Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event

From: Namhyung Kim
Date: Sat Sep 30 2023 - 02:07:05 EST

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In reply to: Stephane Eranian: "Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event"
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Hello,

On Mon, Sep 25, 2023 at 10:37 AM Stephane Eranian <eranian@xxxxxxxxxx> wrote:
>
> On Sun, Sep 24, 2023 at 8:35 PM Yang Jihong <yangjihong1@xxxxxxxxxx> wrote:
> >
> > Hello,
> >
> > On 2023/9/22 23:05, Ian Rogers wrote:
> > > On Thu, Sep 21, 2023 at 10:36 PM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
> > >>
> > >> On 21/09/23 22:26, Namhyung Kim wrote:
> > >>> Hi,
> > >>>
> > >>> On Mon, Sep 18, 2023 at 11:00 PM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
> > >>>>
> > >>>> On 19/09/23 00:48, Ian Rogers wrote:
> > >>>>> On Mon, Sep 18, 2023 at 1:14 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
> > >>>>>>
> > >>>>>> On 16/09/23 07:09, 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 <eranian@xxxxxxxxxx>
> > >>>>>>> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > >>>
> > >>> I'll take the original patch first.

It's in the perf-tools-next.

> > >>>
> > >>>
> > >>>>>>> ---
> > >>>>>>> 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");
> > >>>>>>
> > >>>>>> Note that evsel__config() will put it back to freq if -F is used.
> > >>>>>
> > >>>>> Right, I was looking for a minimal fix in part for the sake of back
> > >>>>> porting. For the -F we could do:
> > >>>>>
> > >>>>> ```
> > >>>>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > >>>>> index d5363d23f5d3..806185a39e17 100644
> > >>>>> --- a/tools/perf/util/evsel.c
> > >>>>> +++ b/tools/perf/util/evsel.c
> > >>>>> @@ -1083,11 +1083,15 @@ void __weak arch__post_evsel_config(struct
> > >>>>> evsel *evsel __maybe_unused,
> > >>>>> static void evsel__set_default_freq_period(struct record_opts *opts,
> > >>>>> struct perf_event_attr *attr)
> > >>>>> {
> > >>>>> - if (opts->freq) {
> > >>>>> + bool is_dummy = attr->type == PERF_TYPE_SOFTWARE &&
> > >>>>> + attr->config == PERF_COUNT_SW_DUMMY;
> > >>>>> +
> > >>>>> + if (opts->freq && !is_dummy) {
> > >>>>> attr->freq = 1;
> > >>>>> attr->sample_freq = opts->freq;
> > >>>>> } else {
> > >>>>> - attr->sample_period = opts->default_interval;
> > >>>>> + attr->freq = 0;
> > >>>>> + attr->sample_period = is_dummy ? 1 : opts->default_interval;
> > >>>>> }
> > >>>>> }
> > >>>>> ```
> > >>>>>
> > >>>>> But this felt like it could potentially have other side-effects.
> > >>>>
> > >>>> Perhaps leave it alone, if the period has already been defined:
> > >>>>
> > >>>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > >>>> index d5363d23f5d3..ad3e12f5ec88 100644
> > >>>> --- a/tools/perf/util/evsel.c
> > >>>> +++ b/tools/perf/util/evsel.c
> > >>>> @@ -1166,7 +1166,8 @@ void evsel__config(struct evsel *evsel, struct record_opts *opts,
> > >>>> if ((evsel->is_libpfm_event && !attr->sample_period) ||
> > >>>> (!evsel->is_libpfm_event && (!attr->sample_period ||
> > >>>> opts->user_freq != UINT_MAX ||
> > >>>> - opts->user_interval != ULLONG_MAX)))
> > >>>> + opts->user_interval != ULLONG_MAX) &&
> > >>>> + !(is_dummy && attr->sample_period)))
> > >>>> evsel__set_default_freq_period(opts, attr);
> > >>>>
> > >>>> /*
> > >>>
> > >>> Or simply like this?
> > >>>
> > >>>
> > >>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > >>> index d5363d23f5d3..6ce832ce62f1 100644
> > >>> --- a/tools/perf/util/evsel.c
> > >>> +++ b/tools/perf/util/evsel.c
> > >>> @@ -1169,6 +1169,9 @@ void evsel__config(struct evsel *evsel, struct
> > >>> record_opts *opts,
> > >>> opts->user_interval != ULLONG_MAX)))
> > >>> evsel__set_default_freq_period(opts, attr);
> > >>>
> > >>> + if (evsel__is_dummy_event(evsel))
> > >>> + attr->freq = 0;
> > >>> +
> > >>> /*
> > >>> * If attr->freq was set (here or earlier), ask for period
> > >>> * to be sampled.
> > >>
> > >> I thought there might be corner cases where it made a difference,
> > >> but I can't find any, so that should do.
> > >>
> > >
> > > It seemed more intention revealing to do it at creation/initialization
> > > than on a later not obviously executed code path - I'm thinking of
> > > future me trying to understand the code. My priority is the clearing
> > > of the flag, so I'm easy.

Yeah I meant to keep the initialization and to prevent updating
it for -F later. I can add a comment in this code like

/*
* The kernel counts events with freq=1 separately to update
* the sample frequency in the timer tick handler. Do not
* confuse the kernel since the dummy event never fires and
* no need to update the frequency.
*/
if (evsel__is_dummy_event(evsel))
attr->freq = 0;

ok?

> > >
> > evsel__apply_config_terms() also sets freq. For example:
> >
> > # perf record -vv -e dummy/freq=100/ true
>
> This example is NOT relevant to the issue.
> Of course, if you want to explicitly use the dummy event with
> frequency mode, you should be allowed to do so.

Agreed.

> The problem we are solving here is different. We are preventing perf
> record internal use of the dummy event
> from using frequency mode for no good reason as any frequency mode
> event adds additional overhead.
> The perf record internal dummy event is used for one goal: to capture
> MMAP records to avoid a race condition
> between synthesize phase and processes being created and not captured
> by synthesize. In that mode, it acts
> as an aggregator of all MMAP records during the entire run of the
> tool. This does not require any frequency mode.
>
>
> > <SNIP>
> > ------------------------------------------------------------
> > perf_event_attr:
> > type 1 (PERF_TYPE_SOFTWARE)
> > size 136
> > config 0x9 (PERF_COUNT_SW_DUMMY)
> > { sample_period, sample_freq } 100
> > sample_type IP|TID|TIME|PERIOD
> > read_format ID|LOST
> > disabled 1
> > inherit 1
> > mmap 1
> > comm 1
> > freq 1
> > enable_on_exec 1
> > task 1
> > sample_id_all 1
> > exclude_guest 1
> > mmap2 1
> > comm_exec 1
> > ksymbol 1
> > bpf_event 1
> > ------------------------------------------------------------
> > <SNIP>
> >
> > Therefore, do we need to perform special processing on dummy events in
> > evsel__apply_config_terms?

As Stephane said, I don't worry about the case as it's explicitly
requested by the user.

Thanks,
Namhyung

Next message: Greg KH: "Re: [PATCH v4 13/15] uapi: hyperv: Add mshv driver headers defining hypervisor ABIs"
Previous message: kernel test robot: "Re: [PATCH v4 15/15] Drivers: hv: Add modules to expose /dev/mshv to VMMs running on Hyper-V"
In reply to: Stephane Eranian: "Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event"
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