linux-kernel - Re: [PATCH] nvme/tcp: Add support to set the tcp worker cpu affinity

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Date:   Tue, 18 Apr 2023 11:39:49 +0800
From:   Li Feng <fengli@...rtx.com>
To:     Sagi Grimberg <sagi@...mberg.me>
Cc:     Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        "open list:NVM EXPRESS DRIVER" <linux-nvme@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] nvme/tcp: Add support to set the tcp worker cpu affinity

Hi Sagi,

> 2023年4月17日 下午9:45，Sagi Grimberg <sagi@...mberg.me> 写道：
> 
> Hey Li,
> 
>> The default worker affinity policy is using all online cpus, e.g. from 0
>> to N-1. However, some cpus are busy for other jobs, then the nvme-tcp will
>> have a bad performance.
>> This patch adds a module parameter to set the cpu affinity for the nvme-tcp
>> socket worker threads.  The parameter is a comma separated list of CPU
>> numbers.  The list is parsed and the resulting cpumask is used to set the
>> affinity of the socket worker threads.  If the list is empty or the
>> parsing fails, the default affinity is used.
> 
> I can see how this may benefit a specific set of workloads, but I have a
> few issues with this.
> 
> - This is exposing a user interface for something that is really
> internal to the driver.
> 
> - This is something that can be misleading and could be tricky to get
> right, my concern is that this would only benefit a very niche case.
Our storage products needs this feature~
If the user doesn’t know what this is, they can keep it default, so I thinks this is
not unacceptable.
> 
> - If the setting should exist, it should not be global.
V2 has fixed it.
> 
> - I prefer not to introduce new modparams.
> 
> - I'd prefer to find a way to support your use-case without introducing
> a config knob for it.
> 
I’m looking forward to it.
 
> - It is not backed by performance improvements, but more importantly
> does not cover any potential regressions in key metrics (bw/iops/lat)
> or lack there of.

I can do more tests if needed.

Thanks,
Feng Li