From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E79C1111E for ; Tue, 31 Oct 2023 05:48:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="kL5jWchS" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D0188C433CA; Tue, 31 Oct 2023 05:47:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1698731280; bh=p+P/xP9EbaQml5yZke5QeF30o+WUdV6v112azu7rFLo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=kL5jWchSNYYCG3LQ6IBTiLQvi1Kwr5DwhYV0HdB1dWKwXD3e56mQGPbDbCzpDlean 8/nQgpmdN69VoOZrKLQsKx9X2W+iBuR8+4kFbdY4y+Vb71arz8+ob9ncW5o4NuRSzS 4mdEiGZjKXHZgqNnm3TWqq5rawamhXR1evc9SsVI= Date: Tue, 31 Oct 2023 06:47:55 +0100 From: Greg KH To: Mingwei Zhang Cc: stable@vger.kernel.org, Ian Rogers , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Adrian Hunter , Kan Liang , linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, Yang Jihong , Stephane Eranian Subject: Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event Message-ID: <2023103145-ruse-statue-9b8f@gregkh> References: <20230916040915.1075620-1-irogers@google.com> Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Mon, Oct 30, 2023 at 01:01:45PM -0700, Mingwei Zhang wrote: > On Mon, Oct 30, 2023 at 12:04 PM Mingwei Zhang wrote: > > > > On Fri, Sep 15, 2023 at 9:10 PM Ian Rogers wrote: > > > > > > 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=0 and > > > period=1. 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=0x3c,period=10000000/. 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 constructor") > > > Reported-by: Stephane Eranian > > > Signed-off-by: Ian Rogers > > > --- > > > 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 = PERF_TYPE_SOFTWARE, > > > .config = PERF_COUNT_SW_DUMMY, > > > .size = sizeof(attr), /* to capture ABI version */ > > > + /* Avoid frequency mode for dummy events to avoid associated timers. */ > > > + .freq = 0, > > > + .sample_period = 1, > > > }; > > > > > > return evsel__new_idx(&attr, evlist->core.nr_entries); > > > @@ -277,8 +280,6 @@ struct evsel *evlist__add_aux_dummy(struct evlist *evlist, bool system_wide) > > > evsel->core.attr.exclude_kernel = 1; > > > evsel->core.attr.exclude_guest = 1; > > > evsel->core.attr.exclude_hv = 1; > > > - evsel->core.attr.freq = 0; > > > - evsel->core.attr.sample_period = 1; > > > evsel->core.system_wide = system_wide; > > > evsel->no_aux_samples = true; > > > evsel->name = strdup("dummy:u"); > > > -- > > > 2.42.0.459.ge4e396fd5e-goog > > > > > > > Hi Greg, > > > > This patch is a critical performance fix for perf and vPMU. Can you > > help us dispatch the commit to all stable kernel versions? > > > > Appreciate your help. Thanks. > > -Mingwei > > Oops... Update target email to: stable@vger.kernel.org This is not the correct way to submit patches for inclusion in the stable kernel tree. Please read: https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html for how to do this properly.