Re: [PATCH 4/4] perf stat: Enable BPF counter with --for-each-cgroup

[Song Liu <songliubraving@fb.com>]

> On Jun 25, 2021, at 12:18 AM, Namhyung Kim <namhyung@kernel.org> wrote:
> 
> Recently bperf was added to use BPF to count perf events for various
> purposes.  This is an extension for the approach and targetting to
> cgroup usages.
> 
> Unlike the other bperf, it doesn't share the events with other
> processes but it'd reduce unnecessary events (and the overhead of
> multiplexing) for each monitored cgroup within the perf session.
> 
> When --for-each-cgroup is used with --bpf-counters, it will open
> cgroup-switches event per cpu internally and attach the new BPF
> program to read given perf_events and to aggregate the results for
> cgroups.  It's only called when task is switched to a task in a
> different cgroup.
> 
> Cc: Song Liu <songliubraving@fb.com>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
> tools/perf/Makefile.perf                    |  17 +-
> tools/perf/util/Build                       |   1 +
> tools/perf/util/bpf_counter.c               |   5 +
> tools/perf/util/bpf_counter_cgroup.c        | 299 ++++++++++++++++++++
> tools/perf/util/bpf_skel/bperf_cgroup.bpf.c | 191 +++++++++++++
> tools/perf/util/cgroup.c                    |   2 +
> tools/perf/util/cgroup.h                    |   1 +
> 7 files changed, 515 insertions(+), 1 deletion(-)
> create mode 100644 tools/perf/util/bpf_counter_cgroup.c
> create mode 100644 tools/perf/util/bpf_skel/bperf_cgroup.bpf.c

[...]

> diff --git a/tools/perf/util/bpf_counter_cgroup.c b/tools/perf/util/bpf_counter_cgroup.c
> new file mode 100644
> index 000000000000..327f97a23a84
> --- /dev/null
> +++ b/tools/perf/util/bpf_counter_cgroup.c
> @@ -0,0 +1,299 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/* Copyright (c) 2019 Facebook */

I am not sure whether this ^^^ is accurate. 

