Re: [PATCH v15 8/8] perf tools: Merge deferred user callchains

[Namhyung Kim <namhyung@kernel.org>]

On Mon, Aug 25, 2025 at 02:06:46PM -0400, Steven Rostedt wrote:
> From: Namhyung Kim <namhyung@kernel.org>
> 
> Save samples with deferred callchains in a separate list and deliver
> them after merging the user callchains.  If users don't want to merge
> they can set tool->merge_deferred_callchains to false to prevent the
> behavior.
> 
> With previous result, now perf script will show the merged callchains.
> 
>   $ perf script
>   perf     801 [000]    18.031793:          1 cycles:P:
>           ffffffff91a14c36 __intel_pmu_enable_all.isra.0+0x56 ([kernel.kallsyms])
>           ffffffff91d373e9 perf_ctx_enable+0x39 ([kernel.kallsyms])
>           ffffffff91d36af7 event_function+0xd7 ([kernel.kallsyms])
>           ffffffff91d34222 remote_function+0x42 ([kernel.kallsyms])
>           ffffffff91c1ebe1 generic_exec_single+0x61 ([kernel.kallsyms])
>           ffffffff91c1edac smp_call_function_single+0xec ([kernel.kallsyms])
>           ffffffff91d37a9d event_function_call+0x10d ([kernel.kallsyms])
>           ffffffff91d33557 perf_event_for_each_child+0x37 ([kernel.kallsyms])
>           ffffffff91d47324 _perf_ioctl+0x204 ([kernel.kallsyms])
>           ffffffff91d47c43 perf_ioctl+0x33 ([kernel.kallsyms])
>           ffffffff91e2f216 __x64_sys_ioctl+0x96 ([kernel.kallsyms])
>           ffffffff9265f1ae do_syscall_64+0x9e ([kernel.kallsyms])
>           ffffffff92800130 entry_SYSCALL_64+0xb0 ([kernel.kallsyms])
>               7fb5fc22034b __GI___ioctl+0x3b (/usr/lib/x86_64-linux-gnu/libc.so.6)
>   ...
> 
> The old output can be get using --no-merge-callchain option.
> Also perf report can get the user callchain entry at the end.
> 
>   $ perf report --no-children --percent-limit=0 --stdio -q -S __intel_pmu_enable_all.isra.0
>   # symbol: __intel_pmu_enable_all.isra.0
>        0.00%  perf     [kernel.kallsyms]
>               |
>               ---__intel_pmu_enable_all.isra.0
>                  perf_ctx_enable
>                  event_function
>                  remote_function
>                  generic_exec_single
>                  smp_call_function_single
>                  event_function_call
>                  perf_event_for_each_child
>                  _perf_ioctl
>                  perf_ioctl
>                  __x64_sys_ioctl
>                  do_syscall_64
>                  entry_SYSCALL_64
>                  __GI___ioctl
> 
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
> Changes since v14: https://lore.kernel.org/20250718164324.925232448@kernel.org
> 
> - Use both TID and cookie to match the request event to the deferred
>   unwind event.
> 
>  tools/perf/Documentation/perf-script.txt |  5 ++
>  tools/perf/builtin-script.c              |  5 +-
>  tools/perf/util/callchain.c              | 24 +++++++++
>  tools/perf/util/callchain.h              |  3 ++
>  tools/perf/util/evlist.c                 |  1 +
>  tools/perf/util/evlist.h                 |  1 +
>  tools/perf/util/session.c                | 64 +++++++++++++++++++++++-
>  tools/perf/util/tool.c                   |  1 +
>  tools/perf/util/tool.h                   |  1 +
>  9 files changed, 103 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/Documentation/perf-script.txt b/tools/perf/Documentation/perf-script.txt
> index 28bec7e78bc8..03d112960632 100644
> --- a/tools/perf/Documentation/perf-script.txt
> +++ b/tools/perf/Documentation/perf-script.txt
> @@ -527,6 +527,11 @@ include::itrace.txt[]
>  	The known limitations include exception handing such as
>  	setjmp/longjmp will have calls/returns not match.
>  
> +--merge-callchains::
> +	Enable merging deferred user callchains if available.  This is the
> +	default behavior.  If you want to see separate CALLCHAIN_DEFERRED
> +	records for some reason, use --no-merge-callchains explicitly.
> +
>  :GMEXAMPLECMD: script
>  :GMEXAMPLESUBCMD:
>  include::guest-files.txt[]
> diff --git a/tools/perf/builtin-script.c b/tools/perf/builtin-script.c
> index d17e0a3d8567..70e7658a61fb 100644
> --- a/tools/perf/builtin-script.c
> +++ b/tools/perf/builtin-script.c
> @@ -3785,6 +3785,7 @@ int cmd_script(int argc, const char **argv)
>  	bool header_only = false;
>  	bool script_started = false;
>  	bool unsorted_dump = false;
> +	bool merge_deferred_callchains = true;
>  	char *rec_script_path = NULL;
>  	char *rep_script_path = NULL;
>  	struct perf_session *session;
> @@ -3938,6 +3939,8 @@ int cmd_script(int argc, const char **argv)
