On 2/17/22 02:29, Chaitanya Kulkarni wrote:
> Only caller of alloc_cmd() is null_submit_bio() unconditionally sets
> second parameter to true and that is statically hard-coded in null_blk.
> There is no point in having statically hardcoded function parameter.
>
> Remove the unnecessary parameter can_wait and adjust the code so it
> can retain existing behavior of waiting when we don't get valid
> nullb_cmd from __alloc_cmd() in alloc_cmd().
>
> The restructured code avoids multiple return statements, multiple
> calls to __alloc_cmd() and resulting a fast path call to
> prepare_to_wait() due to removal of first alloc_cmd() call.
>
> Follow the pattern that we have in bio_alloc() to set the structure
> members in the structure allocation function in alloc_cmd() and pass
> bio to initialize newly allocated cmd->bio member.
>
> Follow the pattern in copy_to_nullb() to use result of one function call
> (null_cache_active()) to be used as a parameter to another function call
> (null_insert_page()), use result of alloc_cmd() as a first parameter to
> the null_handle_cmd() in null_submit_bio() function. This allow us to
> remove the local variable cmd on stack in null_submit_bio() that is in
> fast path.
>
> Signed-off-by: Chaitanya Kulkarni <kch@xxxxxxxxxx>
> ---
> drivers/block/null_blk/main.c | 29 ++++++++++++-----------------
> 1 file changed, 12 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
> index 90b6bd2a114b..29e183719e77 100644
> --- a/drivers/block/null_blk/main.c
> +++ b/drivers/block/null_blk/main.c
> @@ -720,26 +720,25 @@ static struct nullb_cmd *__alloc_cmd(struct nullb_queue *nq)
> return NULL;
> }
>
> -static struct nullb_cmd *alloc_cmd(struct nullb_queue *nq, int can_wait)
> +static struct nullb_cmd *alloc_cmd(struct nullb_queue *nq, struct bio *bio)
> {
> struct nullb_cmd *cmd;
> DEFINE_WAIT(wait);
>
> - cmd = __alloc_cmd(nq);
> - if (cmd || !can_wait)
> - return cmd;
> -
> do {
> - prepare_to_wait(&nq->wait, &wait, TASK_UNINTERRUPTIBLE);
> + /*
> + * This avoids multiple return statements, multiple calls to
> + * __alloc_cmd() and a fast path call to prepare_to_wait().
> + */
> cmd = __alloc_cmd(nq);
> - if (cmd)
> - break;
> -
> + if (cmd) {
> + cmd->bio = bio;
> + return cmd;
> + }
> + prepare_to_wait(&nq->wait, &wait, TASK_UNINTERRUPTIBLE);
> io_schedule();
> + finish_wait(&nq->wait, &wait);
> } while (1);
> -
> - finish_wait(&nq->wait, &wait);
> - return cmd;
> }
>
> static void end_cmd(struct nullb_cmd *cmd)
> @@ -1477,12 +1476,8 @@ static void null_submit_bio(struct bio *bio)
> sector_t nr_sectors = bio_sectors(bio);
> struct nullb *nullb = bio->bi_bdev->bd_disk->private_data;
> struct nullb_queue *nq = nullb_to_queue(nullb);
> - struct nullb_cmd *cmd;
> -
> - cmd = alloc_cmd(nq, 1);
> - cmd->bio = bio;
>
> - null_handle_cmd(cmd, sector, nr_sectors, bio_op(bio));
> + null_handle_cmd(alloc_cmd(nq, bio), sector, nr_sectors, bio_op(bio));
> }
>
> static bool should_timeout_request(struct request *rq)
I would have preferred the simple while () {} loop I suggested (this do
{ } while (1) is ugly...) but anyway, looks good.
Reviewed-by: Damien Le Moal <damien.lemoal@xxxxxxxxxxxxxxxxxx>
--
Damien Le Moal
Western Digital Research
