[dm-devel] should blk-mq halt requeue processing while queue is frozen?

[Mike Snitzer]

On Tue, Sep 13 2016 at  4:01am -0400,
Bart Van Assche <bart.vanassche at gmail.com> wrote:

> On 09/07/2016 06:41 PM, Mike Snitzer wrote:
> >On Fri, Sep 02 2016 at  6:42pm -0400,
> >Bart Van Assche <bart.vanassche at sandisk.com> wrote:
> >>+/**
> >>+ * blk_mq_quiesce_queue - wait until all pending queue_rq calls have finished
> >>+ *
> >>+ * Prevent that new I/O requests are queued and wait until all pending
> >>+ * queue_rq() calls have finished.
> >>+ */
> >>+void blk_mq_quiesce_queue(struct request_queue *q)
> >>+{
> >>+	spin_lock_irq(q->queue_lock);
> >>+	WARN_ON_ONCE(blk_queue_quiescing(q));
> >>+	queue_flag_set(QUEUE_FLAG_QUIESCING, q);
> >>+	spin_unlock_irq(q->queue_lock);
> >>+
> >>+	atomic_inc_return(&q->mq_freeze_depth);
> >>+	blk_mq_run_hw_queues(q, false);
> >>+	synchronize_rcu();
> >
> >Why the synchronize_rcu()?
> 
> Hello Mike,
> 
> Adding read_lock() + read_unlock() in __blk_mq_run_hw_queue() and
> synchronize_rcu() in blk_mq_quiesce_queue() is the lowest overhead
> mechanism I know of to make the latter function wait until the
> former has finished.

OK.

> >Also, you're effectively open-coding blk_mq_freeze_queue_start() minus
> >the q->q_usage_counter mgmt.  Why not add a flag to conditionally manage
> >q->q_usage_counter to blk_mq_freeze_queue_start()?
> 
> I will consider this.
> 
> >But I'm concerned about blk_mq_{quiesce,resume}_queue vs
> >blk_mq_{freeze,unfreeze}_queue -- e.g. if "freeze" is nested after
> >"queue" (but before "resume") it would still need the q->q_usage_counter
> >management.  Your patch as-is would break the blk-mq freeze interface.
> 
> Agreed. blk_mq_{quiesce,resume}_queue() has to manipulate
> q_usage_counter in the same way as blk_mq_{freeze,unfreeze}_queue().
> Once I am back in the office I will rework this patch and send it to
> Jens.

Please base any further work in this area ontop of
http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=devel

And please verify all the mptest tests pass with your changes in place.