On Wed, Oct 23, 2024 at 7:11 PM Tengda Wu <wutengda@xxxxxxxxxxxxxxx> wrote:
>
>
>
> On 2024/10/24 6:45, Namhyung Kim wrote:
> > 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@xxxxxxxxxxxxxxx>
> >>>> ---
> >>>> 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?
>
> This is how perf originally performs. To confirm this, I wrote a workload
> that creates one new thread per second and then stat it, as shown below.
> You can see that in 'TGID w/o inherit' case, perf does not count for the
> newly created threads.
>
> Perf TGID w/ inherit:
> ---
> $ ./perf stat -e cpu-clock --timeout 5000 -- ./new_thread_per_second
> thread 367444: start [main]
> thread 367448: start
> thread 367455: start
> thread 367462: start
> thread 367466: start
> thread 367473: start
> ./new_thread_per_second: Terminated
>
> Performance counter stats for './new_thread_per_second':
>
> 10,017.71 msec cpu-clock
>
> 5.005538048 seconds time elapsed
>
> 10.018777000 seconds user
> 0.000000000 seconds sys
>
> Perf TGID w/o inherit:
> ---
> $ ./perf stat -e cpu-clock --timeout 5000 -i -- ./new_thread_per_second
> thread 366679: start [main]
> thread 366686: start
> thread 366693: start
> thread 366697: start
> thread 366704: start
> thread 366708: start
> ./new_thread_per_second: Terminated
>
> Performance counter stats for './new_thread_per_second':
>
> 4.29 msec cpu-clock
>
> 5.005539338 seconds time elapsed
>
> 10.019673000 seconds user
> 0.000000000 seconds sys
>
>
> Therefore, we also need to distinguish it in bperf so that the collection
> results can be consistent with perf.
>
> Bperf TGID w/o inherit: (BEFORE FIX)
> ---
> $ ./perf stat --bpf-counters -e cpu-clock --timeout 5000 -i -- ./new_thread_per_second
> thread 369127: start [main]
> thread 369134: start
> thread 369141: start
> thread 369145: start
> thread 369152: start
> thread 369156: start
> ./new_thread_per_second: Terminated
>
> Performance counter stats for './new_thread_per_second':
>
> 10,019.05 msec cpu-clock
>
> 5.005567266 seconds time elapsed
>
> 10.018528000 seconds user
> 0.000000000 seconds sys
>
> Bperf TGID w/o inherit: (AFTER FIX)
> ---
> $ ./perf stat --bpf-counters -e cpu-clock --timeout 5000 -i -- ./new_thread_per_second
> thread 366616: start [main]
> thread 366623: start
> thread 366627: start
> thread 366634: start
> thread 366638: start
> thread 366645: start
> ./new_thread_per_second: Terminated
>
> Performance counter stats for './new_thread_per_second':
>
> 4.95 msec cpu-clock
>
> 5.005511173 seconds time elapsed
>
> 10.018790000 seconds user
> 0.000000000 seconds sys
>
>
> Thanks,
> Tengda
>
Thanks for the explanation. Ok I think it's the limitation of the current
implementation of perf_event that works at thread-level. Even if we
can count events at process-level with bperf, it might be important to
keep the compatibility with the existing behavior.
Thanks,
Namhyung
