On Thu, 2017-08-24 at 14:52 +0800, Ming Lei wrote:
> On Tue, Aug 22, 2017 at 09:55:57PM +0000, Bart Van Assche wrote:
> > On Sat, 2017-08-05 at 14:56 +0800, Ming Lei wrote:
> > > + /*
> > > + * if there is q->queue_depth, all hw queues share
> > > + * this queue depth limit
> > > + */
> > > + if (q->queue_depth) {
> > > + queue_for_each_hw_ctx(q, hctx, i)
> > > + hctx->flags |= BLK_MQ_F_SHARED_DEPTH;
> > > + }
> > > +
> > > + if (!q->elevator)
> > > + goto exit;
> >
> > Hello Ming,
> >
> > It seems very fragile to me to set BLK_MQ_F_SHARED_DEPTH if and only if
> > q->queue_depth != 0. Wouldn't it be better to let the block driver tell the
> > block layer whether or not there is a queue depth limit across hardware
> > queues, e.g. through a tag set flag?
>
> One reason for not doing in that way is because q->queue_depth can be
> changed via sysfs interface.
Only the SCSI core allows the queue depth to be changed through sysfs. The
other block drivers I am familiar with set the queue depth when the block
layer queue is created and do not allow the queue depth to be changed later
on.
> Another reason is that better to not exposing this flag to drivers since
> it isn't necessary, that said it is an internal flag actually.
As far as I know only the SCSI core can create request queues that have a
queue depth that is lower than the number of tags in the tag set. So for all
block drivers except the SCSI core it is OK to dispatch all requests at once
to which a hardware tag has been assigned. My proposal is either to let
drivers like the SCSI core set BLK_MQ_F_SHARED_DEPTH or to modify
blk_mq_sched_dispatch_requests() such that it flushes all requests if the
request queue depth is not lower than the hardware tag set size instead of if
q->queue_depth == 0.
Bart.
