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libbpf-tools: use fentry in funclatency
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For tracing kernel functions, funclatency will attempt to use fentry and
fallback to kprobes if that fails.  I left the option to force the use
of kprobes if you'd like.

Under load, fentry is more efficient:

With kprobes:

cpu:/tmp/b# ./funclatency -u -k bpf_spin_lock
Tracing bpf_spin_lock.  Hit Ctrl-C to exit
^C
     usec                : count    distribution
         0 -> 1          : 565159   |****************************************|
         2 -> 3          : 216204   |***************                         |
         4 -> 7          : 306316   |*********************                   |
         8 -> 15         : 416990   |*****************************           |
        16 -> 31         : 415110   |*****************************           |
        32 -> 63         : 170298   |************                            |
        64 -> 127        : 29726    |**                                      |
       128 -> 255        : 427962   |******************************          |
       256 -> 511        : 233207   |****************                        |
       512 -> 1023       : 453      |                                        |
      1024 -> 2047       : 0        |                                        |
Exiting trace of bpf_spin_lock

With fentry:
cpu:/tmp/b# ./funclatency -u bpf_spin_lock
Tracing bpf_spin_lock.  Hit Ctrl-C to exit
^C
     usec                : count    distribution
         0 -> 1          : 6371351  |****************************************+|
         2 -> 3          : 1100093  |******                                  |
         4 -> 7          : 518197   |***                                     |
         8 -> 15         : 174881   |*                                       |
        16 -> 31         : 25022    |                                        |
        32 -> 63         : 1553     |                                        |
        64 -> 127        : 1312     |                                        |
       128 -> 255        : 2376     |                                        |
       256 -> 511        : 0        |                                        |
Exiting trace of bpf_spin_lock

Signed-off-by: Barret Rhoden <[email protected]>
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@brho @yonghong-song
brho authored and yonghong-song committed May 15, 2023
1 parent 7d350d9 commit 3e2a251
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Showing 2 changed files with 97 additions and 26 deletions.
38 changes: 30 additions & 8 deletions libbpf-tools/funclatency.bpf.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -28,27 +28,37 @@ struct {

__u32 hist[MAX_SLOTS] = {};

SEC("kprobe/dummy_kprobe")
int BPF_KPROBE(dummy_kprobe)
static void entry(void)
{
u64 id = bpf_get_current_pid_tgid();
u32 tgid = id >> 32;
u32 pid = id;
u64 nsec;

if (filter_cg && !bpf_current_task_under_cgroup(&cgroup_map, 0))
return 0;
return;

if (targ_tgid && targ_tgid != tgid)
return 0;
return;
nsec = bpf_ktime_get_ns();
bpf_map_update_elem(&starts, &pid, &nsec, BPF_ANY);
}

SEC("fentry/dummy_fentry")
int BPF_PROG(dummy_fentry)
{
entry();
return 0;
}

SEC("kretprobe/dummy_kretprobe")
int BPF_KRETPROBE(dummy_kretprobe)
SEC("kprobe/dummy_kprobe")
int BPF_KPROBE(dummy_kprobe)
{
entry();
return 0;
}

static void exit(void)
{
u64 *start;
u64 nsec = bpf_ktime_get_ns();
Expand All @@ -57,11 +67,11 @@ int BPF_KRETPROBE(dummy_kretprobe)
u64 slot, delta;

if (filter_cg && !bpf_current_task_under_cgroup(&cgroup_map, 0))
return 0;
return;

start = bpf_map_lookup_elem(&starts, &pid);
if (!start)
return 0;
return;

delta = nsec - *start;

Expand All @@ -78,7 +88,19 @@ int BPF_KRETPROBE(dummy_kretprobe)
if (slot >= MAX_SLOTS)
slot = MAX_SLOTS - 1;
__sync_fetch_and_add(&hist[slot], 1);
}

SEC("fexit/dummy_fexit")
int BPF_PROG(dummy_fexit)
{
exit();
return 0;
}

SEC("kretprobe/dummy_kretprobe")
int BPF_KRETPROBE(dummy_kretprobe)
{
exit();
return 0;
}

