Re: [PATCH 0/5] blk-mq: allow to run queue if queue refcount is held

[Ming Lei <ming.lei@redhat.com>]

Hi Dongli,

On Mon, Apr 01, 2019 at 01:05:46PM +0800, Dongli Zhang wrote:
> 
> 
> On 4/1/19 10:52 AM, Ming Lei wrote:
> > On Sun, Mar 31, 2019 at 07:39:17PM -0700, Bart Van Assche wrote:
> >> On 3/31/19 7:00 PM, Ming Lei wrote:
> >>> On Sun, Mar 31, 2019 at 08:27:35AM -0700, Bart Van Assche wrote:
> >>>> I'm not sure the approach of this patch series is really the direction we
> >>>> should pursue. There are many block driver that free resources immediately
> >>>
> >>> Please see scsi_run_queue(), and the queue refcount is always held
> >>> before run queue.
> >>
> >> That's not correct. There is no guarantee that q->q_usage_counter > 0 when
> >> scsi_run_queue() is called from inside scsi_requeue_run_queue().
> > 
> > We don't need the guarantee of 'q->q_usage_counter > 0', I mean the
> > queue's kobj reference counter.
> > 
> > What we need is to allow run queue to work correctly after queue is frozen
> > or cleaned up.
> > 
> >>
> >>>> I'd like to avoid having to modify all block drivers that free resources
> >>>> immediately after blk_cleanup_queue() has returned. Have you considered to
> >>>> modify blk_mq_run_hw_queues() such that it becomes safe to call that
> >>>> function while blk_cleanup_queue() is in progress, e.g. by inserting a
> >>>> percpu_ref_tryget_live(&q->q_usage_counter) /
> >>>> percpu_ref_put(&q->q_usage_counter) pair?
> >>>
> >>> It can't work because blk_mq_run_hw_queues may happen after
> >>> percpu_ref_exit() is done.
> >>>
> >>> However, if we move percpu_ref_exit() into queue's release handler, we
> >>> don't need to grab q->q_usage_counter any more in blk_mq_run_hw_queues(),
> >>> and we still have to free hw queue resources in queue's release handler,
> >>> that is exactly what this patchset is doing.
> >>>
> >>> In short, getting q->q_usage_counter doesn't make a difference on this
> >>> issue.
> >>
> >> percpu_ref_tryget_live() fails if a per-cpu counter is in the "dead" state.
> >> percpu_ref_kill() changes the state of a per-cpu counter to the "dead"
> >> state. blk_freeze_queue_start() calls percpu_ref_kill(). blk_cleanup_queue()
> >> already calls blk_set_queue_dying() and that last function calls
> >> blk_freeze_queue_start(). So I think that what you wrote is not correct and
> >> that inserting a percpu_ref_tryget_live()/percpu_ref_put() pair in
> >> blk_mq_run_hw_queues() or blk_mq_run_hw_queue() would make a difference and
> >> also that moving the percpu_ref_exit() call into blk_release_queue() makes
> >> sense.
> > 
> > If percpu_ref_exit() is moved to blk_release_queue(), we still need to
> > move freeing of hw queue's resource into blk_release_queue() like what
> > the patchset is doing.
> 
> Hi Ming,
> 
> Would you mind help explain why we still need to move freeing of hw queue's
> resource into blk_release_queue() like what the patchset is doing?
> 
> Let's assume there is no deadlock when percpu_ref_tryget_live() is used,

Could you explain why the assumption is true?

We have to run queue after starting to freeze queue for draining
allocated requests and making forward progress. Inside blk_freeze_queue_start(),
percpu_ref_kill() marks this ref as DEAD, then percpu_ref_tryget_live() returns
false, then queue won't be run.


Thanks, 
Ming