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Lbr improvements #61

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merged 2 commits into from
Mar 20, 2024
Merged

Lbr improvements #61

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e7dfde8
mass_attach: ensure we have straight-line execution to LBR capture
anakryiko Mar 20, 2024
1620aa8
retsnoop: add full-lbr debug option to not filter LBR records
anakryiko Mar 20, 2024
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12 changes: 8 additions & 4 deletions src/mass_attach.bpf.c
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,9 @@
#include <bpf/bpf_tracing.h>
#include <bpf/bpf_core_read.h>

#define likely(x) __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)

/* these two are defined by custom BPF code outside of mass_attacher */
extern int handle_func_entry(void *ctx, u32 func_id, u64 func_ip);
extern int handle_func_exit(void *ctx, u32 func_id, u64 func_ip, u64 ret);
Expand Down Expand Up @@ -63,7 +66,8 @@ static __always_inline void capture_lbrs(int cpu)
{
long lbr_sz;

if (!use_lbr)
/* prioritize straight path logic if LBR is requested */
if (unlikely(!use_lbr))
return;

lbr_sz = bpf_get_branch_snapshot(&lbrs[cpu & max_cpu_mask], sizeof(lbrs[0]), 0);
Expand Down Expand Up @@ -131,7 +135,7 @@ int kexit(struct pt_regs *ctx)
u32 id, cpu;
long ip;

if (!ready)
if (unlikely(!ready))
return 0;

cpu = bpf_get_smp_processor_id();
Expand Down Expand Up @@ -232,11 +236,11 @@ static __always_inline int handle_fexit(void *ctx, int arg_cnt, bool is_void_ret
long ip;
u64 res;

if (!ready)
if (unlikely(!ready))
return 0;

cpu = bpf_get_smp_processor_id();
if (!recur_enter(cpu))
if (unlikely(!recur_enter(cpu)))
return 0;

capture_lbrs(cpu);
Expand Down
35 changes: 18 additions & 17 deletions src/retsnoop.c
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Original file line number Diff line number Diff line change
Expand Up @@ -48,6 +48,7 @@ enum symb_mode {
enum debug_feat {
DEBUG_NONE = 0x00,
DEBUG_MULTI_KPROBE = 0x01,
DEBUG_FULL_LBR = 0x02,
};

static struct env {
Expand Down Expand Up @@ -200,7 +201,7 @@ static const struct argp_option opts[] = {
{ "stacks-map-size", OPT_STACKS_MAP_SIZE, "SIZE", 0,
"Stacks map size (default 4096)" },
{ "debug", OPT_DEBUG_FEAT, "FEATURE", 0,
"Enable selected debug features. Any set of: multi-kprobe." },
"Enable selected debug features. Any set of: multi-kprobe, full-lbr." },
{},
};

Expand Down Expand Up @@ -308,6 +309,7 @@ static enum debug_feat parse_debug_arg(const char *arg)
enum debug_feat value;
} table[] = {
{"multi-kprobe", DEBUG_MULTI_KPROBE},
{"full-lbr", DEBUG_FULL_LBR},
};

for (i = 0; i < ARRAY_SIZE(table); i++) {
Expand Down Expand Up @@ -1612,23 +1614,22 @@ static int handle_call_stack(struct ctx *dctx, const struct call_stack *s)
lbr_cnt = s->lbrs_sz / sizeof(struct perf_branch_entry);
lbr_from = lbr_cnt - 1;

if (!env.emit_full_stacks) {
/* Filter out last few irrelevant LBRs that captured
* internal BPF/kprobe/perf jumps. For that, find the
* first LBR record that overlaps with the last traced
* function. All the records after that are assumed
* relevant.
*/
for (i = 0, lbr_to = 0; i < lbr_cnt; i++, lbr_to++) {
if (lbr_matches(s->lbrs[i].from, start, end) ||
lbr_matches(s->lbrs[i].to, start, end)) {
found_useful_lbrs = true;
break;
}
/* Filter out last few irrelevant LBRs that captured
* internal BPF/kprobe/perf jumps. For that, find the
* first LBR record that overlaps with the last traced
* function. All the records after that are assumed
* relevant.
*/
for (i = 0, lbr_to = 0; i < lbr_cnt; i++, lbr_to++) {
if (lbr_matches(s->lbrs[i].from, start, end) ||
lbr_matches(s->lbrs[i].to, start, end)) {
found_useful_lbrs = true;
break;
}
if (!found_useful_lbrs)
lbr_to = 0;
}
if (!found_useful_lbrs ||
env.emit_full_stacks || (env.debug_feats & DEBUG_FULL_LBR))
lbr_to = 0;

if (env.lbr_max_cnt && lbr_from - lbr_to + 1 > env.lbr_max_cnt)
lbr_from = min(lbr_cnt - 1, lbr_to + env.lbr_max_cnt - 1);
Expand All @@ -1647,7 +1648,7 @@ static int handle_call_stack(struct ctx *dctx, const struct call_stack *s)
&stack_items1, rec_cnts1,
&stack_items2, rec_cnts2);

if (!env.emit_full_stacks && !found_useful_lbrs)
if (!found_useful_lbrs)
printf("[LBR] No relevant LBR data were captured, showing unfiltered LBR stack!\n");
}

Expand Down