Expand Down
85 changes: 67 additions & 18 deletions libbpf-tools/funclatency.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -38,8 +38,10 @@ static struct prog_env {
bool timestamp;
char *funcname;
bool verbose;
bool kprobes;
char *cgroupspath;
bool cg;
bool is_kernel_func;
} env = {
.interval = 99999999,
.iterations = 99999999,
Expand Down Expand Up @@ -82,6 +84,7 @@ static const struct argp_option opts[] = {
{ "duration", 'd', "DURATION", 0, "Duration to trace"},
{ "timestamp", 'T', NULL, 0, "Print timestamp"},
{ "verbose", 'v', NULL, 0, "Verbose debug output" },
{ "kprobes", 'k', NULL, 0, "Use kprobes instead of fentry" },
{ NULL, 'h', NULL, OPTION_HIDDEN, "Show the full help"},
{},
};
Expand Down Expand Up @@ -140,6 +143,9 @@ static error_t parse_arg(int key, char *arg, struct argp_state *state)
case 'T':
env->timestamp = true;
break;
case 'k':
env->kprobes = true;
break;
case 'v':
env->verbose = true;
break;
Expand Down Expand Up @@ -191,23 +197,56 @@ static const char *unit_str(void)
return "bad units";
}

static int attach_kprobes(struct funclatency_bpf *obj)
static bool try_fentry(struct funclatency_bpf *obj)
{
long err;

obj->links.dummy_kprobe = bpf_program__attach_kprobe(obj->progs.dummy_kprobe, false,
env.funcname);
if (env.kprobes || !env.is_kernel_func ||
!fentry_can_attach(env.funcname, NULL)) {
goto out_no_fentry;
}

err = bpf_program__set_attach_target(obj->progs.dummy_fentry, 0,
env.funcname);
if (err) {
warn("failed to set attach fentry: %s\n", strerror(-err));
goto out_no_fentry;
}

err = bpf_program__set_attach_target(obj->progs.dummy_fexit, 0,
env.funcname);
if (err) {
warn("failed to set attach fexit: %s\n", strerror(-err));
goto out_no_fentry;
}

bpf_program__set_autoload(obj->progs.dummy_kprobe, false);
bpf_program__set_autoload(obj->progs.dummy_kretprobe, false);

return true;

out_no_fentry:
bpf_program__set_autoload(obj->progs.dummy_fentry, false);
bpf_program__set_autoload(obj->progs.dummy_fexit, false);

return false;
}

static int attach_kprobes(struct funclatency_bpf *obj)
{
obj->links.dummy_kprobe =
bpf_program__attach_kprobe(obj->progs.dummy_kprobe, false,
env.funcname);
if (!obj->links.dummy_kprobe) {
err = -errno;
warn("failed to attach kprobe: %ld\n", err);
warn("failed to attach kprobe: %d\n", -errno);
return -1;
}

obj->links.dummy_kretprobe = bpf_program__attach_kprobe(obj->progs.dummy_kretprobe, true,
env.funcname);
obj->links.dummy_kretprobe =
bpf_program__attach_kprobe(obj->progs.dummy_kretprobe, true,
env.funcname);
if (!obj->links.dummy_kretprobe) {
err = -errno;
warn("failed to attach kretprobe: %ld\n", err);
warn("failed to attach kretprobe: %d\n", -errno);
return -1;
}

Expand Down Expand Up @@ -270,13 +309,6 @@ static int attach_uprobes(struct funclatency_bpf *obj)
return ret;
}

static int attach_probes(struct funclatency_bpf *obj)
{
if (strchr(env.funcname, ':'))
return attach_uprobes(obj);
return attach_kprobes(obj);
}

static volatile bool exiting;

static void sig_hand(int signr)
Expand All @@ -302,11 +334,14 @@ int main(int argc, char **argv)
time_t t;
int idx, cg_map_fd;
int cgfd = -1;
bool used_fentry = false;

err = argp_parse(&argp, argc, argv, 0, NULL, &env);
if (err)
return err;

env.is_kernel_func = !strchr(env.funcname, ':');

sigaction(SIGINT, &sigact, 0);

libbpf_set_print(libbpf_print_fn);
Expand All @@ -327,6 +362,8 @@ int main(int argc, char **argv)
obj->rodata->targ_tgid = env.pid;
obj->rodata->filter_cg = env.cg;

used_fentry = try_fentry(obj);

err = funclatency_bpf__load(obj);
if (err) {
warn("failed to load BPF object\n");
Expand All @@ -353,9 +390,21 @@ int main(int argc, char **argv)
goto cleanup;
}

err = attach_probes(obj);
if (err)
if (!used_fentry) {
if (env.is_kernel_func)
err = attach_kprobes(obj);
else
err = attach_uprobes(obj);
if (err)
goto cleanup;
}

err = funclatency_bpf__attach(obj);
if (err) {
fprintf(stderr, "failed to attach BPF programs: %s\n",
strerror(-err));
goto cleanup;
}

printf("Tracing %s. Hit Ctrl-C to exit\n", env.funcname);

Expand Down

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