpus.
>
> PM_QOS_REQ_AFFINE_CORES: This allows drivers to specify a certain set of cpus
> that the request should be applied on.
>
> PM_QOS_REQ_AFFINE_IRQ: This allows drivers to specify an IRQ to which the QoS
> request can be tracked with. This uses the IRQ SMP affinity notification to set
> the cpumask of the affected cpus internally.
>
> The request defaults to PM_QOS_REQ_ALL_CORES when not explicitly specified and
> applies the request to all cpus.
>
> There is also a provision to read the QoS request value for a constraint, for a
> constraint for a set of cpus or just a cpu. CPUIdle governors can use this
> feature to get a QoS request for the cpu(s) they are interested in.

IMHO, it would be desirable first to implement a per cpu pm_qos without 
adding any extra new request or features, so we can comment it focused 
on how to do that.

> Lina Iyer (3):
>    irq: Allow multiple clients to register for irq affinity notification
>    QoS: Modify data structures and function arguments for scalability.
>    QoS: Enhance framework to support cpu/irq specific QoS requests
>
>   Documentation/power/pm_qos_interface.txt |  18 +++
>   drivers/base/power/qos.c                 |  14 +--
>   drivers/infiniband/hw/qib/qib_iba7322.c  |   4 +-
>   include/linux/interrupt.h                |  12 +-
>   include/linux/irq.h                      |   1 +
>   include/linux/irqdesc.h                  |   6 +-
>   include/linux/pm_qos.h                   |  23 +++-
>   kernel/irq/irqdesc.c                     |   1 +
>   kernel/irq/manage.c                      |  77 ++++++++-----
>   kernel/power/qos.c                       | 184 ++++++++++++++++++++++++++++++-
>   lib/cpu_rmap.c                           |   2 +-
>   11 files changed, 289 insertions(+), 53 deletions(-)
>


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