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SubjectRe: [PATCH v3] block: trace completion of all bios.
On Fri, Mar 24, 2017 at 8:07 AM, NeilBrown <neilb@suse.com> wrote:
>
> Currently only dm and md/raid5 bios trigger
> trace_block_bio_complete(). Now that we have bio_chain() and
> bio_inc_remaining(), it is not possible, in general, for a driver to
> know when the bio is really complete. Only bio_endio() knows that.
>
> So move the trace_block_bio_complete() call to bio_endio().
>
> Now trace_block_bio_complete() pairs with trace_block_bio_queue().
> Any bio for which a 'queue' event is traced, will subsequently
> generate a 'complete' event.
>
> There are a few cases where completion tracing is not wanted.
> 1/ If blk_update_request() has already generated a completion
> trace event at the 'request' level, there is no point generating
> one at the bio level too. In this case the bi_sector and bi_size
> will have changed, so the bio level event would be wrong
>
> 2/ If the bio hasn't actually been queued yet, but is being aborted
> early, then a trace event could be confusing. Some filesystems
> call bio_endio() but do not want tracing.
>
> 3/ The bio_integrity code interposes itself by replacing bi_end_io,
> then restoring it and calling bio_endio() again. This would produce
> two identical trace events if left like that.
>
> To handle these, we introduce a flag BIO_TRACE_COMPLETION and only
> produce the trace event when this is set.
> We address point 1 above by clearing the flag in blk_update_request().
> We address point 2 above by only setting the flag when
> generic_make_request() is called.
> We address point 3 above by clearing the flag after generating a
> completion event.
>
> When bio_split() is used on a bio, particularly in blk_queue_split(),
> there is an extra complication. A new bio is split off the front, and
> may be handle directly without going through generic_make_request().
> The old bio, which has been advanced, is passed to
> generic_make_request(), so it will trigger a trace event a second
> time.
> Probably the best result when a split happens is to see a single
> 'queue' event for the whole bio, then multiple 'complete' events - one
> for each component. To achieve this was can:
> - copy the BIO_TRACE_COMPLETION flag to the new bio in bio_split()
> - avoid generating a 'queue' event if BIO_TRACE_COMPLETION is already set.
> This way, the split-off bio won't create a queue event, the original
> won't either even if it re-submitted to generic_make_request(),
> but both will produce completion events, each for their own range.
>
> So if generic_make_request() is called (which generates a QUEUED
> event), then bi_endio() will create a single COMPLETE event for each
> range that the bio is split into, unless the driver has explicitly
> requested it not to.
>
> Signed-off-by: NeilBrown <neilb@suse.com>
> ---
> block/bio.c | 13 +++++++++++++
> block/blk-core.c | 10 +++++++++-
> drivers/md/dm.c | 1 -
> drivers/md/raid5.c | 8 --------
> include/linux/blk_types.h | 4 +++-
> 5 files changed, 25 insertions(+), 11 deletions(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index 5eec5e08417f..c1272986133e 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1818,6 +1818,11 @@ static inline bool bio_remaining_done(struct bio *bio)
> * bio_endio() will end I/O on the whole bio. bio_endio() is the preferred
> * way to end I/O on a bio. No one should call bi_end_io() directly on a
> * bio unless they own it and thus know that it has an end_io function.
> + *
> + * bio_endio() can be called several times on a bio that has been chained
> + * using bio_chain(). The ->bi_end_io() function will only be call the
> + * last time. At this point the BLK_TA_COMPLETE tracing event will be
> + * generated if BIO_TRACE_COMPLETION is set.
> **/
