linux-kernel - Re: [PATCHSET v4] blk-mq-scheduling framework

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Date:   Thu, 22 Dec 2016 09:12:24 -0800
From:   Omar Sandoval <osandov@...ndov.com>
To:     Bart Van Assche <Bart.VanAssche@...disk.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "osandov@...com" <osandov@...com>, "axboe@...com" <axboe@...com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "paolo.valente@...aro.org" <paolo.valente@...aro.org>
Subject: Re: [PATCHSET v4] blk-mq-scheduling framework

On Thu, Dec 22, 2016 at 04:57:36PM +0000, Bart Van Assche wrote:
> On Thu, 2016-12-22 at 08:52 -0800, Omar Sandoval wrote:
> > This approach occurred to us, but we couldn't figure out a way to make
> > blk_mq_tag_to_rq() work with it. From skimming over the patches, I
> > didn't see a solution to that problem.
> 
> Hello Omar,
> 
> Can you clarify your comment? Since my patches initialize both tags->rqs[]
> and sched_tags->rqs[] the function blk_mq_tag_to_rq() should still work.
> 
> Bart.

Sorry, you're right, it does work, but tags->rqs[] ends up being the
extra lookup table. I suspect that the runtime overhead of keeping that
up to date could be worse than copying the rq fields if you have lots of
CPUs but only one hardware queue.