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

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Message-ID: <2379088e-e5d0-4766-9968-756aad04f9a3@arm.com>
Date: Mon, 28 Jul 2025 11:09:22 +0100
From: Christian Loehle <christian.loehle@....com>
To: Zhongqiu Han <quic_zhonhan@...cinc.com>, 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
Cc: linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/5] PM QoS: Add CPU affinity latency QoS support and
 resctrl integration

On 7/21/25 13:40, 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.

So what's needed for the UFS use-case additionally?
Would adding that here be too much?