[dm-devel] [RFC PATCH 4/4] dm: support bio polling
Ming Lei
ming.lei at redhat.com
Thu Jun 17 02:14:30 UTC 2021
On Wed, Jun 16, 2021 at 12:05:13PM -0400, Mike Snitzer wrote:
> On Wed, Jun 16 2021 at 9:05P -0400,
> Ming Lei <ming.lei at redhat.com> wrote:
>
> > Support bio(REQ_POLLED) polling in the following approach:
> >
> > 1) setup one list in instance of 'struct dm_io', adds every 'struct
> > dm_target_io' instance cloned for current dm bio into this list;
> > store the list in 1) into bio->bi_bio_drv_data
> >
> > 2) hold one refcnt on io->io_count after submitting this dm bio with
> > REQ_POLLED
> >
> > 4) implement .poll_bio() callback, and iterate over the list in 1) and
> > polled on each ->clone of 'dm_target_io' instance; call dec_pending()
> > if all target ios are done in .poll_bio().
> >
> > 4) enable QUEUE_FLAG_POLL if all underlying queues enable QUEUE_FLAG_POLL,
> > which is based on Jeffle's previous patch.
> >
> > Signed-off-by: Ming Lei <ming.lei at redhat.com>
>
> Thanks for refreshing this DM bio polling support Ming.
>
> In general I'm really happy to see polling switch over to using bios,
> nice job Christoph! Are you hoping for all this to land in time for
> 5.14 merge?
>
> Once Ming responds to my review inlined below, and I Acked-by his
> set, would you be willing to fold it at the end of your patchset so
> that I don't need to rebase on block to get these changes in, etc?
>
> Mike
>
> > ---
> > drivers/md/dm-table.c | 24 ++++++++++++++++++
> > drivers/md/dm.c | 59 ++++++++++++++++++++++++++++++++++++++++---
> > 2 files changed, 79 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> > index ee47a332b462..b14b379442d2 100644
> > --- a/drivers/md/dm-table.c
> > +++ b/drivers/md/dm-table.c
> > @@ -1491,6 +1491,12 @@ struct dm_target *dm_table_find_target(struct dm_table *t, sector_t sector)
> > return &t->targets[(KEYS_PER_NODE * n) + k];
> > }
> >
> > +static int device_not_poll_capable(struct dm_target *ti, struct dm_dev *dev,
> > + sector_t start, sector_t len, void *data)
> > +{
> > + return !blk_queue_poll(bdev_get_queue(dev->bdev));
> > +}
> > +
> > /*
> > * type->iterate_devices() should be called when the sanity check needs to
> > * iterate and check all underlying data devices. iterate_devices() will
> > @@ -1541,6 +1547,11 @@ static int count_device(struct dm_target *ti, struct dm_dev *dev,
> > return 0;
> > }
> >
> > +static int dm_table_supports_poll(struct dm_table *t)
> > +{
> > + return !dm_table_any_dev_attr(t, device_not_poll_capable, NULL);
> > +}
> > +
> > /*
> > * Check whether a table has no data devices attached using each
> > * target's iterate_devices method.
> > @@ -2078,6 +2089,19 @@ void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
> >
> > dm_update_keyslot_manager(q, t);
> > blk_queue_update_readahead(q);
> > +
> > + /*
> > + * Check for request-based device is remained to
> > + * dm_mq_init_request_queue()->blk_mq_init_allocated_queue().
> > + * For bio-based device, only set QUEUE_FLAG_POLL when all underlying
> > + * devices supporting polling.
