On Thu, Apr 01, 2021 at 10:19:23AM +0800, Ming Lei wrote:
> Currently bio based IO polling needs to poll all hw queue blindly, this
> way is very inefficient, and one big reason is that we can't pass any
> bio submission result to blk_poll().
>
> In IO submission context, track associated underlying bios by per-task
> submission queue and store returned 'cookie' in
> bio->bi_iter.bi_private_data, and return current->pid to caller of
> submit_bio() for any bio based driver's IO, which is submitted from FS.
>
> In IO poll context, the passed cookie tells us the PID of submission
> context, then we can find bios from the per-task io pull context of
> submission context. Moving bios from submission queue to poll queue of
> the poll context, and keep polling until these bios are ended. Remove
> bio from poll queue if the bio is ended. Add bio flags of BIO_DONE and
> BIO_END_BY_POLL for such purpose.
>
> In was found in Jeffle Xu's test that kfifo doesn't scale well for a
> submission queue as queue depth is increased, so a new mechanism for
> tracking bios is needed. So far bio's size is close to 2 cacheline size,
> and it may not be accepted to add new field into bio for solving the
> scalability issue by tracking bios via linked list, switch to bio group
> list for tracking bio, the idea is to reuse .bi_end_io for linking bios
> into a linked list for all sharing same .bi_end_io(call it bio group),
> which is recovered before ending bio really, since BIO_END_BY_POLL is
> added for enhancing this point. Usually .bi_end_bio is same for all
> bios in same layer, so it is enough to provide very limited groups, such
> as 16 or less for fixing the scalability issue.
>
> Usually submission shares context with io poll. The per-task poll context
> is just like stack variable, and it is cheap to move data between the two
> per-task queues.
>
> Also when the submission task is exiting, drain pending IOs in the context
> until all are done.
>
> Tested-by: Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx>
> Reviewed-by: Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> ---
> block/bio.c | 5 +
> block/blk-core.c | 155 +++++++++++++++++++++++-
> block/blk-ioc.c | 3 +
> block/blk-mq.c | 244 +++++++++++++++++++++++++++++++++++++-
> block/blk.h | 10 ++
> include/linux/blk_types.h | 26 +++-
> 6 files changed, 439 insertions(+), 4 deletions(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index 26b7f721cda8..04c043dc60fc 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1402,6 +1402,11 @@ static inline bool bio_remaining_done(struct bio *bio)
> **/
> void bio_endio(struct bio *bio)
> {
> + /* BIO_END_BY_POLL has to be set before calling submit_bio */
> + if (bio_flagged(bio, BIO_END_BY_POLL)) {
> + bio_set_flag(bio, BIO_DONE);
> + return;
> + }
Why can't driver that implements bio based polling call a separate
bio_endio_polled instead?
> +static inline void *bio_grp_data(struct bio *bio)
> +{
> + return bio->bi_poll;
> +}
What is the purpose of this helper? And why does it have to lose the
type information?
