[PATCH v5 11/11] alpha/perf: sample only retired instructions for precise events

[Matt Turner <mattst88@gmail.com>]

ProfileMe samples include instructions killed on a wrong path, which
show up as hot spots in perf report although they did no work. On an
EV7 loop with a randomly mispredicted branch, 19% of the samples were
killed before being mapped and another 48% later.

PEBS and AMD IBS op samples are of retired instructions only, so make
precise_ip imply the retired filter, unless filters are given. The
profileme bit still selects every sample. With cycles:p, both kinds of
killed sample went to 0%.

DTB miss samples, whose instruction is aborted, are then dropped too:
their data source now needs cpu/cycles,trapped/p.

Suggested-by: Ian Rogers <irogers@google.com>
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Matt Turner <mattst88@gmail.com>
---
 arch/alpha/kernel/perf_event.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/alpha/kernel/perf_event.c b/arch/alpha/kernel/perf_event.c
index bed671fe726d..c108b5488a50 100644
--- a/arch/alpha/kernel/perf_event.c
+++ b/arch/alpha/kernel/perf_event.c
@@ -825,6 +825,11 @@ static int __hw_perf_event_init(struct perf_event *event)
 	if (attr->config1 && !profileme)
 		return -EINVAL;
 	hwc->config = attr->config1;
+	/* As with PEBS, precise samples are of retired instructions. */
+	if (attr->precise_ip && !attr->config1 &&
+	    !(attr->type == PERF_TYPE_RAW &&
+	      (attr->config & ALPHA_PERF_CONFIG_PROFILEME)))
+		hwc->config = ALPHA_PERF_CONFIG1_RETIRED;
 
 	if (attr->type == PERF_TYPE_RAW &&
 	    (attr->config & ALPHA_PERF_CONFIG_RANDOMIZE)) {

-- 
2.55.0