From 5d1ab659fb93eed85d6d8b2937013360157032f4 Mon Sep 17 00:00:00 2001 From: Ian Rogers Date: Mon, 9 Feb 2026 22:03:59 -0800 Subject: [PATCH] perf stat: Add no-affinity flag Add flag that disables affinity behavior. Using sched_setaffinity() to place a perf thread on a CPU can avoid certain interprocessor interrupts but may introduce a delay due to the scheduling, particularly on loaded machines. Add a command line option to disable the behavior. This behavior is less present in other tools like `perf record`, as it uses a ring buffer and doesn't make repeated system calls. Signed-off-by: Ian Rogers Cc: Adrian Hunter Cc: Alexander Shishkin Cc: Andi Kleen Cc: Andres Freund Cc: Dapeng Mi Cc: Dr. David Alan Gilbert Cc: Ingo Molnar Cc: James Clark Cc: Jiri Olsa Cc: Namhyung Kim Cc: Peter Zijlstra Cc: Thomas Falcon Cc: Thomas Richter Cc: Yang Li Signed-off-by: Arnaldo Carvalho de Melo --- tools/perf/Documentation/perf-stat.txt | 5 +++++ tools/perf/builtin-stat.c | 6 ++++++ tools/perf/util/evlist.c | 6 +----- tools/perf/util/evlist.h | 1 + 4 files changed, 13 insertions(+), 5 deletions(-) diff --git a/tools/perf/Documentation/perf-stat.txt b/tools/perf/Documentation/perf-stat.txt index 1a766d4a2233..7cccc3a847d1 100644 --- a/tools/perf/Documentation/perf-stat.txt +++ b/tools/perf/Documentation/perf-stat.txt @@ -382,6 +382,11 @@ color the metric's computed value. Don't print output, warnings or messages. This is useful with perf stat record below to only write data to the perf.data file. +--no-affinity:: +Don't change scheduler CPU affinities when iterating over +CPUs. Disables an optimization aimed at minimizing interprocessor +interrupts. + STAT RECORD ----------- Stores stat data into perf data file. diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c index c1bb40b99176..73c2ba7e3076 100644 --- a/tools/perf/builtin-stat.c +++ b/tools/perf/builtin-stat.c @@ -2426,6 +2426,7 @@ static int parse_tpebs_mode(const struct option *opt, const char *str, int cmd_stat(int argc, const char **argv) { struct opt_aggr_mode opt_mode = {}; + bool affinity = true, affinity_set = false; struct option stat_options[] = { OPT_BOOLEAN('T', "transaction", &transaction_run, "hardware transaction statistics"), @@ -2554,6 +2555,8 @@ int cmd_stat(int argc, const char **argv) "don't print 'summary' for CSV summary output"), OPT_BOOLEAN(0, "quiet", &quiet, "don't print any output, messages or warnings (useful with record)"), + OPT_BOOLEAN_SET(0, "affinity", &affinity, &affinity_set, + "enable (default) or disable affinity optimizations to reduce IPIs"), OPT_CALLBACK(0, "cputype", &evsel_list, "hybrid cpu type", "Only enable events on applying cpu with this type " "for hybrid platform (e.g. core or atom)", @@ -2611,6 +2614,9 @@ int cmd_stat(int argc, const char **argv) } else stat_config.csv_sep = DEFAULT_SEPARATOR; + if (affinity_set) + evsel_list->no_affinity = !affinity; + if (argc && strlen(argv[0]) > 2 && strstarts("record", argv[0])) { argc = __cmd_record(stat_options, &opt_mode, argc, argv); if (argc < 0) diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c index 45833244daf3..591bdf0b3e2a 100644 --- a/tools/perf/util/evlist.c +++ b/tools/perf/util/evlist.c @@ -369,11 +369,7 @@ static bool evlist__use_affinity(struct evlist *evlist) struct perf_cpu_map *used_cpus = NULL; bool ret = false; - /* - * With perf record core.user_requested_cpus is usually NULL. - * Use the old method to handle this for now. - */ - if (!evlist->core.user_requested_cpus || + if (evlist->no_affinity || !evlist->core.user_requested_cpus || cpu_map__is_dummy(evlist->core.user_requested_cpus)) return false; diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h index 30dff7484d3c..d17c3b57a409 100644 --- a/tools/perf/util/evlist.h +++ b/tools/perf/util/evlist.h @@ -59,6 +59,7 @@ struct event_enable_timer; struct evlist { struct perf_evlist core; bool enabled; + bool no_affinity; int id_pos; int is_pos; int nr_br_cntr;