Re: [PATCH 2/2] blk-mq: add flag for drivers wanting blocking ->queue_rq()

On Thu, Sep 22, 2016 at 08:53:00AM -0600, Jens Axboe wrote:

If a driver sets BLK_MQ_F_BLOCKING, it is allowed to block in its
->queue_rq() handler. For that case, blk-mq ensures that we always
calls it from a safe context.

First can you provide a more useful defintion of blocking?  Lots of current
drivers will already block in ->queue_rq, mostly to allocate memory.

Second we looked at something similar a few times ago, mosty notably
when converting loop and rbd, and came to the conclusion that
performance sucks when we only have that single per-hctx work struct
for a busy device.  I think Ming has done a lot of the benchmarking,
so I've added him to Cc.