On Sat, Apr 17, 2021 at 10:06:53PM +0800, JeffleXu wrote: > > > On 4/16/21 5:07 PM, Ming Lei wrote: > > On Fri, Apr 16, 2021 at 04:00:37PM +0800, Jeffle Xu wrote: > >> Hi, > >> How about this patch to remove the extra poll_capable() method? > >> > >> And the following 'dm: support IO polling for bio-based dm device' needs > >> following change. > >> > >> ``` > >> + /* > >> + * Check for request-based device is remained to > >> + * dm_mq_init_request_queue()->blk_mq_init_allocated_queue(). > >> + * For bio-based device, only set QUEUE_FLAG_POLL when all underlying > >> + * devices supporting polling. > >> + */ > >> + if (__table_type_bio_based(t->type)) { > >> + if (dm_table_supports_poll(t)) { > >> + blk_queue_flag_set(QUEUE_FLAG_POLL_CAP, q); > >> + blk_queue_flag_set(QUEUE_FLAG_POLL, q); > >> + } > >> + else { > >> + blk_queue_flag_clear(QUEUE_FLAG_POLL, q); > >> + blk_queue_flag_clear(QUEUE_FLAG_POLL_CAP, q); > >> + } > >> + } > >> ``` > > > > Frankly speaking, I don't see any value of using QUEUE_FLAG_POLL_CAP for > > DM, and the result is basically subset of treating DM as always being capable > > of polling. > > > > Also underlying queue change(either limits or flag) won't be propagated > > to DM/MD automatically. Strictly speaking it doesn't matter if all underlying > > queues are capable of supporting polling at the exact time of 'write sysfs/poll', > > cause any of them may change in future. > > > > So why not start with the simplest approach(always capable of polling) > > which does meet normal bio based polling requirement? > > > > I find one scenario where this issue may matter. Consider the scenario > where HIPRI bios are submitted to DM device though **all** underlying > devices has been disabled for polling. In this case, a **valid** cookie > (pid of current submitting process) is still returned. Then if @spin of > the following blk_poll() is true, blk_poll() will get stuck in dead loop > because blk_mq_poll() always returns 0, since previously submitted bios > are all enqueued into IRQ hw queue. > > Maybe you need to re-remove the bio from the poll context if the > returned cookie is BLK_QC_T_NONE? It won't be one issue, see blk_bio_poll_preprocess() which is called from submit_bio_checks(), so any bio's HIPRI will be cleared if the queue doesn't support POLL, that code does cover underlying bios. > > > Something like: > > -static blk_qc_t __submit_bio_noacct(struct bio *bio) > +static blk_qc_t __submit_bio_noacct_ctx(struct bio *bio, struct > io_context *ioc) > { > struct bio_list bio_list_on_stack[2]; > blk_qc_t ret = BLK_QC_T_NONE; > @@ -1047,7 +1163,15 @@ static blk_qc_t __submit_bio_noacct(struct bio *bio) > bio_list_on_stack[1] = bio_list_on_stack[0]; > bio_list_init(&bio_list_on_stack[0]); > > if (ioc && queue_is_mq(q) && (bio->bi_opf & REQ_HIPRI)) { REQ_HIPRI won't be set for underlying bios which queue doesn't support poll, so this branch won't be reached. And the submission queue will be empty, and blk_poll() for DM/MD(bio based queue) checks nothing, but the polling won't be stopped until the iocb is completed. And this handling is actually same with current polling on IRQ based queue. > bool queued = blk_bio_poll_prep_submit(ioc, bio); > > ret = __submit_bio(bio); > + if (queued && !blk_qc_t_valid(ret)) > /* TODO:remove bio from poll_context */ > > bio_set_private_data(bio, ret); > } else { > ret = __submit_bio(bio); > } > > > Then if you'd like fix this in this way, the returned value of > .submit_bio() of DM/MD also needs to return BLK_QC_T_NONE now. Currently > .submit_bio() of DM actually returns the cookie of the last split bio > (to underlying mq deivice). I am a bit confused, this patch requires .submit_bio() of DM/MD(bio based queue) to return either 0 or pid of the submission task. Thanks, Ming -- dm-devel mailing list dm-devel@xxxxxxxxxx https://listman.redhat.com/mailman/listinfo/dm-devel