Re: [PATCH 4.19] perf/x86/intel/pt: Fix buffer full but size is 0 case

[Greg KH <greg@kroah.com>]

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@intel.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: stable@vger.kernel.org
> Link: https://lkml.kernel.org/r/20241022155920.17511-2-adrian.hunter@intel.com
> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
> ---
>  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.