linux-kernel - Re: [PATCH blk-next 1/2] blk-mq-rdma: Delete not-used multi-queue RDMA map queue code

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Message-ID: <20201005083817.GA14908@lst.de>
Date:   Mon, 5 Oct 2020 10:38:17 +0200
From:   Christoph Hellwig <hch@....de>
To:     Sagi Grimberg <sagi@...mberg.me>
Cc:     Christoph Hellwig <hch@....de>, Leon Romanovsky <leon@...nel.org>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <kbusch@...nel.org>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-rdma@...r.kernel.org
Subject: Re: [PATCH blk-next 1/2] blk-mq-rdma: Delete not-used multi-queue
 RDMA map queue code

On Fri, Oct 02, 2020 at 01:20:35PM -0700, Sagi Grimberg wrote:
>> Well, why would they change it?  The whole point of the infrastructure
>> is that there is a single sane affinity setting for a given setup. Now
>> that setting needed some refinement from the original series (e.g. the
>> current series about only using housekeeping cpus if cpu isolation is
>> in use).  But allowing random users to modify affinity is just a receipe
>> for a trainwreck.
>
> Well allowing people to mangle irq affinity settings seem to be a hard
> requirement from the discussions in the past.
>
>> So I think we need to bring this back ASAP, as doing affinity right
>> out of the box is an absolute requirement for sane performance without
>> all the benchmarketing deep magic.
>
> Well, it's hard to say that setting custom irq affinity settings is
> deemed non-useful to anyone and hence should be prevented. I'd expect
> that irq settings have a sane default that works and if someone wants to
> change it, it can but there should be no guarantees on optimal
> performance. But IIRC this had some dependencies on drivers and some
> more infrastructure to handle dynamic changes...

The problem is that people change random settings.  We need to generalize
it into a sane API (e.g. the housekeeping CPUs thing which totally makes
sense).