linux-kernel - Re: [PATCH v2 0/5] PM QoS: Add CPU affinity latency QoS support and resctrl integration

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Message-ID: <c4d4c458-bc64-4ca9-9c65-3e942825eac5@quicinc.com>
Date: Fri, 15 Aug 2025 20:48:43 +0800
From: Zhongqiu Han <quic_zhonhan@...cinc.com>
To: <rafael@...nel.org>, <lenb@...nel.org>, <pavel@...nel.org>,
        <tony.luck@...el.com>, <reinette.chatre@...el.com>,
        <Dave.Martin@....com>, <james.morse@....com>, <ulf.hansson@...aro.org>,
        <amit.kucheria@...aro.org>, <christian.loehle@....com>
CC: <linux-pm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <zhongqiu.han@....qualcomm.com>
Subject: Re: [PATCH v2 0/5] PM QoS: Add CPU affinity latency QoS support and
 resctrl integration

On 7/21/2025 8:40 PM, Zhongqiu Han wrote:
> Hi all,
> 
> This patch series introduces support for CPU affinity-based latency
> constraints in the PM QoS framework. The motivation is to allow
> finer-grained power management by enabling latency QoS requests to target
> specific CPUs, rather than applying system-wide constraints.
> 
> The current PM QoS framework supports global and per-device CPU latency
> constraints. However, in many real-world scenarios, such as IRQ affinity
> or CPU-bound kernel threads, only a subset of CPUs are
> performance-critical. Applying global constraints in such cases
> unnecessarily prevents other CPUs from entering deeper C-states, leading
> to increased power consumption.
> 
> This series addresses that limitation by introducing a new interface that
> allows latency constraints to be applied to a CPU mask. This is
> particularly useful on heterogeneous platforms (e.g., big.LITTLE) and
> embedded systems where power efficiency is critical for example:
> 
>                          driver A       rt kthread B      module C
>    CPU IDs (mask):         0-3              2-5              6-7
>    target latency(us):     20               30               100
>                            |                |                |
>                            v                v                v
>                            +---------------------------------+
>                            |        PM  QoS  Framework       |
>                            +---------------------------------+
>                            |                |                |
>                            v                v                v
>    CPU IDs (mask):        0-3            2-3,4-5            6-7
>    runtime latency(us):   20             20, 30             100
> 
> The current implementation includes only cpu_affinity_latency_qos_add()
> and cpu_affinity_latency_qos_remove() interfaces. An update interface is
> planned for future submission, along with PM QoS optimizations in the UFS
> subsystem.
> 
> Patch1 introduces the core support for CPU affinity latency QoS in the PM
> QoS framework.
> 
> Patch2 removes redundant KERN_ERR prefixes in WARN() calls in the global
> CPU PM QoS interface. This change addresses issues in existing code and is
> not related to the new interface introduced in this patch series.
> 
> Patch3 adds documentation for the new interface.
> 
> Patch4 fixes a minor documentation issue related to the return type of
> cpu_latency_qos_request_active(). This change addresses issues in existing
> doc and is not related to the new interface introduced in this patch
> series.
> 
> Patch5 updates the resctrl pseudo-locking logic to use the new CPU
> affinity latency QoS helpers, improving clarity and consistency. The only
> functional and beneficial change is that the new interface actively wakes
> up CPUs whose latency QoS values have changed, ensuring the latency limit
> takes effect immediately.

Hi Rafael,
I hope you're doing well. I just wanted to kindly check in regarding
current patch I submitted a while ago.

I understand things can get busy, and there's absolutely no rush — just
wanted to make sure it hasn't been missed.

Thank you~


> 
> Changes since v1:
> - Rebased on top of current next.
> - Resolve the compilation warning due to a missing static function
>    declaration.
> - Remove the conditional compilation based on CONFIG_CPU_IDLE and make it
>    depend solely on CONFIG_PM.
> - Add support for cpu_affinity_latency_qos_active.
> - Remove cpu_affinity_latency_qos_update; will reintroduce it when needed
>    in the future.
> - Optimize the code, for example by using cpu_affinity_latency_qos_active
>    inside the add/remove functions to enhance robustness.
> - Refine the commit message and fix a few minor issues unrelated to this
>    series.
> - Refactor the CPU latency PM QoS logic of resctrl pseudo_lock using the
>    interfaces provided by this series.
> - Link to v1: https://lore.kernel.org/all/20250424095228.1112558-1-quic_zhonhan@quicinc.com/
> 
> Zhongqiu Han (5):
>    PM: QoS: Add support for CPU affinity latency PM QoS
>    PM: QOS: Remove unnecessary KERN_ERR on WARN() calls
>    Documentation: PM: QoS: Add CPU affinity latency PM QoS Interface
>      documentation
>    Documentation: PM: QoS: Fix return type and return value description
>    resctrl: Replace PM QoS logic with cpu_affinity_latency_qos_* helpers
> 
>   Documentation/power/pm_qos_interface.rst |  63 ++++++++-
>   fs/resctrl/pseudo_lock.c                 |  51 +------
>   include/linux/pm_qos.h                   |  40 ++++++
>   include/linux/resctrl.h                  |   3 +-
>   kernel/power/qos.c                       | 166 ++++++++++++++++++++++-
>   5 files changed, 268 insertions(+), 55 deletions(-)
> 
> 
> base-commit: 024e09e444bd2b06aee9d1f3fe7b313c7a2df1bb


-- 
Thx and BRs,
Zhongqiu Han