>  		    "Guest code can be found in hypervisor process"),
>  	OPT_BOOLEAN('\0', "stitch-lbr", &script.stitch_lbr,
>  		    "Enable LBR callgraph stitching approach"),
> +	OPT_BOOLEAN('\0', "merge-callchains", &merge_deferred_callchains,
> +		    "Enable merge deferred user callchains"),
>  	OPTS_EVSWITCH(&script.evswitch),
>  	OPT_END()
>  	};
> @@ -4194,7 +4197,7 @@ int cmd_script(int argc, const char **argv)
>  	script.tool.throttle		 = process_throttle_event;
>  	script.tool.unthrottle		 = process_throttle_event;
>  	script.tool.ordering_requires_timestamps = true;
> -	script.tool.merge_deferred_callchains = false;
> +	script.tool.merge_deferred_callchains = merge_deferred_callchains;
>  	session = perf_session__new(&data, &script.tool);
>  	if (IS_ERR(session))
>  		return PTR_ERR(session);
> diff --git a/tools/perf/util/callchain.c b/tools/perf/util/callchain.c
> index d7b7eef740b9..d2d672f1d6ba 100644
> --- a/tools/perf/util/callchain.c
> +++ b/tools/perf/util/callchain.c
> @@ -1828,3 +1828,27 @@ int sample__for_each_callchain_node(struct thread *thread, struct evsel *evsel,
>  	}
>  	return 0;
>  }
> +
> +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_deferred = sample_callchain->callchain->nr;
> +	struct ip_callchain *callchain;
> +
> +	callchain = calloc(1 + nr_orig + nr_deferred, sizeof(u64));
> +	if (callchain == NULL) {
> +		sample_orig->deferred_callchain = false;
> +		return -ENOMEM;
> +	}
> +
> +	callchain->nr = nr_orig + nr_deferred;
> +	/* copy except for the last PERF_CONTEXT_USER_DEFERRED */
> +	memcpy(callchain->ips, sample_orig->callchain->ips, nr_orig * sizeof(u64));
> +	/* copy deferred use callchains */
> +	memcpy(&callchain->ips[nr_orig], sample_callchain->callchain->ips,
> +	       nr_deferred * sizeof(u64));
> +
> +	sample_orig->callchain = callchain;
> +	return 0;
> +}
> diff --git a/tools/perf/util/callchain.h b/tools/perf/util/callchain.h
> index 86ed9e4d04f9..89785125ed25 100644
> --- a/tools/perf/util/callchain.h
> +++ b/tools/perf/util/callchain.h
> @@ -317,4 +317,7 @@ int sample__for_each_callchain_node(struct thread *thread, struct evsel *evsel,
>  				    struct perf_sample *sample, int max_stack,
>  				    bool symbols, callchain_iter_fn cb, void *data);
>  
> +int sample__merge_deferred_callchain(struct perf_sample *sample_orig,
> +				     struct perf_sample *sample_callchain);
> +
>  #endif	/* __PERF_CALLCHAIN_H */
> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> index 80d8387e6b97..9518b45af393 100644
> --- a/tools/perf/util/evlist.c
> +++ b/tools/perf/util/evlist.c
> @@ -85,6 +85,7 @@ void evlist__init(struct evlist *evlist, struct perf_cpu_map *cpus,
>  	evlist->ctl_fd.pos = -1;
>  	evlist->nr_br_cntr = -1;
>  	metricgroup__rblist_init(&evlist->metric_events);
> +	INIT_LIST_HEAD(&evlist->deferred_samples);
>  }
>  
>  struct evlist *evlist__new(void)
> diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
> index 5e71e3dc6042..309ef8d78495 100644
> --- a/tools/perf/util/evlist.h
> +++ b/tools/perf/util/evlist.h
> @@ -92,6 +92,7 @@ struct evlist {
>  	 * of struct metric_expr.
>  	 */
>  	struct rblist	metric_events;
> +	struct list_head deferred_samples;
>  };
>  
>  struct evsel_str_handler {
> diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
> index a071006350f5..ef1902309395 100644
> --- a/tools/perf/util/session.c
> +++ b/tools/perf/util/session.c
> @@ -1283,6 +1283,57 @@ static int evlist__deliver_sample(struct evlist *evlist, const struct perf_tool
>  					    per_thread);
>  }
>  
> +struct deferred_event {
> +	struct list_head list;
> +	union perf_event *event;
> +};
> +
> +static int evlist__deliver_deferred_samples(struct evlist *evlist,
> +					    const struct perf_tool *tool,
> +					    union  perf_event *event,
> +					    struct perf_sample *sample,
> +					    struct machine *machine)
> +{
> +	struct deferred_event *de, *tmp;
> +	struct evsel *evsel;
> +	int ret = 0;
> +
> +	if (!tool->merge_deferred_callchains) {
> +		evsel = evlist__id2evsel(evlist, sample->id);
> +		return tool->callchain_deferred(tool, event, sample,
> +						evsel, machine);
> +	}
> +
> +	list_for_each_entry_safe(de, tmp, &evlist->deferred_samples, list) {
> +		struct perf_sample orig_sample;
> +
> +		ret = evlist__parse_sample(evlist, de->event, &orig_sample);
> +		if (ret < 0) {
> +			pr_err("failed to parse original sample\n");
> +			break;
> +		}
> +
> +		if (sample->tid != orig_sample.tid ||
> +		    event->callchain_deferred.cookie != orig_sample.deferred_cookie)
> +			continue;