> +/* Copyright (c) 2021 Google */
> +
> +#include <assert.h>
> +#include <limits.h>
> +#include <unistd.h>
> +#include <sys/file.h>
> +#include <sys/time.h>
> +#include <sys/resource.h>
> +#include <linux/err.h>
> +#include <linux/zalloc.h>
> +#include <linux/perf_event.h>
> +#include <api/fs/fs.h>
> +#include <perf/bpf_perf.h>
> +
> +#include "affinity.h"
> +#include "bpf_counter.h"
> +#include "cgroup.h"
> +#include "counts.h"
> +#include "debug.h"
> +#include "evsel.h"
> +#include "evlist.h"
> +#include "target.h"
> +#include "cpumap.h"
> +#include "thread_map.h"
> +
> +#include "bpf_skel/bperf_cgroup.skel.h"
> +
> +static struct perf_event_attr cgrp_switch_attr = {
> +	.type = PERF_TYPE_SOFTWARE,
> +	.config = PERF_COUNT_SW_CGROUP_SWITCHES,
> +	.size = sizeof(cgrp_switch_attr),
> +	.sample_period = 1,
> +	.disabled = 1,
> +};
> +
> +static struct evsel *cgrp_switch;
> +static struct bperf_cgroup_bpf *skel;
> +
> +#define FD(evt, cpu) (*(int *)xyarray__entry(evt->core.fd, cpu, 0))
> +
> +static int bperf_load_program(struct evlist *evlist)
> +{
> +	struct bpf_link *link;
> +	struct evsel *evsel;
> +	struct cgroup *cgrp, *leader_cgrp;
> +	__u32 i, cpu;
> +	int nr_cpus = evlist->core.all_cpus->nr;
> +	int total_cpus = cpu__max_cpu();
> +	int map_size, map_fd;
> +	int prog_fd, err;
> +
> +	skel = bperf_cgroup_bpf__open();
> +	if (!skel) {
> +		pr_err("Failed to open cgroup skeleton\n");
> +		return -1;
> +	}
> +
> +	skel->rodata->num_cpus = total_cpus;
> +	skel->rodata->num_events = evlist->core.nr_entries / nr_cgroups;
> +
> +	BUG_ON(evlist->core.nr_entries % nr_cgroups != 0);
> +
> +	/* we need one copy of events per cpu for reading */
> +	map_size = total_cpus * evlist->core.nr_entries / nr_cgroups;
> +	bpf_map__resize(skel->maps.events, map_size);
> +	bpf_map__resize(skel->maps.cgrp_idx, nr_cgroups);
> +	/* previous result is saved in a per-cpu array */
> +	map_size = evlist->core.nr_entries / nr_cgroups;
> +	bpf_map__resize(skel->maps.prev_readings, map_size);
> +	/* cgroup result needs all events (per-cpu) */
> +	map_size = evlist->core.nr_entries;
> +	bpf_map__resize(skel->maps.cgrp_readings, map_size);
> +
> +	set_max_rlimit();
> +
> +	err = bperf_cgroup_bpf__load(skel);
> +	if (err) {
> +		pr_err("Failed to load cgroup skeleton\n");
> +		goto out;
> +	}
> +
> +	if (cgroup_is_v2("perf_event") > 0)
> +		skel->bss->use_cgroup_v2 = 1;
> +
> +	err = -1;
> +
> +	cgrp_switch = evsel__new(&cgrp_switch_attr);
> +	if (evsel__open_per_cpu(cgrp_switch, evlist->core.all_cpus, -1) < 0) {
> +		pr_err("Failed to open cgroup switches event\n");
> +		goto out;
> +	}
> +
> +	for (i = 0; i < nr_cpus; i++) {
> +		link = bpf_program__attach_perf_event(skel->progs.on_cgrp_switch,
> +						      FD(cgrp_switch, i));
> +		if (IS_ERR(link)) {
> +			pr_err("Failed to attach cgroup program\n");
> +			err = PTR_ERR(link);
> +			goto out;
> +		}
> +	}
> +
> +	/*
> +	 * Update cgrp_idx map from cgroup-id to event index.
> +	 */
> +	cgrp = NULL;
> +	i = 0;
> +
> +	evlist__for_each_entry(evlist, evsel) {
> +		if (cgrp == NULL || evsel->cgrp == leader_cgrp) {
> +			leader_cgrp = evsel->cgrp;
> +			evsel->cgrp = NULL;
> +
> +			/* open single copy of the events w/o cgroup */
> +			err = evsel__open_per_cpu(evsel, evlist->core.all_cpus, -1);
> +			if (err) {
> +				pr_err("Failed to open first cgroup events\n");
> +				goto out;
> +			}
> +
> +			map_fd = bpf_map__fd(skel->maps.events);
> +			for (cpu = 0; cpu < nr_cpus; cpu++) {
> +				int fd = FD(evsel, cpu);
> +				__u32 idx = evsel->idx * total_cpus +
> +					evlist->core.all_cpus->map[cpu];
> +
> +				err = bpf_map_update_elem(map_fd, &idx, &fd,
> +							  BPF_ANY);
> +				if (err < 0) {
> +					pr_err("Failed to update perf_event fd\n");
> +					goto out;
> +				}
> +			}
> +
> +			evsel->cgrp = leader_cgrp;
> +		}
> +		evsel->supported = true;
> +
> +		if (evsel->cgrp == cgrp)
> +			continue;
> +
> +		cgrp = evsel->cgrp;
> +
> +		if (read_cgroup_id(cgrp) < 0) {
> +			pr_err("Failed to get cgroup id\n");
> +			err = -1;
> +			goto out;
> +		}
> +
> +		map_fd = bpf_map__fd(skel->maps.cgrp_idx);
> +		err = bpf_map_update_elem(map_fd, &cgrp->id, &i, BPF_ANY);
> +		if (err < 0) {
> +			pr_err("Failed to update cgroup index map\n");
> +			goto out;
> +		}
> +
> +		i++;
> +	}
> +
> +	/*
> +	 * bperf uses BPF_PROG_TEST_RUN to get accurate reading. Check
> +	 * whether the kernel support it
> +	 */
> +	prog_fd = bpf_program__fd(skel->progs.trigger_read);
> +	err = bperf_trigger_reading(prog_fd, 0);
> +	if (err) {
> +		pr_debug("The kernel does not support test_run for raw_tp BPF programs.\n"
> +			 "Therefore, --for-each-cgroup might show inaccurate readings\n");

I think this should be a warning, and we should set err = 0 to continue? 

> +	}
> +
> +out:
> +	return err;
> +}
> +

[...]

> +
> +/*
> + * trigger the leader prog on each cpu, so the cgrp_reading map could get
> + * the latest results.
> + */
> +static int bperf_cgrp__sync_counters(struct evlist *evlist)
> +{
> +	int i, cpu;
> +	int nr_cpus = evlist->core.all_cpus->nr;
> +	int prog_fd = bpf_program__fd(skel->progs.trigger_read);
> +
> +	for (i = 0; i < nr_cpus; i++) {
> +		cpu = evlist->core.all_cpus->map[i];
> +		bperf_trigger_reading(prog_fd, cpu);
> +	}
> +
> +	return 0;
> +}
> +
> +static int bperf_cgrp__enable(struct evsel *evsel)
> +{

Do we need to call bperf_cgrp__sync_counters() before setting enabled to 1? 
If we don't, we may count some numbers before setting enabled to 1, no? 

> +	skel->bss->enabled = 1;
> +	return 0;
> +}

[...]