Re: [PATCH v2 2/2] powerpc/perf: Return regs->nip as instruction pointer value when SIAR is 0

On 8/13/21 3:04 PM, Christophe Leroy wrote:

Le 13/08/2021 à 10:24, Kajol Jain a écrit :

quoted

Incase of random sampling, there can be scenarios where SIAR is not
latching sample address and results in 0 value. Since current code
directly returning the siar value, we could see multiple instruction
pointer values as 0 in perf report.
Patch resolves this issue by adding a ternary condition to return
regs->nip incase SIAR is 0.

Your description seems rather similar to https://github.com/linuxppc/linux/commit/2ca13a4cc56c920a6c9fc8ee45d02bccacd7f46c

Does it mean that the problem occurs on more than the power10 DD1 ?

In that case, can the solution be common instead of doing something for power10 DD1 and something for others ?

Hi Christophe,
    Yes its better to have common check. I will make these updates.

Thanks,
Kajol Jain

quoted

Fixes: 75382aa72f06 ("powerpc/perf: Move code to select SIAR or pt_regs
into perf_read_regs")
Signed-off-by: Kajol Jain <redacted>
---
  arch/powerpc/perf/core-book3s.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 1b464aad29c4..aeecaaf6810f 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c

@@ -2260,7 +2260,7 @@ unsigned long perf_instruction_pointer(struct pt_regs *regs)

          else
              return regs->nip;
      } else if (use_siar && siar_valid(regs))
-        return siar + perf_ip_adjust(regs);
+        return siar ? siar + perf_ip_adjust(regs) : regs->nip;
      else if (use_siar)
          return 0;        // no valid instruction pointer
      else