On Tue, Sep 13 2016 at 4:01am -0400,
Bart Van Assche <bart.vanassche@xxxxxxxxx> 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@xxxxxxxxxxx> 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.
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