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

Ming Lei
2023-09-16T09:25:18Z

On Wed, Sep 13, 2023 at 03:16:12PM +0000, chengming.zhou@linux.dev wrote:
> From: Chengming Zhou <zhouchengming@bytedance.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