Re: [PATCH v4 4/7] perf record: Track sideband events for all CPUs when tracing selected CPUs

From: Adrian Hunter
Date: Thu Aug 03 2023 - 02:29:45 EST

On 2/08/23 10:49, Yang Jihong wrote:
> User space tasks can migrate between CPUs, we need to track side-band
> events for all CPUs.
>
> The specific scenarios are as follows:
>
> CPU0 CPU1
> perf record -C 0 start
> taskA starts to be created and executed
> -> PERF_RECORD_COMM and PERF_RECORD_MMAP
> events only deliver to CPU1
> ......
> |
> migrate to CPU0
> |
> Running on CPU0 <----------/
> ...
>
> perf record -C 0 stop
>
> Now perf samples the PC of taskA. However, perf does not record the
> PERF_RECORD_COMM and PERF_RECORD_MMAP events of taskA.
> Therefore, the comm and symbols of taskA cannot be parsed.
>
> The solution is to record sideband events for all CPUs when tracing
> selected CPUs. Because this modifies the default behavior, add related
> comments to the perf record man page.
>
> The sys_perf_event_open invoked is as follows:
>
> # perf --debug verbose=3 record -e cpu-clock -C 1 true
> <SNIP>
> Opening: cpu-clock
> ------------------------------------------------------------
> perf_event_attr:
> type 1 (PERF_TYPE_SOFTWARE)
> size 136
> config 0 (PERF_COUNT_SW_CPU_CLOCK)
> { sample_period, sample_freq } 4000
> sample_type IP|TID|TIME|CPU|PERIOD|IDENTIFIER
> read_format ID|LOST
> disabled 1
> inherit 1
> freq 1
> sample_id_all 1
> exclude_guest 1
> ------------------------------------------------------------
> sys_perf_event_open: pid -1 cpu 1 group_fd -1 flags 0x8 = 5
> Opening: dummy:u
> ------------------------------------------------------------
> perf_event_attr:
> type 1 (PERF_TYPE_SOFTWARE)
> size 136
> config 0x9 (PERF_COUNT_SW_DUMMY)
> { sample_period, sample_freq } 1
> sample_type IP|TID|TIME|CPU|IDENTIFIER
> read_format ID|LOST
> inherit 1
> exclude_kernel 1
> exclude_hv 1
> mmap 1
> comm 1
> task 1
> sample_id_all 1
> exclude_guest 1
> mmap2 1
> comm_exec 1
> ksymbol 1
> bpf_event 1
> ------------------------------------------------------------
> sys_perf_event_open: pid -1 cpu 0 group_fd -1 flags 0x8 = 6
> sys_perf_event_open: pid -1 cpu 1 group_fd -1 flags 0x8 = 7
> sys_perf_event_open: pid -1 cpu 2 group_fd -1 flags 0x8 = 9
> sys_perf_event_open: pid -1 cpu 3 group_fd -1 flags 0x8 = 10
> sys_perf_event_open: pid -1 cpu 4 group_fd -1 flags 0x8 = 11
> sys_perf_event_open: pid -1 cpu 5 group_fd -1 flags 0x8 = 12
> sys_perf_event_open: pid -1 cpu 6 group_fd -1 flags 0x8 = 13
> sys_perf_event_open: pid -1 cpu 7 group_fd -1 flags 0x8 = 14
> <SNIP>
>
> Signed-off-by: Yang Jihong <yangjihong1@xxxxxxxxxx>
> ---
> tools/perf/Documentation/perf-record.txt | 3 ++
> tools/perf/builtin-record.c | 45 +++++++++++++++++++++++-
> 2 files changed, 47 insertions(+), 1 deletion(-)
>
> diff --git a/tools/perf/Documentation/perf-record.txt b/tools/perf/Documentation/perf-record.txt
> index 680396c56bd1..dac53ece51ab 100644
> --- a/tools/perf/Documentation/perf-record.txt
> +++ b/tools/perf/Documentation/perf-record.txt
> @@ -388,6 +388,9 @@ comma-separated list with no space: 0,1. Ranges of CPUs are specified with -: 0-
> In per-thread mode with inheritance mode on (default), samples are captured only when
> the thread executes on the designated CPUs. Default is to monitor all CPUs.
>
> +User space tasks can migrate between CPUs, so when tracing selected CPUs,
> +a dummy event is created to track sideband for all CPUs.
> +
> -B::
> --no-buildid::
> Do not save the build ids of binaries in the perf.data files. This skips
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index 3ff9d972225e..468afeaac52d 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -908,10 +908,45 @@ static int record__config_off_cpu(struct record *rec)
> return off_cpu_prepare(rec->evlist, &rec->opts.target, &rec->opts);
> }
>
> +static bool record__tracking_system_wide(struct record *rec)
> +{
> + struct record_opts *opts = &rec->opts;
> + struct evlist *evlist = rec->evlist;
> + struct evsel *evsel;
> +
> + /*
> + * If all (non-dummy) evsel have exclude_user,
> + * system_wide is not needed.
> + *
> + * all_kernel and all_user will overwrite exclude_kernel and
> + * exclude_user of attr in evsel__config(), here need to check
> + * all the three items.
> + *
> + * Sideband system wide if one of the following conditions is met:
> + *
> + * - all_user is set, and there is a non-dummy event
> + * - all_user and all_kernel are not set, and there is
> + * a non-dummy event without exclude_user
> + */

Could start with:

if (opts->all_kernel)
return false;

> + evlist__for_each_entry(evlist, evsel) {
> + if (!evsel__is_dummy_event(evsel)) {
> + if (opts->all_user)
> + return true;
> +
> + if (!opts->all_user && !opts->all_kernel &&

!opts->all_user is always true here

> + !evsel->core.attr.exclude_user)
> + return true;

So then this could simply be:

if (opts->all_user || !evsel->core.attr.exclude_user)
return true;


> + }
> + }
> +
> + return false;
> +}
> +
> static int record__config_tracking_events(struct record *rec)
> {
> struct record_opts *opts = &rec->opts;
> struct evlist *evlist = rec->evlist;
> + bool system_wide = false;
> struct evsel *evsel;
>
> /*
> @@ -921,7 +956,15 @@ static int record__config_tracking_events(struct record *rec)
> */
> if (opts->target.initial_delay || target__has_cpu(&opts->target) ||
> perf_pmus__num_core_pmus() > 1) {
> - evsel = evlist__findnew_tracking_event(evlist, false);
> +
> + /*
> + * User space tasks can migrate between CPUs, so when tracing
> + * selected CPUs, sideband for all CPUs is still needed.
> + */
> + if (!!opts->target.cpu_list && record__tracking_system_wide(rec))
> + system_wide = true;
> +
> + evsel = evlist__findnew_tracking_event(evlist, system_wide);
> if (!evsel)
> return -ENOMEM;
>