linux-kernel - Re: [PATCH v2 1/5] blk-mq: account active requests when get driver tag

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Date:   Sat, 16 Sep 2023 17:23:10 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     chengming.zhou@...ux.dev
Cc:     axboe@...nel.dk, hch@....de, bvanassche@....org, kbusch@...nel.org,
        mst@...hat.com, damien.lemoal@...nsource.wdc.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Chengming Zhou <zhouchengming@...edance.com>,
        ming.lei@...hat.com
Subject: Re: [PATCH v2 1/5] blk-mq: account active requests when get driver
 tag

On Wed, Sep 13, 2023 at 03:16:12PM +0000, chengming.zhou@...ux.dev wrote:
> From: Chengming Zhou <zhouchengming@...edance.com>
> 
> There is a limit that batched queue_rqs() can't work on shared tags
> queue, since the account of active requests can't be done there.
> 
> Now we account the active requests only in blk_mq_get_driver_tag(),
> which is not the time we get driver tag actually (with none elevator).
> 
> To support batched queue_rqs() on shared tags queue, we move the
> account of active requests to where we get the driver tag:
> 
> 1. none elevator: blk_mq_get_tags() and blk_mq_get_tag()
> 2. other elevator: __blk_mq_alloc_driver_tag()
> 
> This is clearer and match with the unaccount side, which just happen
> when we put the driver tag.
> 
> The other good point is that we don't need RQF_MQ_INFLIGHT trick
> anymore, which used to avoid double account of flush request.
> Now we only account when actually get the driver tag, so all is good.
> We will remove RQF_MQ_INFLIGHT in the next patch.

RQF_MQ_INFLIGHT is only set for BLK_MQ_F_TAG_QUEUE_SHARED, so we can
avoid the extra atomic accounting for !BLK_MQ_F_TAG_QUEUE_SHARED.

But now your patch switches to account unconditionally by removing
RQF_MQ_INFLIGHT, not friendly for !BLK_MQ_F_TAG_QUEUE_SHARED.

Thanks, 
Ming