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Message-ID: <Zxl8iRgHi0ZZKMf-@google.com>
Date: Wed, 23 Oct 2024 15:45:29 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Tengda Wu <wutengda@...weicloud.com>
Cc: Peter Zijlstra <peterz@...radead.org>, song@...nel.org,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
Adrian Hunter <adrian.hunter@...el.com>, kan.liang@...ux.intel.com,
linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
bpf@...r.kernel.org
Subject: Re: [PATCH -next v5 1/2] perf stat: Support inherit events during
fork() for bperf
On Tue, Oct 22, 2024 at 05:39:23PM +0800, Tengda Wu wrote:
>
>
> On 2024/10/22 12:08, Namhyung Kim wrote:
> > Hello,
> >
> > On Mon, Oct 21, 2024 at 11:02:00AM +0000, Tengda Wu wrote:
> >> bperf has a nice ability to share PMUs, but it still does not support
> >> inherit events during fork(), resulting in some deviations in its stat
> >> results compared with perf.
> >>
> >> perf stat result:
> >> $ ./perf stat -e cycles,instructions -- ./perf test -w sqrtloop
> >> Performance counter stats for './perf test -w sqrtloop':
> >>
> >> 2,316,038,116 cycles
> >> 2,859,350,725 instructions
> >>
> >> 1.009603637 seconds time elapsed
> >>
> >> 1.004196000 seconds user
> >> 0.003950000 seconds sys
> >>
> >> bperf stat result:
> >> $ ./perf stat --bpf-counters -e cycles,instructions -- \
> >> ./perf test -w sqrtloop
> >>
> >> Performance counter stats for './perf test -w sqrtloop':
> >>
> >> 18,762,093 cycles
> >> 23,487,766 instructions
> >>
> >> 1.008913769 seconds time elapsed
> >>
> >> 1.003248000 seconds user
> >> 0.004069000 seconds sys
> >>
> >> In order to support event inheritance, two new bpf programs are added
> >> to monitor the fork and exit of tasks respectively. When a task is
> >> created, add it to the filter map to enable counting, and reuse the
> >> `accum_key` of its parent task to count together with the parent task.
> >> When a task exits, remove it from the filter map to disable counting.
> >>
> >> After support:
> >> $ ./perf stat --bpf-counters -e cycles,instructions -- \
> >> ./perf test -w sqrtloop
> >>
> >> Performance counter stats for './perf test -w sqrtloop':
> >>
> >> 2,316,252,189 cycles
> >> 2,859,946,547 instructions
> >>
> >> 1.009422314 seconds time elapsed
> >>
> >> 1.003597000 seconds user
> >> 0.004270000 seconds sys
> >>
> >> Signed-off-by: Tengda Wu <wutengda@...weicloud.com>
> >> ---
> >> tools/perf/builtin-stat.c | 1 +
> >> tools/perf/util/bpf_counter.c | 35 +++++--
> >> tools/perf/util/bpf_skel/bperf_follower.bpf.c | 98 +++++++++++++++++--
> >> tools/perf/util/bpf_skel/bperf_u.h | 5 +
> >> tools/perf/util/target.h | 1 +
> >> 5 files changed, 126 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
> >> index 3e6b9f216e80..8bc880479417 100644
> >> --- a/tools/perf/builtin-stat.c
> >> +++ b/tools/perf/builtin-stat.c
> >> @@ -2620,6 +2620,7 @@ int cmd_stat(int argc, const char **argv)
> >> } else if (big_num_opt == 0) /* User passed --no-big-num */
> >> stat_config.big_num = false;
> >>
> >> + target.inherit = !stat_config.no_inherit;
> >> err = target__validate(&target);
> >> if (err) {
> >> target__strerror(&target, err, errbuf, BUFSIZ);
> >> diff --git a/tools/perf/util/bpf_counter.c b/tools/perf/util/bpf_counter.c
> >> index 7a8af60e0f51..73fcafbffc6a 100644
> >> --- a/tools/perf/util/bpf_counter.c
> >> +++ b/tools/perf/util/bpf_counter.c
> >> @@ -394,6 +394,7 @@ static int bperf_check_target(struct evsel *evsel,
> >> }
> >>
> >> static struct perf_cpu_map *all_cpu_map;
> >> +static __u32 filter_entry_cnt;
> >>
> >> static int bperf_reload_leader_program(struct evsel *evsel, int attr_map_fd,
> >> struct perf_event_attr_map_entry *entry)
> >> @@ -444,12 +445,32 @@ static int bperf_reload_leader_program(struct evsel *evsel, int attr_map_fd,