> void bio_endio(struct bio *bio)
> {
> @@ -1838,6 +1843,11 @@ void bio_endio(struct bio *bio)
> goto again;
> }
>
> + if (bio->bi_bdev && bio_flagged(bio, BIO_TRACE_COMPLETION)) {
> + trace_block_bio_complete(bdev_get_queue(bio->bi_bdev),
> + bio, bio->bi_error);
> + bio_clear_flag(bio, BIO_TRACE_COMPLETION);
> + }
> if (bio->bi_end_io)
> bio->bi_end_io(bio);
> }
> @@ -1876,6 +1886,9 @@ struct bio *bio_split(struct bio *bio, int sectors,
>
> bio_advance(bio, split->bi_iter.bi_size);
>
> + if (bio_flagged(bio, BIO_TRACE_COMPLETION))
> + bio_set_flag(bio, BIO_TRACE_COMPLETION);
> +
> return split;
> }
> EXPORT_SYMBOL(bio_split);
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 0eeb99ef654f..b34b5b1b1bbf 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1936,7 +1936,13 @@ generic_make_request_checks(struct bio *bio)
> if (!blkcg_bio_issue_check(q, bio))
> return false;
>
> - trace_block_bio_queue(q, bio);
> + if (!bio_flagged(bio, BIO_TRACE_COMPLETION)) {
> + trace_block_bio_queue(q, bio);
> + /* Now that enqueuing has been traced, we need to trace
> + * completion as well.
> + */
> + bio_set_flag(bio, BIO_TRACE_COMPLETION);
> + }
> return true;
>
> not_supported:
> @@ -2595,6 +2601,8 @@ bool blk_update_request(struct request *req, int error, unsigned int nr_bytes)
> if (bio_bytes == bio->bi_iter.bi_size)
> req->bio = bio->bi_next;
>
> + /* Completion has already been traced */
> + bio_clear_flag(bio, BIO_TRACE_COMPLETION);
> req_bio_endio(req, bio, bio_bytes, error);
>
> total_bytes += bio_bytes;
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index f4ffd1eb8f44..f5f09ace690a 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -810,7 +810,6 @@ static void dec_pending(struct dm_io *io, int error)
> queue_io(md, bio);
> } else {
> /* done with normal IO or empty flush */
> - trace_block_bio_complete(md->queue, bio, io_error);
> bio->bi_error = io_error;
> bio_endio(bio);
> }
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 9a3b7da34137..f684cb566721 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -5141,8 +5141,6 @@ static void raid5_align_endio(struct bio *bi)
> rdev_dec_pending(rdev, conf->mddev);
>
> if (!error) {
> - trace_block_bio_complete(bdev_get_queue(raid_bi->bi_bdev),
> - raid_bi, 0);
> bio_endio(raid_bi);
> if (atomic_dec_and_test(&conf->active_aligned_reads))
> wake_up(&conf->wait_for_quiescent);
> @@ -5727,10 +5725,6 @@ static void raid5_make_request(struct mddev *mddev, struct bio * bi)
> md_write_end(mddev);
> remaining = raid5_dec_bi_active_stripes(bi);
> if (remaining == 0) {
> -
> -
> - trace_block_bio_complete(bdev_get_queue(bi->bi_bdev),
> - bi, 0);
> bio_endio(bi);
> }
> }
> @@ -6138,8 +6132,6 @@ static int retry_aligned_read(struct r5conf *conf, struct bio *raid_bio)
> }
> remaining = raid5_dec_bi_active_stripes(raid_bio);
> if (remaining == 0) {
> - trace_block_bio_complete(bdev_get_queue(raid_bio->bi_bdev),
> - raid_bio, 0);
> bio_endio(raid_bio);
> }
> if (atomic_dec_and_test(&conf->active_aligned_reads))
> diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
> index d703acb55d0f..db7a57ee0e58 100644
> --- a/include/linux/blk_types.h
> +++ b/include/linux/blk_types.h
> @@ -29,7 +29,7 @@ struct bio {
> * top bits REQ_OP. Use
> * accessors.
> */
> - unsigned short bi_flags; /* status, command, etc */
> + unsigned short bi_flags; /* status, etc */
> unsigned short bi_ioprio;
>
> struct bvec_iter bi_iter;
> @@ -102,6 +102,8 @@ struct bio {
> #define BIO_REFFED 8 /* bio has elevated ->bi_cnt */
> #define BIO_THROTTLED 9 /* This bio has already been subjected to
> * throttling rules. Don't do it again. */
> +#define BIO_TRACE_COMPLETION 10 /* bio_endio() should trace the final completion
> + * of this bio. */

This may not be a good idea, since the flag space is quite small(12).

Thanks,
Ming

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