On Fri, May 21, 2021 at 01:20:16PM +0200, Jan Kara wrote:
> On Fri 21-05-21 09:29:33, Ming Lei wrote:
> > On Thu, May 20, 2021 at 01:25:28PM +0200, Jan Kara wrote:
> > > Provided the device driver does not implement dispatch budget accounting
> > > (which only SCSI does) the loop in __blk_mq_do_dispatch_sched() pulls
> > > requests from the IO scheduler as long as it is willing to give out any.
> > > That defeats scheduling heuristics inside the scheduler by creating
> > > false impression that the device can take more IO when it in fact
> > > cannot.
> >
> > So hctx->dispatch_busy isn't set as true in this case?
>
> No. blk_mq_update_dispatch_busy() has:
>
> if (hctx->queue->elevator)
> return;
ooops, the above check should have been killed in commit 6e6fcbc27e77
("blk-mq: support batching dispatch in case of io"), :-(
Thanks,
Ming
