linux-kernel - Re: [PATCH v2] nvme/tcp: Add support to set the tcp worker cpu affinity

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Date:   Mon, 17 Apr 2023 16:05:18 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Li Feng <lifeng1519@...il.com>
Cc:     Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        "open list:NVM EXPRESS DRIVER" <linux-nvme@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, ming.lei@...hat.com
Subject: Re: [PATCH v2] nvme/tcp: Add support to set the tcp worker cpu
 affinity

On Mon, Apr 17, 2023 at 03:50:46PM +0800, Li Feng wrote:
> 
> 
> > 2023年4月17日 下午3:37，Ming Lei <ming.lei@...hat.com> 写道：
> > 
> > On Thu, Apr 13, 2023 at 09:29:41PM +0800, Li Feng wrote:
> >> 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.
> > 
> > Can you explain in detail how nvme-tcp performs worse in this situation?
> > 
> > If some of CPUs are knows as busy, you can submit the nvme-tcp io jobs
> > on other non-busy CPUs via taskset, or scheduler is supposed to choose
> > proper CPUs for you. And usually nvme-tcp device should be saturated
> > with limited io depth or jobs/cpus.
> > 
> > 
> > Thanks, 
> > Ming
> > 
> 
> Taskset can’t work on nvme-tcp io-queues, because the worker cpu has decided at the nvme-tcp ‘connect’ stage,
> not the sending io stage. Assume there is only one io-queue, the binding cpu is CPU0, no matter io jobs
> run other cpus.

OK, looks the problem is on queue->io_cpu, see nvme_tcp_queue_request().

But I am wondering why nvme-tcp doesn't queue the io work on the current
cpu? And why is queue->io_cpu introduced? Given blk-mq defines cpu
affinities for each hw queue, driver is supposed to submit IO request
to hardware on the local CPU.

Sagi and Guys, any ideas about introducing queue->io_cpu?


Thanks,
Ming