Re: [md PATCH 09/14] md/raid10: stop using bi_phys_segments

Jack Wang
2017-02-16T14:26:45Z

2017-02-16 5:39 GMT+01:00 NeilBrown <neilb@suse.com>:
> raid10 currently repurposes bi_phys_segments on each
> incoming bio to count how many r10bio was used to encode the
> request.
>
> We need to know when the number of attached r10bio reaches
> zero to:
> 1/ call bio_endio() when all IO on the bio is finished
> 2/ decrement ->nr_pending so that resync IO can proceed.
>
> Now that the bio has its own __bi_remaining counter, that
> can be used instead. We can call bio_inc_remaining to
> increment the counter and call bio_endio() every time an
> r10bio completes, rather than only when bi_phys_segments
> reaches zero.
>
> This addresses point 1, but not point 2. bio_endio()
> doesn't (and cannot) report when the last r10bio has
> finished, so a different approach is needed.
>
> So: instead of counting bios in ->nr_pending, count r10bios.
> i.e. every time we attach a bio, increment nr_pending.
> Every time an r10bio completes, decrement nr_pending.
>
> Normally we only increment nr_pending after first checking
> that ->barrier is zero, or some other non-trivial tests and
> possible waiting. When attaching multiple r10bios to a bio,
> we only need the tests and the waiting once. After the
> first increment, subsequent increments can happen
> unconditionally as they are really all part of the one
> request.
>
> So introduce inc_pending() which can be used when we know
> that nr_pending is already elevated.
>
> Signed-off-by: NeilBrown <neilb@suse.com>
> ---
> drivers/md/raid10.c | 76 +++++++++++++++++----------------------------------
> 1 file changed, 25 insertions(+), 51 deletions(-)
>
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 9258cbe233bb..6b4d8643c574 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -301,27 +301,18 @@ static void reschedule_retry(struct r10bio *r10_bio)
> static void raid_end_bio_io(struct r10bio *r10_bio)
> {
> struct bio *bio = r10_bio->master_bio;
> - int done;
> struct r10conf *conf = r10_bio->mddev->private;
>
> - if (bio->bi_phys_segments) {
> - unsigned long flags;
> - spin_lock_irqsave(&conf->device_lock, flags);
> - bio->bi_phys_segments--;
> - done = (bio->bi_phys_segments == 0);
> - spin_unlock_irqrestore(&conf->device_lock, flags);
> - } else
> - done = 1;
> if (!test_bit(R10BIO_Uptodate, &r10_bio->state))
> bio->bi_error = -EIO;
> - if (done) {
> - bio_endio(bio);
> - /*
> - * Wake up any possible resync thread that waits for the device
> - * to go idle.
> - */
> - allow_barrier(conf);
> - }
> +
> + /*
> + * Wake up any possible resync thread that waits for the device
> + * to go idle.
> + */
> + allow_barrier(conf);
> + bio_endio(bio);
> +

Hi Neil,

Why do you switch the order of above 2 lines, is there a reason
behind, I notice in raid1 you kept the order?

Regards,
Jack

> free_r10bio(r10_bio);
> }
>
> @@ -984,6 +975,15 @@ static void wait_barrier(struct r10conf *conf)
> spin_unlock_irq(&conf->resync_lock);
> }
>
> +static void inc_pending(struct r10conf *conf)
> +{
> + /* The current request requires multiple r10_bio, so
> + * we need to increment the pending count.
> + */
> + WARN_ON(!atomic_read(&conf->nr_pending));
> + atomic_inc(&conf->nr_pending);
> +}
> +
> static void allow_barrier(struct r10conf *conf)
> {
> if ((atomic_dec_and_test(&conf->nr_pending)) ||
> @@ -1161,12 +1161,8 @@ static void raid10_read_request(struct mddev *mddev, struct bio *bio,
> sectors_handled = (r10_bio->sector + max_sectors
> - bio->bi_iter.bi_sector);
> r10_bio->sectors = max_sectors;
> - spin_lock_irq(&conf->device_lock);
> - if (bio->bi_phys_segments == 0)
> - bio->bi_phys_segments = 2;
> - else
> - bio->bi_phys_segments++;
> - spin_unlock_irq(&conf->device_lock);
> + inc_pending(conf);
> + bio_inc_remaining(bio);
> /*
> * Cannot call generic_make_request directly as that will be
> * queued in __generic_make_request and subsequent
> @@ -1261,9 +1257,7 @@ static void raid10_write_request(struct mddev *mddev, struct bio *bio,
> * on which we have seen a write error, we want to avoid
> * writing to those blocks. This potentially requires several
> * writes to write around the bad blocks. Each set of writes
> - * gets its own r10_bio with a set of bios attached. The number
> - * of r10_bios is recored in bio->bi_phys_segments just as with
> - * the read case.
> + * gets its own r10_bio with a set of bios attached.
> */
>
> r10_bio->read_slot = -1; /* make sure repl_bio gets freed */
> @@ -1481,15 +1475,9 @@ static void raid10_write_request(struct mddev *mddev, struct bio *bio,
> */
>
> if (sectors_handled < bio_sectors(bio)) {
> - /* We need another r10_bio and it needs to be counted
> - * in bio->bi_phys_segments.
> - */
> - spin_lock_irq(&conf->device_lock);
> - if (bio->bi_phys_segments == 0)
> - bio->bi_phys_segments = 2;
> - else
> - bio->bi_phys_segments++;
> - spin_unlock_irq(&conf->device_lock);
> + /* We need another r10_bio and it needs to be counted */
> + inc_pending(conf);
> + bio_inc_remaining(bio);
> one_write_done(r10_bio);
> r10_bio = mempool_alloc(conf->r10bio_pool, GFP_NOIO);
>
> @@ -1518,16 +1506,6 @@ static void __make_request(struct mddev *mddev, struct bio *bio)
> r10_bio->sector = bio->bi_iter.bi_sector;
> r10_bio->state = 0;
>
> - /*
> - * We might need to issue multiple reads to different devices if there
> - * are bad blocks around, so we keep track of the number of reads in
> - * bio->bi_phys_segments. If this is 0, there is only one r10_bio and
> - * no locking will be needed when the request completes. If it is
> - * non-zero, then it is the number of not-completed requests.
> - */
> - bio->bi_phys_segments = 0;
> - bio_clear_flag(bio, BIO_SEG_VALID);
> -
> if (bio_data_dir(bio) == READ)
> raid10_read_request(mddev, bio, r10_bio);
> else
> @@ -2662,12 +2640,8 @@ static void handle_read_error(struct mddev *mddev, struct r10bio *r10_bio)
> r10_bio->sector + max_sectors
> - mbio->bi_iter.bi_sector;
> r10_bio->sectors = max_sectors;
> - spin_lock_irq(&conf->device_lock);
> - if (mbio->bi_phys_segments == 0)
> - mbio->bi_phys_segments = 2;
> - else
> - mbio->bi_phys_segments++;
> - spin_unlock_irq(&conf->device_lock);
> + bio_inc_remaining(mbio);
> + inc_pending(conf);
> generic_make_request(bio);
>
> r10_bio = mempool_alloc(conf->r10bio_pool,
>
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html