SQPOLL / uring_cmd_iopoll

[Bernd Schubert <bschubert@ddn.com>]

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?

Thanks,
Bernd