From nobody Thu Feb 12 09:23:40 2026 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8C264C46CA1 for ; Sat, 16 Sep 2023 04:10:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238285AbjIPEJv (ORCPT ); Sat, 16 Sep 2023 00:09:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60656 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237962AbjIPEJZ (ORCPT ); Sat, 16 Sep 2023 00:09:25 -0400 Received: from mail-yw1-x1149.google.com (mail-yw1-x1149.google.com [IPv6:2607:f8b0:4864:20::1149]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1D3BB1BD1 for ; Fri, 15 Sep 2023 21:09:20 -0700 (PDT) Received: by mail-yw1-x1149.google.com with SMTP id 00721157ae682-59c0327b75dso23340357b3.2 for ; Fri, 15 Sep 2023 21:09:20 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1694837359; x=1695442159; darn=vger.kernel.org; h=to:from:subject:mime-version:message-id:date:from:to:cc:subject :date:message-id:reply-to; bh=zipWfEEsnum9dauMSnXBQTgBSpY3twUkBA45BcLxABI=; b=idKeyoRr24pRaWecrAw7DJ7O9deBaDjKus4zQYhZ9wDfziiLGXTkkEEyR1UTiuwbL4 dyQWK0hFXLkmTtNYLEGAL1zunC0pg2o/4bpzGYZnFFemnXi8v6FzVuI+XqdK5F5qvz4c uRkSBDmavtyTe3CQINeKdB9+sbwzFFQhjqgegufmE/50U3B8qAdB1y6HL6OUQPEEfm5h ECoMKeMO8t122ofA247EgWpRt8bNcMlGqclnRKDDMvJqozDsJNCoMOsNP/EVzdzXaXKM ++fVWTEa4Tlw5QrMra9BcjsWyzQBPK2IUi6qhouRKFgc6KwPU19xK0WyfawPQZtmCnRT sbEQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1694837359; x=1695442159; h=to:from:subject:mime-version:message-id:date:x-gm-message-state :from:to:cc:subject:date:message-id:reply-to; bh=zipWfEEsnum9dauMSnXBQTgBSpY3twUkBA45BcLxABI=; b=jPM9RIuNpYyj5I0XYIJvmsfn1kYuVXg4tfO0V2To7cmPalf4J4oUbSj/8D2e7yattV TpYqMF0rRzzGwQoae0oxbF/kb9LjXi3OYot0I+l9tBI4v0GcpxstTt0I1Tu1iogLNCqR y/eCGpkHBk4Mp0oCDV0EzH+kZKfgt3OO7EEP0ujOMmbU90jsFUxdrYaR2K3sgJljn3Cq sT7QFabcI+Pqo8OccCWEZX44wpG2euRANKzzA0vEN4+n1hj98DUSHvWqR814RfyZgPe1 mxr+E9BaiqY8EpfZShPWTjJrKqmkx6A0z4jR9CHAG+69PajiQZZEjTgizs3wVwc+AAEh ZTEQ== X-Gm-Message-State: AOJu0Yz6qqCfWu14zW0FoApJJoQlARdHBMrurN9tBSYMhOhuPnJcF3+4 MrrOX5dAdxWmh9n/XUTOcU2s+qG/bjPI X-Google-Smtp-Source: AGHT+IFCMvRhEqBZ5w3q2IXMRP5+eviNQUZ+p7MKuvjjo68Vr2FD7mxfcCmn3VsoAaiTr1ZuqeKTJZYfjMnZ X-Received: from irogers.svl.corp.google.com ([2620:15c:2a3:200:de72:1a43:1938:6165]) (user=irogers job=sendgmr) by 2002:a81:ac60:0:b0:59b:e669:c940 with SMTP id z32-20020a81ac60000000b0059be669c940mr96303ywj.5.1694837359199; Fri, 15 Sep 2023 21:09:19 -0700 (PDT) Date: Fri, 15 Sep 2023 21:09:15 -0700 Message-Id: <20230916040915.1075620-1-irogers@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.42.0.459.ge4e396fd5e-goog Subject: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event From: Ian Rogers To: Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Ian Rogers , Adrian Hunter , Kan Liang , linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, Yang Jihong , Stephane Eranian Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Dummy events are created with an attribute where the period and freq are zero. evsel__config will then see the uninitialized values and initialize them in evsel__default_freq_period. As fequency mode is used by default the dummy event would be set to use frequency mode. However, this has no effect on the dummy event but does cause unnecessary timers/interrupts. Avoid this overhead by setting the period to 1 for dummy events. evlist__add_aux_dummy calls evlist__add_dummy then sets freq=3D0 and period=3D1. This isn't necessary after this change and so the setting is removed. From Stephane: The dummy event is not counting anything. It is used to collect mmap records and avoid a race condition during the synthesize mmap phase of perf record. As such, it should not cause any overhead during active profiling. Yet, it did. Because of a bug the dummy event was programmed as a sampling event in frequency mode. Events in that mode incur more kernel overheads because on timer tick, the kernel has to look at the number of samples for each event and potentially adjust the sampling period to achieve the desired frequency. The dummy event was therefore adding a frequency event to task and ctx contexts we may otherwise not have any, e.g., perf record -a -e cpu/event=3D0x3c,period=3D10000000/. On each timer tick the perf_adjust_freq_unthr_context() is invoked and if ctx->nr_freq is non-zero, then the kernel will loop over ALL the events of the context looking for frequency mode ones. In doing, so it locks the context, and enable/disable the PMU of each hw event. If all the events of the context are in period mode, the kernel will have to traverse the list for nothing incurring overhead. The overhead is multiplied by a very large factor when this happens in a guest kernel. There is no need for the dummy event to be in frequency mode, it does not count anything and therefore should not cause extra overhead for no reason. Fixes: 5bae0250237f ("perf evlist: Introduce perf_evlist__new_dummy constru= ctor") Reported-by: Stephane Eranian Signed-off-by: Ian Rogers Acked-by: Adrian Hunter --- tools/perf/util/evlist.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c index 25c3ebe2c2f5..e36da58522ef 100644 --- a/tools/perf/util/evlist.c +++ b/tools/perf/util/evlist.c @@ -251,6 +251,9 @@ static struct evsel *evlist__dummy_event(struct evlist = *evlist) .type =3D PERF_TYPE_SOFTWARE, .config =3D PERF_COUNT_SW_DUMMY, .size =3D sizeof(attr), /* to capture ABI version */ + /* Avoid frequency mode for dummy events to avoid associated timers. */ + .freq =3D 0, + .sample_period =3D 1, }; =20 return evsel__new_idx(&attr, evlist->core.nr_entries); @@ -277,8 +280,6 @@ struct evsel *evlist__add_aux_dummy(struct evlist *evli= st, bool system_wide) evsel->core.attr.exclude_kernel =3D 1; evsel->core.attr.exclude_guest =3D 1; evsel->core.attr.exclude_hv =3D 1; - evsel->core.attr.freq =3D 0; - evsel->core.attr.sample_period =3D 1; evsel->core.system_wide =3D system_wide; evsel->no_aux_samples =3D true; evsel->name =3D strdup("dummy:u"); --=20 2.42.0.459.ge4e396fd5e-goog