linux-kernel - Re: [PATCH 0/6] blk-mq: optimize the queue

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Date:   Thu, 24 Aug 2023 10:02:30 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     chengming.zhou@...ux.dev, axboe@...nel.dk, hch@....de,
        ming.lei@...hat.com, kbusch@...nel.org
Cc:     mst@...hat.com, sagi@...mberg.me, damien.lemoal@...nsource.wdc.com,
        kch@...dia.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, zhouchengming@...edance.com
Subject: Re: [PATCH 0/6] blk-mq: optimize the queue_rqs() support

On 8/24/23 07:43, chengming.zhou@...ux.dev wrote:
> From: Chengming Zhou <zhouchengming@...edance.com>
> 
> The current queue_rqs() support has limitation that it can't work on
> shared tags queue, which is resolved by patch 1-3. We move the account
> of active requests to where we really allocate the driver tag.
> 
> This is clearer and matched with the unaccount side which now happen
> when we put the driver tag. And we can remove RQF_MQ_INFLIGHT, which
> was used to avoid double account problem of flush request.
> 
> Another problem is that the driver that support queue_rqs() has to
> set inflight request table by itself, which is resolved in patch 4.
> 
> The patch 5 fixes a potential race problem which may cause false
> timeout because of the reorder of rq->state and rq->deadline.
> 
> The patch 6 add support queue_rqs() for null_blk, which showed a
> 3.6% IOPS improvement in fio/t/io_uring benchmark on my test VM.
> And we also use it for testing queue_rqs() on shared tags queue.

Hi Jens and Christoph,

This patch series would be simplified significantly if the code for
fair tag allocation would be removed first
(https://lore.kernel.org/linux-block/20230103195337.158625-1-bvanassche@acm.org/, 
January 2023).
It has been proposed to improve fair tag sharing but the complexity of
the proposed alternative is scary
(https://lore.kernel.org/linux-block/20230618160738.54385-1-yukuai1@huaweicloud.com/, 
June 2023).
  Does everyone agree with removing the code for fair tag sharing - code
that significantly hurts performance of UFS devices and code that did
not exist in the legacy block layer?

Thanks,

Bart.