linux-kernel - Re: [PATCH v2 5/5] block/null_blk: add queue

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Date:   Sat, 23 Sep 2023 15:01:30 +0800
From:   Chengming Zhou <chengming.zhou@...ux.dev>
To:     Ming Lei <ming.lei@...hat.com>
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>
Subject: Re: [PATCH v2 5/5] block/null_blk: add queue_rqs() support

On 2023/9/22 16:54, Ming Lei wrote:
> On Wed, Sep 13, 2023 at 03:16:16PM +0000, chengming.zhou@...ux.dev wrote:
>> From: Chengming Zhou <zhouchengming@...edance.com>
>>
>> Add batched mq_ops.queue_rqs() support in null_blk for testing. The
>> implementation is much easy since null_blk doesn't have commit_rqs().
>>
>> We simply handle each request one by one, if errors are encountered,
>> leave them in the passed in list and return back.
>>
>> There is about 3.6% improvement in IOPS of fio/t/io_uring on null_blk
>> with hw_queue_depth=256 on my test VM, from 1.09M to 1.13M.
> 
> I guess you pass 'shared_tags' to null_blk for the verification?
IIRC it should be "modprobe null_blk hw_queue_depth=256 nr_devices=2 shared_tags=1".

> 
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
>> ---
>>  drivers/block/null_blk/main.c | 20 ++++++++++++++++++++
>>  1 file changed, 20 insertions(+)
>>
>> diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
>> index 968090935eb2..79d6cd3c3d41 100644
>> --- a/drivers/block/null_blk/main.c
>> +++ b/drivers/block/null_blk/main.c
>> @@ -1750,6 +1750,25 @@ static blk_status_t null_queue_rq(struct blk_mq_hw_ctx *hctx,
>>  	return null_handle_cmd(cmd, sector, nr_sectors, req_op(rq));
>>  }
>>  
>> +static void null_queue_rqs(struct request **rqlist)
>> +{
>> +	struct request *requeue_list = NULL;
>> +	struct request **requeue_lastp = &requeue_list;
>> +	struct blk_mq_queue_data bd = { };
>> +	blk_status_t ret;
>> +
>> +	do {
>> +		struct request *rq = rq_list_pop(rqlist);
>> +
>> +		bd.rq = rq;
>> +		ret = null_queue_rq(rq->mq_hctx, &bd);
>> +		if (ret != BLK_STS_OK)
>> +			rq_list_add_tail(&requeue_lastp, rq);
>> +	} while (!rq_list_empty(*rqlist));
>> +
>> +	*rqlist = requeue_list;
>> +}
>> +
> 
> null_blk may not be one perfect example for showing queue_rqs()
> which is usually for handling request in batch, but for test or
> demo purpose, it is fine:
> 
> Reviewed-by: Ming Lei <ming.lei@...hat.com>
> 

Yes, some other "real" drivers should be better choice that we can
handle more things in batch to improve performance. Maybe ublk driver
can benefit from this too.

Thanks!