linux-kernel - Re: [PATCH v3 1/3] blk-mq: always use __blk_mq_alloc

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Date:   Thu, 29 Jun 2023 15:40:03 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Christoph Hellwig <hch@....de>, chengming.zhou@...ux.dev
Cc:     axboe@...nel.dk, tj@...nel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, ming.lei@...hat.com
Subject: Re: [PATCH v3 1/3] blk-mq: always use __blk_mq_alloc_requests() to
 alloc and init rq

On 2023/6/29 13:28, Christoph Hellwig wrote:
> On Wed, Jun 28, 2023 at 08:45:44PM +0800, chengming.zhou@...ux.dev wrote:
>> After these cleanup, __blk_mq_alloc_requests() is the only entry to
>> alloc and init rq.
> 
> I find the code a little hard to follow now, due to the optional
> setting of the ctx.  We also introduce really odd behavior here
> if the caller for a hctx-specific allocation doesn't have free
> tags, as we'll now run into the normal retry path.
> 
> Is this really needed for your timestamp changes?  If not I'd prefer
> to skip it.
> 

Thanks for your review!

Since hctx-specific allocation path always has BLK_MQ_REQ_NOWAIT flag,
it won't retry.

But I agree, this makes the general __blk_mq_alloc_requests() more complex.

The reason is blk_mq_rq_ctx_init() has some data->rq_flags initialization:

```
if (data->flags & BLK_MQ_REQ_PM)
	data->rq_flags |= RQF_PM;
if (blk_queue_io_stat(q))
	data->rq_flags |= RQF_IO_STAT;
rq->rq_flags = data->rq_flags;
```

Because we need this data->rq_flags to tell if we need start_time_ns,
we need to put these initialization in the callers of blk_mq_rq_ctx_init().

Now we basically have two callers, the 1st is general __blk_mq_alloc_requests(),
the 2nd is the special blk_mq_alloc_request_hctx(). So I change the 2nd caller
to reuse the 1st __blk_mq_alloc_requests().

Or we put these data->rq_flags initialization in blk_mq_alloc_request_hctx() too?

Thanks.