> > + */
> > + if (__table_type_bio_based(t->type)) {
> > + if (dm_table_supports_poll(t))
> > + blk_queue_flag_set(QUEUE_FLAG_POLL, q);
> > + else
> > + blk_queue_flag_clear(QUEUE_FLAG_POLL, q);
> > + }
> > }
> >
> > unsigned int dm_table_get_num_targets(struct dm_table *t)
> > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > index 363f12a285ce..0a0e4a38f435 100644
> > --- a/drivers/md/dm.c
> > +++ b/drivers/md/dm.c
> > @@ -84,6 +84,7 @@ struct dm_target_io {
> > struct dm_target *ti;
> > unsigned target_bio_nr;
> > unsigned *len_ptr;
> > + struct list_head list;
> > bool inside_dm_io;
> > struct bio clone;
> > };
> > @@ -99,6 +100,7 @@ struct dm_io {
> > blk_status_t status;
> > atomic_t io_count;
> > struct bio *orig_bio;
> > + struct list_head poll_head;
> > unsigned long start_time;
> > spinlock_t endio_lock;
> > struct dm_stats_aux stats_aux;
> > @@ -655,6 +657,11 @@ static struct dm_io *alloc_io(struct mapped_device *md, struct bio *bio)
> > io->md = md;
> > spin_lock_init(&io->endio_lock);
> >
> > + if (bio->bi_opf & REQ_POLLED) {
> > + bio->bi_bio_drv_data = io;
> > + INIT_LIST_HEAD(&io->poll_head);
> > + }
> > +
> > start_io_acct(io);
> >
> > return io;
> > @@ -692,6 +699,8 @@ static struct dm_target_io *alloc_tio(struct clone_info *ci, struct dm_target *t
> >
> > static void free_tio(struct dm_target_io *tio)
> > {
> > + list_del_init(&tio->list);
> > +
> > if (tio->inside_dm_io)
> > return;
> > bio_put(&tio->clone);
> > @@ -936,10 +945,15 @@ static void dec_pending(struct dm_io *io, blk_status_t error)
> > io_error = io->status;
> > bio = io->orig_bio;
> > end_io_acct(io);
> > +
> > free_io(md, io);
> >
> > - if (io_error == BLK_STS_DM_REQUEUE)
> > + if (io_error == BLK_STS_DM_REQUEUE) {
> > + /* not poll any more in case of requeue */
> > + if (bio->bi_opf & REQ_POLLED)
> > + bio->bi_opf &= ~REQ_POLLED;
> > return;
> > + }
> >
> > if ((bio->bi_opf & REQ_PREFLUSH) && bio->bi_iter.bi_size) {
> > /*
> > @@ -1043,7 +1057,9 @@ static void clone_endio(struct bio *bio)
> > up(&md->swap_bios_semaphore);
> > }
> >
> > - free_tio(tio);
> > + /* Any cloned bio submitted as POLLED, free them all after dm_io is done */
> > + if (list_empty(&tio->list))
> > + free_tio(tio);
> > dec_pending(io, error);
> > }
> >
> > @@ -1300,6 +1316,11 @@ static void __map_bio(struct dm_target_io *tio)
> > struct dm_io *io = tio->io;
> > struct dm_target *ti = tio->ti;
> >
> > + if (clone->bi_opf & REQ_POLLED)
> > + list_add_tail(&tio->list, &io->poll_head);
> > + else
> > + INIT_LIST_HEAD(&tio->list);
> > +
>
> Why not INIT_LIST_HEAD() at end of alloc_tio()? Shouldn't that be done
> even if you have this else claue here because you can clear REQ_POLLED
> on BLK_STS_DM_REQUEUE? (otherwise you're calling list_add_tail on list
> that wasn't ever INIT_LIST_HEAD).
It is fine to add one un-initialized list node via list_add_tail(), just
the list itself has to be initialized well.
>
> > clone->bi_end_io = clone_endio;
> >
> > /*
> > @@ -1666,8 +1687,9 @@ static void __split_and_process_bio(struct mapped_device *md,
> > }
> > }
> >
> > - /* drop the extra reference count */
> > - dec_pending(ci.io, errno_to_blk_status(error));
> > + /* drop the extra reference count for non-POLLED bio */
> > + if (!(bio->bi_opf & REQ_POLLED))
> > + dec_pending(ci.io, errno_to_blk_status(error));
> > }
> >
> > static void dm_submit_bio(struct bio *bio)
> > @@ -1707,6 +1729,34 @@ static void dm_submit_bio(struct bio *bio)
> > dm_put_live_table(md, srcu_idx);
> > }
> >
> > +static int dm_poll_bio(struct bio *bio, unsigned int flags)
> > +{
> > + struct dm_io *io = bio->bi_bio_drv_data;
> > + struct dm_target_io *tio;
> > +
> > + if (!(bio->bi_opf & REQ_POLLED) || !io)
> > + return 0;
>
> Should this be a WARN_ON()? Cannot see why this would ever happen
> other than a bug? Or is there some race that makes it more likely?
REQ_POLLED can be cleared in case of requeue or blk_queue_split(), however
the upper layer still keeps polling. And we need to return simply for
bios which will be completed via IRQ.
Thanks,
Ming
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