Re: [PATCH 04/14] blk-mq: pass in request/bio flags to queue mapping

From: Jens Axboe
Date: Mon Oct 29 2018 - 13:33:39 EST

On 10/29/18 11:30 AM, Bart Van Assche wrote:
> On Mon, 2018-10-29 at 10:37 -0600, Jens Axboe wrote:
>> @@ -400,9 +402,15 @@ void blk_mq_sched_insert_requests(struct request_queue *q,
>> struct blk_mq_ctx *ctx,
>> struct list_head *list, bool run_queue_async)
>> {
>> - struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu);
>> - struct elevator_queue *e = hctx->queue->elevator;
>> + struct blk_mq_hw_ctx *hctx;
>> + struct elevator_queue *e;
>> + struct request *rq;
>> +
>> + /* For list inserts, requests better be on the same hw queue */
>> + rq = list_first_entry(list, struct request, queuelist);
>> + hctx = blk_mq_map_queue(q, rq->cmd_flags, ctx->cpu);
> Passing all request cmd_flags bits to blk_mq_map_queue() makes it possible
> for that function to depend on every single cmd_flags bit even if different
> requests have different cmd_flags. Have you considered to pass the hw_ctx
> type only to blk_mq_map_queue() to avoid that that function would start
> depending on other cmd_flags?

The core only knows about the number of types, not what each type means
nor how to map it outside of using the mapping functions. So I don't
want to expose this is an explicit type, as that would then mean that
blk-mq had to know about them.

> Additionally, what guarantees that all requests in queuelist have the same
> hw_ctx type? If a later patch will guarantee that, please mention that in
> the comment about list_first_entry().

When the code is introduced, it's always the same hctx. Later on when we
do support multiple sets, the user of the list insert (plugging) explicitly
makes sure that a list only contains requests for the same hardware queue.

I'll improve the comment.

Jens Axboe