Re: [PATCH 2/9] perf thread-stack: Report branch stack hw_idx as not available
Adrian Hunter <[email protected]>
| Newsgroups | org.kernel.vger.linux-perf-users,org.infradead.lists.linux-arm-kernel,org.kernel.vger.linux-doc |
|---|---|
| Organization | Intel Finland Oy, Registered Address: c/o Alberga Business Park, 6 krs, Bertel Jungin Aukio 5, 02600 Espoo, Business Identity Code: 0357606 - 4, Domiciled in Helsinki |
| Message-ID | <[email protected]> |
On 11/08/2026 18:58, Adrian Hunter wrote: > On 03/08/2026 12:06, Amir Ayupov wrote: >> thread_stack__br_sample() and thread_stack__br_sample_late() fill a >> caller-supplied branch_stack that is typically allocated with zalloc(), >> leaving hw_idx as 0. Zero is a valid hardware index, so consumers that >> honour PERF_SAMPLE_BRANCH_HW_INDEX see a reconstructed branch stack >> claiming to start at LBR TOS entry 0. >> >> These branch stacks are reconstructed from instruction trace and have no >> hardware index at all. Set hw_idx to -1ULL, which is the established way >> to say "not available" and matches what intel-pt and cs-etm already put >> in the branch stacks they synthesise directly. >> >> Signed-off-by: Amir Ayupov <[email protected]> > > Fixes tag? > > Otherwise: > > Reviewed-by: Adrian Hunter <[email protected]> On second thoughts, it seems that hw_idx is only used for stitching LBRs which is anyway disabled by default and only enabled by --stitch-lbr. Setting -1ULL will prevent has_stitched_lbr() making a match, but we can rely on the user to decide that for themselves via --stitch-lbr. So, in fact, it doesn't look like this change should be needed? > >> --- >> tools/perf/util/thread-stack.c | 2 ++ >> 1 file changed, 2 insertions(+) >> >> diff --git a/tools/perf/util/thread-stack.c b/tools/perf/util/thread-stack.c >> index c5ce741b07446..1a3dffa83bde2 100644 >> --- a/tools/perf/util/thread-stack.c >> +++ b/tools/perf/util/thread-stack.c >> @@ -624,6 +624,7 @@ void thread_stack__br_sample(struct thread *thread, int cpu, >> unsigned int nr; >> >> dst->nr = 0; >> + dst->hw_idx = -1ULL; >> >> if (!ts) >> return; >> @@ -686,6 +687,7 @@ void thread_stack__br_sample_late(struct thread *thread, int cpu, >> bool start = false; >> >> dst->nr = 0; >> + dst->hw_idx = -1ULL; >> >> if (!ts) >> return; >