On Wed, Dec 04, 2024 at 08:11:26PM +0200, Adrian Hunter wrote:
> 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]
>
> Fixes: 52ca9ced3f70 ("perf/x86/intel/pt: Add Intel PT PMU driver")
> Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Link: https://lkml.kernel.org/r/20241022155920.17511-2-adrian.hunter@xxxxxxxxx
> Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> ---
> arch/x86/events/intel/pt.c | 11 ++++++++---
> arch/x86/events/intel/pt.h | 2 ++
> 2 files changed, 10 insertions(+), 3 deletions(-)
Sorry, but 4.19.y is now end-of-life.
