linux-kernel - Re: [PATCH v16 0/4] perf tool: Support the deferred unwinding infrastructure

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Message-ID: <20251024130107.GB3245006@noisy.programming.kicks-ass.net>
Date: Fri, 24 Oct 2025 15:01:07 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	bpf@...r.kernel.org, x86@...nel.org,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrii Nakryiko <andrii@...nel.org>,
	Indu Bhagat <indu.bhagat@...cle.com>,
	"Jose E. Marchesi" <jemarch@....org>,
	Beau Belgrave <beaub@...ux.microsoft.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Florian Weimer <fweimer@...hat.com>, Sam James <sam@...too.org>,
	Kees Cook <kees@...nel.org>, Carlos O'Donell <codonell@...hat.com>
Subject: Re: [PATCH v16 0/4] perf tool: Support the deferred unwinding
 infrastructure



Per the hackery I did:

  https://lkml.kernel.org/r/20251023150002.GR4067720@noisy.programming.kicks-ass.net

The userspace bits need something like so on top..

---
 tools/perf/util/callchain.c               |    2 +-
 tools/perf/util/evsel.c                   |   10 +++++++---
 tools/perf/util/perf_event_attr_fprintf.c |    1 +
 3 files changed, 9 insertions(+), 4 deletions(-)

--- a/tools/perf/util/callchain.c
+++ b/tools/perf/util/callchain.c
@@ -1832,7 +1832,7 @@ int sample__for_each_callchain_node(stru
 int sample__merge_deferred_callchain(struct perf_sample *sample_orig,
 				     struct perf_sample *sample_callchain)
 {
-	u64 nr_orig = sample_orig->callchain->nr - PERF_DEFERRED_ITEMS;
+	u64 nr_orig = sample_orig->callchain->nr - 1;
 	u64 nr_deferred = sample_callchain->callchain->nr;
 	struct ip_callchain *callchain;
 
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -1520,6 +1520,7 @@ void evsel__config(struct evsel *evsel,
 	attr->mmap2    = track && !perf_missing_features.mmap2;
 	attr->comm     = track;
 	attr->build_id = track && opts->build_id;
+	attr->defer_output = track && !perf_missing_features.defer_callchain;
 
 	/*
 	 * ksymbol is tracked separately with text poke because it needs to be
@@ -2206,8 +2207,10 @@ static int __evsel__prepare_open(struct
 
 static void evsel__disable_missing_features(struct evsel *evsel)
 {
-	if (perf_missing_features.defer_callchain)
+	if (perf_missing_features.defer_callchain) {
 		evsel->core.attr.defer_callchain = 0;
+		evsel->core.attr.defer_output = 0;
+	}
 	if (perf_missing_features.inherit_sample_read && evsel->core.attr.inherit &&
 	    (evsel->core.attr.sample_type & PERF_SAMPLE_READ))
 		evsel->core.attr.inherit = 0;
@@ -2489,6 +2492,7 @@ static bool evsel__detect_missing_featur
 	perf_missing_features.defer_callchain = true;
 	pr_debug2("switching off deferred callchain support\n");
 	attr.defer_callchain = false;
+	attr.defer_output = false;
 	attr.sample_type = 0;
 
 	attr.inherit = true;
@@ -3255,8 +3259,8 @@ int evsel__parse_sample(struct evsel *ev
 			return -EFAULT;
 		sz = data->callchain->nr * sizeof(u64);
 		if (evsel->core.attr.defer_callchain &&
-		    data->callchain->nr >= PERF_DEFERRED_ITEMS &&
-		    data->callchain->ips[data->callchain->nr - PERF_DEFERRED_ITEMS] == PERF_CONTEXT_USER_DEFERRED) {
+		    data->callchain->nr >= 2 &&
+		    data->callchain->ips[data->callchain->nr - 2] == PERF_CONTEXT_USER_DEFERRED) {
 			data->deferred_callchain = true;
 			data->deferred_cookie = data->callchain->ips[data->callchain->nr - 1];
 		}
--- a/tools/perf/util/perf_event_attr_fprintf.c
+++ b/tools/perf/util/perf_event_attr_fprintf.c
@@ -344,6 +344,7 @@ int perf_event_attr__fprintf(FILE *fp, s
 	PRINT_ATTRf(remove_on_exec, p_unsigned);
 	PRINT_ATTRf(sigtrap, p_unsigned);
 	PRINT_ATTRf(defer_callchain, p_unsigned);
+	PRINT_ATTRf(defer_output, p_unsigned);
 
 	PRINT_ATTRn("{ wakeup_events, wakeup_watermark }", wakeup_events, p_unsigned, false);
 	PRINT_ATTRf(bp_type, p_unsigned);