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
