perf/x86/intel/pt: Fix buffer full but size is 0 case

commit 5b590160d2cf776b304eb054afafea2bd55e3620 upstream.

If the trace data buffer becomes full, a truncated flag [T] is reported in PERF_RECORD_AUX. In some cases, the size reported is 0, even though data must have been added to make the buffer full.

That happens when the buffer fills up from empty to full before the Intel PT driver has updated the buffer position. Then the driver calculates the new buffer position before calculating the data size. If the old and new positions are the same, the data size is reported as 0, even though it is really the whole buffer size.

Fix by detecting when the buffer position is wrapped, and adjust the data size calculation accordingly.

Example

Use a very small buffer size (8K) and observe the size of truncated [T] data. Before the fix, it is possible to see records of 0 size.

Before:

    $ perf record -m,8K -e intel_pt// uname
    Linux
    [ perf record: Woken up 2 times to write data ]
    [ perf record: Captured and wrote 0.105 MB perf.data ]
    $ perf script -D --no-itrace | grep AUX | grep -F '[T]'
    Warning:
    AUX data lost 2 times out of 3!

    5 19462712368111 0x19710 [0x40]: PERF_RECORD_AUX offset: 0 size: 0 flags: 0x1 [T]
    5 19462712700046 0x19ba8 [0x40]: PERF_RECORD_AUX offset: 0x170 size: 0xe90 flags: 0x1 [T]

After:

    $ perf record -m,8K -e intel_pt// uname
    Linux
    [ perf record: Woken up 3 times to write data ]
    [ perf record: Captured and wrote 0.040 MB perf.data ]
    $ perf script -D --no-itrace | grep AUX | grep -F '[T]'
    Warning:
    AUX data lost 2 times out of 3!

    1 113720802995 0x4948 [0x40]: PERF_RECORD_AUX offset: 0 size: 0x2000 flags: 0x1 [T]
    1 113720979812 0x6b10 [0x40]: PERF_RECORD_AUX offset: 0x2000 size: 0x2000 flags: 0x1 [T]

---
 arch/x86/events/intel/pt.c | 11 ++++++++---
 arch/x86/events/intel/pt.h |  2 ++
 2 files changed, 10 insertions(+), 3 deletions(-)

index 99b286b6c200..622bfb9a2fe7 100644
@@ -782,11 +782,13 @@ static void pt_buffer_advance(struct pt_buffer *buf)
 	buf->cur_idx++;
 
 	if (buf->cur_idx == buf->cur->last) {
-		if (buf->cur == buf->last)
+		if (buf->cur == buf->last) {
 			buf->cur = buf->first;
-		else
+			buf->wrapped = true;
+		} else {
 			buf->cur = list_entry(buf->cur->list.next, struct topa,
 					      list);
+		}
 		buf->cur_idx = 0;
 	}
 }

@@ -800,8 +802,11 @@ static void pt_buffer_advance(struct pt_buffer *buf)
 static void pt_update_head(struct pt *pt)
 {
 	struct pt_buffer *buf = perf_get_aux(&pt->handle);
+	bool wrapped = buf->wrapped;
 	u64 topa_idx, base, old;
 
+	buf->wrapped = false;
+
 	/* offset of the first region in this table from the beginning of buf */
 	base = buf->cur->offset + buf->output_off;
 

@@ -814,7 +819,7 @@ static void pt_update_head(struct pt *pt)
 	} else {
 		old = (local64_xchg(&buf->head, base) &
 		       ((buf->nr_pages << PAGE_SHIFT) - 1));
-		if (base < old)
+		if (base < old || (base == old && wrapped))
 			base += buf->nr_pages << PAGE_SHIFT;
 
 		local_add(base - old, &buf->data_size);

index c2d00a072952..51674454c69d 100644
@@ -64,6 +64,7 @@ struct pt_pmu {
  * @lost:	if data was lost/truncated
  * @head:	logical write offset inside the buffer
  * @snapshot:	if this is for a snapshot/overwrite counter
+ * @wrapped:	buffer advance wrapped back to the first topa table
  * @stop_pos:	STOP topa entry index
  * @intr_pos:	INT topa entry index
  * @stop_te:	STOP topa entry pointer

@@ -80,6 +81,7 @@ struct pt_buffer {
 	local_t			data_size;
 	local64_t		head;
 	bool			snapshot;
+	bool			wrapped;
 	long			stop_pos, intr_pos;
 	struct topa_entry	*stop_te, *intr_te;
 	void			**data_pages;

2.43.0

[ Sasha's backport helper bot ]

Hi,

The upstream commit SHA1 provided is correct: 5b590160d2cf776b304eb054afafea2bd55e3620

Status in newer kernel trees:
6.12.y | Present (different SHA1: bd0081617661) 6.11.y | Present (different SHA1: 549225e02e9b) 6.6.y | Present (different SHA1: 1488d93e3e1f) 6.1.y | Present (different SHA1: bda8868213ee) 5.15.y | Present (different SHA1: 1b843f820f7a) 5.10.y | Present (different SHA1: b243226da582) 5.4.y | Not found

Note: The patch differs from the upstream commit:
---
1: 5b590160d2cf7 ! 1: 907198d6c6223 perf/x86/intel/pt: Fix buffer full but size is 0 case
@@ Metadata
## Commit message ##
perf/x86/intel/pt: Fix buffer full but size is 0 case

+ commit 5b590160d2cf776b304eb054afafea2bd55e3620 upstream.
+
If the trace data buffer becomes full, a truncated flag [T] is reported in PERF_RECORD_AUX. In some cases, the size reported is 0, even though data must have been added to make the buffer full.
@@ Commit message

+ Signed-off-by: Adrian Hunter <[email protected]>

## arch/x86/events/intel/pt.c ##
@@ arch/x86/events/intel/pt.c: static void pt_buffer_advance(struct pt_buffer *buf)
@@ arch/x86/events/intel/pt.c: static void pt_buffer_advance(struct pt_buffer *buf)

+ buf->wrapped = false;
+
- if (buf->single) {
- local_set(&buf->data_size, buf->output_off);
- return;
+ /* offset of the first region in this table from the beginning of buf */
+ base = buf->cur->offset + buf->output_off;
+
@@ arch/x86/events/intel/pt.c: static void pt_update_head(struct pt *pt)
} else {
old = (local64_xchg(&buf->head, base) &
@@ arch/x86/events/intel/pt.c: static void pt_update_head(struct pt *pt)

## arch/x86/events/intel/pt.h ##
@@ arch/x86/events/intel/pt.h: struct pt_pmu {
+ * @lost: if data was lost/truncated
* @head: logical write offset inside the buffer
* @snapshot: if this is for a snapshot/overwrite counter
- * @single: use Single Range Output instead of ToPA
+ * @wrapped: buffer advance wrapped back to the first topa table
* @stop_pos: STOP topa entry index
* @intr_pos: INT topa entry index
* @stop_te: STOP topa entry pointer
@@ arch/x86/events/intel/pt.h: struct pt_buffer {
+ local_t data_size;
local64_t head;
bool snapshot;
- bool single;
+ bool wrapped;
long stop_pos, intr_pos;
struct topa_entry *stop_te, *intr_te;
---

Results of testing on various branches:

| Branch                    | Patch Apply | Build Test |
|---------------------------|-------------|------------|
| stable/linux-5.4.y        |  Success    |  Success   |