Re: [dm-devel] [RFC PATCH 2/2] block: support to freeze bio based request queue

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

On Tue, Apr 20, 2021 at 03:21:55PM +0800, JeffleXu wrote:
> 
> 
> On 4/19/21 9:50 PM, Ming Lei wrote:
> > On Mon, Apr 19, 2021 at 08:05:46PM +0800, JeffleXu wrote:
> >>
> >>
> >> On 4/15/21 6:33 PM, Ming Lei wrote:
> >>> For bio based request queue, the queue usage refcnt is only grabbed
> >>> during submission, which isn't consistent with request base queue.
> >>>
> >>> Queue freezing has been used widely, and turns out it is very useful
> >>> to quiesce queue activity.
> >>>
> >>> Support to freeze bio based request queue by the following approach:
> >>>
> >>> 1) grab two queue usage refcount for blk-mq before submitting blk-mq
> >>> bio, one is for bio, anther is for request;
> >>
> >>
> >> Hi, I can't understand the sense of grabbing two refcounts on the
> >> @q_usage_count of the underlying blk-mq device, while
> >> @q_usage_count of the MD/DM device is kept untouched.
> > 
> > Follows the point:
> > 
> > 1) for blk-mq, we hold one refcount for bio and another for request, and
> > release one after ending bio or completing request.
> 
> Blk-mq has already implemented queue freezing semantics, even without
> this 'grabbing two refcount'. So is this just for the code consisdency
> with the bio-based queue?

Right.

> 
> 
> > 
> > 2) for bio based queue, just holding one refcount for bio, and release it
> > after the bio is ended.
> 
> OK.
> 
> > 
> > As I mentioned to you, the current in-tree code only grabs the refcount
> > during submitting bio for bio base queue, and the refcount is released
> > after returning from submission, see __submit_bio().
> 
> Yes. I ignored that the refcount grabbed in the entry of bio submission
> has been returned back when the submission completes for bio-based queue.
> 
> > 
> >>
> >> In the following calling stack
> >>
> >> ```
> >> queue_poll_store
> >> 	blk_mq_freeze_queue(q)
> >> ```
> >>
> >> Is the input @q still the request queue of MD/DM device?
> > 
> > It can be either one after bio based io polling is supported,
> > queue/io_poll is exposed for both blk-mq and bio based queue.
> > 
> > However, I guess bio based polling doesn't need such strict bio queue
> > freezing, cause QUEUE_FLAG_POLL is only read in submission path, so
> > looks current freezing just during submission is enough.
> 
> Not actually.
> 
> blk_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
>  	struct blk_mq_hw_ctx *hctx;
>  	long state;
> 
> -	if (!blk_qc_t_valid(cookie) || !blk_queue_poll(q))
> +	if (!blk_queue_poll(q) || (queue_is_mq(q) && !blk_qc_t_valid(cookie)))
> 
> Here QUEUE_FLAG_POLL is still checked in blk_poll() for bio-based queue,
> at least in your latest patch for bio-based polling.

OK, we can simply drop it.


Thanks,
Ming


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