[PATCH 0/2] Trace completion of all bios

[PATCH 0/2] Trace completion of all bios

[NeilBrown]

Hi Jens,
 I think all objections to this patch have been answered so I'm
 resending.
 I've added a small cleanup patch first which makes some small
 enhancements to the documentation and #defines for the bio->flags
 field.

Thanks,
NeilBrown

---

NeilBrown (2):
      block: simple improvements for bio->flags
      block: trace completion of all bios.


 block/bio.c               |   13 +++++++++++++
 block/blk-core.c          |   10 +++++++++-
 drivers/md/dm.c           |    1 -
 drivers/md/raid5.c        |    2 --
 include/linux/blk_types.h |   24 +++++++++++++++---------
 5 files changed, 37 insertions(+), 13 deletions(-)

--
Signature

[PATCH 1/2] block: simple improvements for bio->flags

[NeilBrown]

The comment for the 'flags' field of 'bio' mentions
"command" which is no longer stored there, and doesn't
mention the bvec pool number, which is.

BIO_RESET_BITS is set in such a way that it would need to be
updated if new bits were added, which is easy to miss.

BVEC_POOL_BITS is larger than needed.  The BVEC_POOL_IDX()
ranges from 0 to 6, so 3 bits are sufficient.

This patch make improvements in each of these areas.

Signed-off-by: NeilBrown <neilb@suse.com>
---
 include/linux/blk_types.h |   22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index d703acb55d0f..ff07d891f38f 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 and bvec pool number */
 	unsigned short		bi_ioprio;
 
 	struct bvec_iter	bi_iter;
@@ -102,12 +102,7 @@ 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. */
-
-/*
- * Flags starting here get preserved by bio_reset() - this includes
- * BVEC_POOL_IDX()
- */
-#define BIO_RESET_BITS	10
+/* See BVEC_POOL_OFFSET below before adding new flags */
 
 /*
  * We support 6 different bvec pools, the last one is magic in that it
@@ -117,13 +112,22 @@ struct bio {
 #define BVEC_POOL_MAX		(BVEC_POOL_NR - 1)
 
 /*
- * Top 4 bits of bio flags indicate the pool the bvecs came from.  We add
+ * Top 3 bits of bio flags indicate the pool the bvecs came from.  We add
  * 1 to the actual index so that 0 indicates that there are no bvecs to be
  * freed.
  */
-#define BVEC_POOL_BITS		(4)
+#define BVEC_POOL_BITS		(3)
 #define BVEC_POOL_OFFSET	(16 - BVEC_POOL_BITS)
 #define BVEC_POOL_IDX(bio)	((bio)->bi_flags >> BVEC_POOL_OFFSET)
+#if (1<< BVEC_POOL_BITS) < (BVEC_POOL_NR+1)
+# error "BVEC_POOL_BITS is too small"
+#endif
+
+/*
+ * Flags starting here get preserved by bio_reset() - this includes
+ * only BVEC_POOL_IDX()
+ */
+#define BIO_RESET_BITS	BVEC_POOL_OFFSET
 
 /*
  * Operations and flags common to the bio and request structures.

[PATCH 2/2] block: trace completion of all bios.

[NeilBrown]

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        |    2 --
 include/linux/blk_types.h |    2 ++
 5 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 12c2837c4277..3db98fc3ee87 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1789,6 +1789,11 @@ 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 called 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)
 {
@@ -1809,6 +1814,11 @@ 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);
 }
@@ -1847,6 +1857,9 @@ 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 d772c221cc17..2a7993063a7e 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:
@@ -2601,6 +2607,8 @@ 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 dfb75979e455..cd93a3b9ceca 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 d8bd25f235dc..14557259dd69 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -5138,8 +5138,6 @@ 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);
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index ff07d891f38f..56c2a8180ce1 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -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. */
 /* See BVEC_POOL_OFFSET below before adding new flags */
 
 /*

[PATCH 2/2] block: trace completion of all bios.

[NeilBrown]

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        |    2 --
 include/linux/blk_types.h |    2 ++
 5 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 12c2837c4277..3db98fc3ee87 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1789,6 +1789,11 @@ 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 called 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)
 {
@@ -1809,6 +1814,11 @@ 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);
 }
@@ -1847,6 +1857,9 @@ 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 d772c221cc17..2a7993063a7e 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:
@@ -2601,6 +2607,8 @@ 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 dfb75979e455..cd93a3b9ceca 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 d8bd25f235dc..14557259dd69 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -5138,8 +5138,6 @@ 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);
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index ff07d891f38f..56c2a8180ce1 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -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. */
 /* See BVEC_POOL_OFFSET below before adding new flags */
 
 /*

Re: [PATCH 0/2] Trace completion of all bios

[Jens Axboe]

On 04/06/2017 07:10 PM, NeilBrown wrote:
> Hi Jens,
>  I think all objections to this patch have been answered so I'm
>  resending.
>  I've added a small cleanup patch first which makes some small
>  enhancements to the documentation and #defines for the bio->flags
>  field.

Added for 4.12. I hand applied 2/2, since it did not apply directly to
the 4.12 branch.

-- 
Jens Axboe

Re: [PATCH 0/2] Trace completion of all bios

[Jens Axboe]

On 04/06/2017 07:10 PM, NeilBrown wrote:
> Hi Jens,
>  I think all objections to this patch have been answered so I'm
>  resending.
>  I've added a small cleanup patch first which makes some small
>  enhancements to the documentation and #defines for the bio->flags
>  field.

Added for 4.12. I hand applied 2/2, since it did not apply directly to
the 4.12 branch.

-- 
Jens Axboe