linux-kernel - Re: [PATCH] block: kyber: make kyber more friendly with merging

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Message-ID: <CACVXFVNLq7CiBMTDPRcHLNZBWLYXWCpdb4G0iqBUon9f-GpsAA@mail.gmail.com>
Date:   Wed, 30 May 2018 17:13:00 +0800
From:   Ming Lei <tom.leiming@...il.com>
To:     "jianchao.wang" <jianchao.w.wang@...cle.com>
Cc:     Omar Sandoval <osandov@...ndov.com>, Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] block: kyber: make kyber more friendly with merging

On Wed, May 30, 2018 at 4:36 PM, jianchao.wang
<jianchao.w.wang@...cle.com> wrote:
> Hi ming
>
> Thanks for your kindly response.
>
> On 05/30/2018 04:22 PM, Ming Lei wrote:
>>>> you could keep the software queues as-is but add our own version of
>>>> flush_busy_ctxs() that only removes requests of the domain that we want.
>>>> If one domain gets backed up, that might get messy with long iterations,
>>>> though.
>>> Yes, I also considered this approach :)
>>> But the long iterations on every ctx->rq_list looks really inefficient.
>> Right, this list can be quite long if dispatch token is used up.
>>
>> You might try to introduce per-domain list into ctx directly, then 'none'
>> may benefit from this change too since bio merge should be done
>> on the per-domain list actually.
>
> Yes, it maybe good for merging of 'none', because the rq_list is split into 3
> lists, and not need to iterate the whole rq_list any more.
> But what's about the dispatch when there is no io scheduler.

blk_mq_flush_busy_ctxs() and blk_mq_dequeue_from_ctx() should work
fine in case of 'none' if per-domain list is added to ctx. Then we can make
none to be a bit fair on READ/WRITE.

> We will dispatch request from ctx one by one at the moment.
> If we have per-domain list in ctx, we have to introduce some policies to determine
> which domain to dispatch, and these policies should be in io scheduler actually.

The policy is done by IO scheduler, and you can just pick up request
from ctx/domain list easily by introducing one blk-mq core API.

Thanks,
Ming Lei