linux-kernel - Re: [PATCH] PM: QoS: Add support for CPU affinity mask-based CPUs latency QoS

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Message-ID: <fe68d36d-f8b2-42ac-88d9-aeabf3499fd0@quicinc.com>
Date: Fri, 25 Apr 2025 19:20:10 +0800
From: Zhongqiu Han <quic_zhonhan@...cinc.com>
To: Christian Loehle <christian.loehle@....com>, <rafael@...nel.org>,
        <pavel@...nel.org>, <len.brown@...el.com>
CC: <linux-pm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PM: QoS: Add support for CPU affinity mask-based CPUs
 latency QoS

On 4/24/2025 6:25 PM, Christian Loehle wrote:
> On 4/24/25 10:52, Zhongqiu Han wrote:
>> Currently, the PM QoS framework supports global CPU latency QoS and
>> per-device CPU latency QoS requests. An example of using global CPU
>> latency QoS is a commit 2777e73fc154 ("scsi: ufs: core: Add CPU latency
>> QoS support for UFS driver") that improved random io performance by 15%
>> for ufs on specific device platform.
>>
>> However, this prevents all CPUs in the system from entering C states.
>> Typically, some threads or drivers know which specific CPUs they are
>> interested in. For example, drivers with IRQ affinity only want interrupts
>> to wake up and be handled on specific CPUs. Similarly, kernel thread bound
>> to specific CPUs through affinity only care about the latency of those
>> particular CPUs.
>>
>> This patch introduces support for partial CPUs PM QoS using a CPU affinity
>> mask, allowing flexible and more precise latency QoS settings for specific
>> CPUs. This can help save power, especially on heterogeneous platforms with
>> big and little cores, as well as some power-conscious embedded systems for
>> example:
>>
>>                    driver A       rt kthread B      module C
>> QoS cpu mask:       0-3              2-5              6-7
>> target latency:     20               30               50
>>                      |                |                |
>>                      v                v                v
>>                      +---------------------------------+
>>                      |        PM  QoS  Framework       |
>>                      +---------------------------------+
>>                      |                |                |
>>                      v                v                v
>> cpu mask:          0-3            2-3,4-5            6-7
>> actual latency:    20             20, 30             50
>>
>> Implement this support based on per-device CPU latency PM QoS.
>>
>> Signed-off-by: Zhongqiu Han <quic_zhonhan@...cinc.com>
> 
> I like the idea!
> The interface does need an in-tree user, why not convert the UFS driver?
> 

Thanks Christian for the review.

As far as I know, the UFS IRQ affinity varies across different
platforms, so a universal solution is needed (we need to investigate
whether there is already a solution or add parameters like intr_mask to
represent the IRQ affinity mask). Let me investigate it, or raise other
user patches as a patch series in PATCH v2 as soon as possible. Thanks


-- 
Thx and BRs,
Zhongqiu Han