On Sat, Apr 22, 2023 at 08:08:41AM -0600, Jens Axboe wrote: > On 4/22/23 6:55?AM, Ming Lei wrote: > > On Fri, Apr 21, 2023 at 10:09:36PM +0000, Bernd Schubert wrote: > >> Hello, > >> > >> I was wondering if I could set up SQPOLL for fuse/IORING_OP_URING_CMD > >> and what would be the latency win. Now I get a bit confused what the > >> f_op->uring_cmd_iopoll() function is supposed to do. > >> > >> Is it just there to check if SQEs are can be completed as CQE? In rw.c > >> io_do_iopoll() it looks like this. I don't follow all code paths in > >> __io_sq_thread yet, but it looks a like it already checks if the ring > >> has new entries > >> > >> to_submit = io_sqring_entries(ctx); > >> ... > >> ret = io_submit_sqes(ctx, to_submit); > >> > >> --> it will eventually call into ->uring_cmd() ? > >> > >> And then io_do_iopoll -> file->f_op->uring_cmd_iopoll is supposed to > >> check for available cq entries and will submit these? I.e. I just return > >> 1 if when the request is ready? And also ensure that > >> req->iopoll_completed is set? > >> > >> > >> I'm also not sure what I should do with struct io_comp_batch * - I don't > >> have struct request *req_list anywhere in my fuse-uring changes, seems > >> to be blk-mq specific? So I should just ignore that parameter? > >> > >> > >> Btw, this might be useful for ublk as well? > > > > For the in-tree ublk driver, we need to copy data inside ->uring_cmd() > > between block request pages and user buffer, so SQPOLL may not be done > > because it isn't efficient for the kthread to copy on remote task mm > > space. However, ublk user copy feature[1](posted recently) doesn't > > need the copy in ->uring_cmd() any more, so SQPOLL becomes possible for > > ublk uring cmd. > > That hasn't been true for a long time, and isn't even true in > 5.10-stable anymore or anything newer. The SQPOLL thread is not a > kthread, and it doesn't need to do anything to copy the data that the > inline submission wouldn't also do. There is no "remote task mm". The > cost would be the same, outside of caching effects. OK, thanks for the clarification, and create_io_thread() does pass CLONE_VM, so there isn't remote task mm problem. However, ublk still can't use SETUP_SQPOLL so far, and problem is that ublk driver has to be bound with the user task for canceling pending commands when the ctx is gone[1]. When this issue is solved, SETUP_SQPOLL should work just fine. Given fuse takes similar approach with ublk, I believe fuse has similar limit too. Actually I was working on adding notifiers in io_uring[2] for addressing this issue so that driver needn't to use the trick for tracking io_uring context destroying. Just see one request double free issue(same request freed in io_submit_flush_completions<-io_fallback_req_func() twice) in case of DEFER_TASKRUN only, but driver actually calls io_uring_cmd_done() just once. Will investigate the issue further. [1] https://lore.kernel.org/linux-fsdevel/ZBxTdCj60+s1aZqA@xxxxxxxxxxxxxxxxxxxxxxxxx/ [2] https://github.com/ming1/linux/commits/for-6.4/io_uring_block Thanks, Ming
