On Thu, Apr 10, 2025 at 6:13 AM Jens Axboe <axboe@xxxxxxxxx> wrote: > > On 4/10/25 3:24 AM, Christoph Hellwig wrote: > > On Tue, Apr 08, 2025 at 08:49:54PM -0600, Caleb Sander Mateos wrote: > >> The ublk driver calls blk_mq_tag_to_rq() in several places. > >> blk_mq_tag_to_rq() tolerates an invalid tag for the tagset, checking it > >> against the number of tags and returning NULL if it is out of bounds. > >> But all the calls from the ublk driver have already verified the tag > >> against the ublk queue's queue depth. In ublk_commit_completion(), > >> ublk_handle_need_get_data(), and case UBLK_IO_COMMIT_AND_FETCH_REQ, the > >> tag has already been checked in __ublk_ch_uring_cmd(). In > >> ublk_abort_queue(), the loop bounds the tag by the queue depth. In > >> __ublk_check_and_get_req(), the tag has already been checked in > >> __ublk_ch_uring_cmd(), in the case of ublk_register_io_buf(), or in > >> ublk_check_and_get_req(). > >> > >> So just index the tagset's rqs array directly in the ublk driver. > >> Convert the tags to unsigned, as blk_mq_tag_to_rq() does. > > > > Poking directly into block layer internals feels like a really bad > > idea. If this is important enough we'll need a non-checking helper > > in the core code, but as with all these kinds of micro-optimizations > > it better have a really good justification. > > FWIW, I agree, and I also have a hard time imagining this making much of > a measurable difference. Caleb, was this based "well this seems > pointless" or was it something you noticed in profiling/testing? That's true, the nr_tags check doesn't show up super prominently in a CPU profile. The atomic reference counting in __ublk_check_and_get_req() or ublk_commit_completion() is significantly more expensive. Still, it seems like unnecessary work. nr_tags is in a different cache line from rqs, so there is the potential for a cache miss. And the prefetch() is another unnecessary cache miss in the cases where ublk doesn't access any of struct request's fields. I am happy to add a "blk_mq_tag_to_rq_unchecked()" helper to avoid accessing the blk-mq internals. Best, Caleb