I think we should handle original samples with different cookies too.  
They are before LOST records so we don't merge the callchain though.

> +
> +		evsel = evlist__id2evsel(evlist, orig_sample.id);
> +		sample__merge_deferred_callchain(&orig_sample, sample);
> +		ret = evlist__deliver_sample(evlist, tool, de->event,
> +					     &orig_sample, evsel, machine);

Something like this.

@@ -1313,12 +1313,13 @@ static int evlist__deliver_deferred_samples(struct evlist *evlist,
                        break;
                }

-               if (sample->tid != orig_sample.tid ||
-                   event->callchain_deferred.cookie != orig_sample.deferred_cookie)
+               if (sample->tid != orig_sample.tid)
                        continue;

+               if (event->callchain_deferred.cookie == orig_sample.deferred_cookie)
+                       sample__merge_deferred_callchain(&orig_sample, sample);
+
                evsel = evlist__id2evsel(evlist, orig_sample.id);
-               sample__merge_deferred_callchain(&orig_sample, sample);
                ret = evlist__deliver_sample(evlist, tool, de->event,
                                             &orig_sample, evsel, machine);


Thanks,
Namhyung

> +
> +		if (orig_sample.deferred_callchain)
> +			free(orig_sample.callchain);
> +
> +		list_del(&de->list);
> +		free(de);
> +
> +		if (ret)
> +			break;
> +	}
> +	return ret;
> +}
> +
>  static int machines__deliver_event(struct machines *machines,
>  				   struct evlist *evlist,
>  				   union perf_event *event,
> @@ -1311,6 +1362,16 @@ static int machines__deliver_event(struct machines *machines,
>  			return 0;
>  		}
>  		dump_sample(evsel, event, sample, perf_env__arch(machine->env));
> +		if (sample->deferred_callchain && tool->merge_deferred_callchains) {
> +			struct deferred_event *de = malloc(sizeof(*de));
> +
> +			if (de == NULL)
> +				return -ENOMEM;
> +
> +			de->event = event;
> +			list_add_tail(&de->list, &evlist->deferred_samples);
> +			return 0;
> +		}
>  		return evlist__deliver_sample(evlist, tool, event, sample, evsel, machine);
>  	case PERF_RECORD_MMAP:
>  		return tool->mmap(tool, event, sample, machine);
> @@ -1370,7 +1431,8 @@ static int machines__deliver_event(struct machines *machines,
>  		return tool->aux_output_hw_id(tool, event, sample, machine);
>  	case PERF_RECORD_CALLCHAIN_DEFERRED:
>  		dump_deferred_callchain(evsel, event, sample);
> -		return tool->callchain_deferred(tool, event, sample, evsel, machine);
> +		return evlist__deliver_deferred_samples(evlist, tool, event,
> +							sample, machine);
>  	default:
>  		++evlist->stats.nr_unknown_events;
>  		return -1;
> diff --git a/tools/perf/util/tool.c b/tools/perf/util/tool.c
> index 8bf86af1ca90..9ab9e231b5d5 100644
> --- a/tools/perf/util/tool.c
> +++ b/tools/perf/util/tool.c
> @@ -258,6 +258,7 @@ void perf_tool__init(struct perf_tool *tool, bool ordered_events)
>  	tool->cgroup_events = false;
>  	tool->no_warn = false;
>  	tool->show_feat_hdr = SHOW_FEAT_NO_HEADER;
> +	tool->merge_deferred_callchains = true;
>  
>  	tool->sample = process_event_sample_stub;
>  	tool->mmap = process_event_stub;
> diff --git a/tools/perf/util/tool.h b/tools/perf/util/tool.h
> index 2676d84da80c..7165a487a485 100644
> --- a/tools/perf/util/tool.h
> +++ b/tools/perf/util/tool.h
> @@ -88,6 +88,7 @@ struct perf_tool {
>  	bool		cgroup_events;
>  	bool		no_warn;
>  	bool		dont_split_sample_group;
> +	bool		merge_deferred_callchains;
>  	enum show_feature_header show_feat_hdr;
>  };
>  
> -- 
> 2.50.1
> 
>