> >> return err;
> >> }
> >>
> >> +static int bperf_attach_follower_program(struct bperf_follower_bpf *skel,
> >> + enum bperf_filter_type filter_type,
> >> + bool inherit)
> >> +{
> >> + struct bpf_link *link;
> >> + int err = 0;
> >> +
> >> + if ((filter_type == BPERF_FILTER_PID ||
> >> + filter_type == BPERF_FILTER_TGID) && inherit)
> >> + /* attach all follower bpf progs to enable event inheritance */
> >> + err = bperf_follower_bpf__attach(skel);
> >> + else {
> >> + link = bpf_program__attach(skel->progs.fexit_XXX);
> >> + if (IS_ERR(link))
> >> + err = PTR_ERR(link);
> >> + }
> >> +
> >> + return err;
> >> +}
> >> +
> >> static int bperf__load(struct evsel *evsel, struct target *target)
> >> {
> >> struct perf_event_attr_map_entry entry = {0xffffffff, 0xffffffff};
> >> int attr_map_fd, diff_map_fd = -1, err;
> >> enum bperf_filter_type filter_type;
> >> - __u32 filter_entry_cnt, i;
> >> + __u32 i;
> >>
> >> if (bperf_check_target(evsel, target, &filter_type, &filter_entry_cnt))
> >> return -1;
> >> @@ -529,9 +550,6 @@ static int bperf__load(struct evsel *evsel, struct target *target)
> >> /* set up reading map */
> >> bpf_map__set_max_entries(evsel->follower_skel->maps.accum_readings,
> >> filter_entry_cnt);
> >> - /* set up follower filter based on target */
> >> - bpf_map__set_max_entries(evsel->follower_skel->maps.filter,
> >> - filter_entry_cnt);
> >> err = bperf_follower_bpf__load(evsel->follower_skel);
> >> if (err) {
> >> pr_err("Failed to load follower skeleton\n");
> >> @@ -543,6 +561,7 @@ static int bperf__load(struct evsel *evsel, struct target *target)
> >> for (i = 0; i < filter_entry_cnt; i++) {
> >> int filter_map_fd;
> >> __u32 key;
> >> + struct bperf_filter_value fval = { i, 0 };
> >>
> >> if (filter_type == BPERF_FILTER_PID ||
> >> filter_type == BPERF_FILTER_TGID)
> >> @@ -553,12 +572,14 @@ static int bperf__load(struct evsel *evsel, struct target *target)
> >> break;
> >>
> >> filter_map_fd = bpf_map__fd(evsel->follower_skel->maps.filter);
> >> - bpf_map_update_elem(filter_map_fd, &key, &i, BPF_ANY);
> >> + bpf_map_update_elem(filter_map_fd, &key, &fval, BPF_ANY);
> >> }
> >>
> >> evsel->follower_skel->bss->type = filter_type;
> >> + evsel->follower_skel->bss->inherit = target->inherit;
> >>
> >> - err = bperf_follower_bpf__attach(evsel->follower_skel);
> >> + err = bperf_attach_follower_program(evsel->follower_skel, filter_type,
> >> + target->inherit);
> >>
> >> out:
> >> if (err && evsel->bperf_leader_link_fd >= 0)
> >> @@ -623,7 +644,7 @@ static int bperf__read(struct evsel *evsel)
> >> bperf_sync_counters(evsel);
> >> reading_map_fd = bpf_map__fd(skel->maps.accum_readings);
> >>
> >> - for (i = 0; i < bpf_map__max_entries(skel->maps.accum_readings); i++) {
> >> + for (i = 0; i < filter_entry_cnt; i++) {
> >> struct perf_cpu entry;
> >> __u32 cpu;
> >>
> >> diff --git a/tools/perf/util/bpf_skel/bperf_follower.bpf.c b/tools/perf/util/bpf_skel/bperf_follower.bpf.c
> >> index f193998530d4..0595063139a3 100644
> >> --- a/tools/perf/util/bpf_skel/bperf_follower.bpf.c
> >> +++ b/tools/perf/util/bpf_skel/bperf_follower.bpf.c
> >> @@ -5,6 +5,8 @@
> >> #include <bpf/bpf_tracing.h>
> >> #include "bperf_u.h"
> >>
> >> +#define MAX_ENTRIES 102400
> >> +
> >> struct {
> >> __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
> >> __uint(key_size, sizeof(__u32));
> >> @@ -22,25 +24,29 @@ struct {
> >> struct {
> >> __uint(type, BPF_MAP_TYPE_HASH);
> >> __uint(key_size, sizeof(__u32));
> >> - __uint(value_size, sizeof(__u32));
> >> + __uint(value_size, sizeof(struct bperf_filter_value));
> >> + __uint(max_entries, MAX_ENTRIES);
> >> + __uint(map_flags, BPF_F_NO_PREALLOC);
> >> } filter SEC(".maps");
> >>
