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? > > 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. -- Thanks, Jeffle -- dm-devel mailing list dm-devel@xxxxxxxxxx https://listman.redhat.com/mailman/listinfo/dm-devel