> >> enum bperf_filter_type type = 0;
> >> int enabled = 0;
> >> +int inherit;
> >>
> >> SEC("fexit/XXX")
> >> int BPF_PROG(fexit_XXX)
> >> {
> >> struct bpf_perf_event_value *diff_val, *accum_val;
> >> __u32 filter_key, zero = 0;
> >> - __u32 *accum_key;
> >> + __u32 accum_key;
> >> + struct bperf_filter_value *fval;
> >>
> >> if (!enabled)
> >> return 0;
> >>
> >> switch (type) {
> >> case BPERF_FILTER_GLOBAL:
> >> - accum_key = &zero;
> >> + accum_key = zero;
> >> goto do_add;
> >> case BPERF_FILTER_CPU:
> >> filter_key = bpf_get_smp_processor_id();
> >> @@ -49,22 +55,34 @@ int BPF_PROG(fexit_XXX)
> >> filter_key = bpf_get_current_pid_tgid() & 0xffffffff;
> >> break;
> >> case BPERF_FILTER_TGID:
> >> - filter_key = bpf_get_current_pid_tgid() >> 32;
> >> + /* Use pid as the filter_key to exclude new task counts
> >> + * when inherit is disabled. Don't worry about the existing
> >> + * children in TGID losing their counts, bpf_counter has
> >> + * already added them to the filter map via perf_thread_map
> >> + * before this bpf prog runs.
> >> + */
> >> + filter_key = inherit ?
> >> + bpf_get_current_pid_tgid() >> 32 :
> >> + bpf_get_current_pid_tgid() & 0xffffffff;
> >
> > I'm not sure why this is needed. Isn't the existing code fine?
>
> No, it's not. If I don't modify here, all child threads will always be counted
> when inherit is disabled.
>
>
> Before explaining this modification, we may need to first clarify what is included
> in the filter map.
>
> 1. The fexit_XXX prog determines whether to count by filter_key during each
> context switch. If the key is found in the filter map, it will be counted,
> otherwise not.
> 2. The keys in the filter map are synchronized from the perf_thread_map when
> bperf__load().
> 3. The threads in perf_thread_map are added through cmd_stat()->evlist__create_maps()
> before bperf__load().
> 4. evlist__create_maps() fills perf_thread_map by traversing the /proc/%d/task
> directory, and these pids belong to the same tgid.
>
> Therefore, when the bperf command is issued, the filter map already holds all
> existing threads with the same tgid as the specified process.
>
>
> Now, let's take a look at the TGID case. We hope the behavior is as follows:
>
> * TGID w/ inherit : specified process + all children from the processes
> * TGID w/o inherit: specified process (all threads in the process) only
>
> Assuming that a new thread is created during bperf stats, the new thread should
> exhibit the following behavior in the fexit_XXX prog:
>
> * TGID w/ inherit : do_add
> * TGID w/o inherit: skip and return
>
> Let's now test the code before and after modification.
>
> Before modification: (filter_key = tgid)
>
> * TGID w/ inherit:
> create : new thread
> enter : fexit_XXX prog
> assign : filter_key = new thread's tgid
> match : bpf_map_lookup_elem(&filter, &filter_key)
> do_add
> (PASS)
>
> * TGID w/o inherit:
> [...] /* like above */
> do_add
> (FAILED, expect skip and return)
>
> After modification: (filter_key = inherit ? tgid : pid)
>
> * TGID w/ inherit:
> create : new thread
> enter : fexit_XXX prog
> assign : filter_key = new thread's tgid
> match : bpf_map_lookup_elem(&filter, &filter_key)
> do_add
> (PASS)
>
> * TGID w/o inherit:
> create : new thread
> enter : fexit_XXX prog
> assign : filter_key = new thread's pid
> mismatch: bpf_map_lookup_elem(&filter, &filter_key)
> skip and return
> (PASS)
>
> As we can see, filter_key=tgid counts incorrectly in TGID w/o inherit case,
> and we need to change it to filter_key=pid to fix it.
I'm sorry but I don't think I'm following. A new thread in TGID mode
(regardless inherit) should be counted always, right? Why do you
expect to skip it?
Thanks,
Namhyung
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