[PATCH] perf lock contention: Do not use BPF task local storage

Namhyung Kim
2022-11-18T19:01:43Z

It caused some troubles when a lock inside kmalloc is contended
because task local storage would allocate memory using kmalloc.
It'd create a recusion and even crash in my system.

There could be a couple of workarounds but I think the simplest
one is to use a pre-allocated hash map. We could fix the task
local storage to use the safe BPF allocator, but it takes time
so let's change this until it happens actually.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
tools/perf/util/bpf_lock_contention.c | 1 +
.../perf/util/bpf_skel/lock_contention.bpf.c | 34 ++++++++++++-------
2 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
index 0deec1178778..4db9ad3d50c4 100644
--- a/tools/perf/util/bpf_lock_contention.c
+++ b/tools/perf/util/bpf_lock_contention.c
@@ -39,6 +39,7 @@ int lock_contention_prepare(struct lock_contention *con)
bpf_map__set_value_size(skel->maps.stacks, con->max_stack * sizeof(u64));
bpf_map__set_max_entries(skel->maps.stacks, con->map_nr_entries);
bpf_map__set_max_entries(skel->maps.lock_stat, con->map_nr_entries);
+ bpf_map__set_max_entries(skel->maps.tstamp, con->map_nr_entries);

if (target__has_cpu(target))
ncpus = perf_cpu_map__nr(evlist->core.user_requested_cpus);
diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
index 1bb8628e7c9f..9681cb59b0df 100644
--- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
+++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
@@ -40,10 +40,10 @@ struct {

/* maintain timestamp at the beginning of contention */
struct {
- __uint(type, BPF_MAP_TYPE_TASK_STORAGE);
- __uint(map_flags, BPF_F_NO_PREALLOC);
+ __uint(type, BPF_MAP_TYPE_HASH);
__type(key, int);
__type(value, struct tstamp_data);
+ __uint(max_entries, MAX_ENTRIES);
} tstamp SEC(".maps");

/* actual lock contention statistics */
@@ -103,18 +103,28 @@ static inline int can_record(void)
SEC("tp_btf/contention_begin")
int contention_begin(u64 *ctx)
{
- struct task_struct *curr;
+ __u32 pid;
struct tstamp_data *pelem;

if (!enabled || !can_record())
return 0;

- curr = bpf_get_current_task_btf();
- pelem = bpf_task_storage_get(&tstamp, curr, NULL,
- BPF_LOCAL_STORAGE_GET_F_CREATE);
- if (!pelem || pelem->lock)
+ pid = bpf_get_current_pid_tgid();
+ pelem = bpf_map_lookup_elem(&tstamp, &pid);
+ if (pelem && pelem->lock)
return 0;

+ if (pelem == NULL) {
+ struct tstamp_data zero = {};
+
+ bpf_map_update_elem(&tstamp, &pid, &zero, BPF_ANY);
+ pelem = bpf_map_lookup_elem(&tstamp, &pid);
+ if (pelem == NULL) {
+ lost++;
+ return 0;
+ }
+ }
+
pelem->timestamp = bpf_ktime_get_ns();
pelem->lock = (__u64)ctx[0];
pelem->flags = (__u32)ctx[1];
@@ -128,7 +138,7 @@ int contention_begin(u64 *ctx)
SEC("tp_btf/contention_end")
int contention_end(u64 *ctx)
{
- struct task_struct *curr;
+ __u32 pid;
struct tstamp_data *pelem;
struct contention_key key;
struct contention_data *data;
@@ -137,8 +147,8 @@ int contention_end(u64 *ctx)
if (!enabled)
return 0;

- curr = bpf_get_current_task_btf();
- pelem = bpf_task_storage_get(&tstamp, curr, NULL, 0);
+ pid = bpf_get_current_pid_tgid();
+ pelem = bpf_map_lookup_elem(&tstamp, &pid);
if (!pelem || pelem->lock != ctx[0])
return 0;

@@ -156,7 +166,7 @@ int contention_end(u64 *ctx)
};

bpf_map_update_elem(&lock_stat, &key, &first, BPF_NOEXIST);
- pelem->lock = 0;
+ bpf_map_delete_elem(&tstamp, &pid);
return 0;
}

@@ -169,7 +179,7 @@ int contention_end(u64 *ctx)
if (data->min_time > duration)
data->min_time = duration;

- pelem->lock = 0;
+ bpf_map_delete_elem(&tstamp, &pid);
return 0;
}

--
2.38.1.584.g0f3c55d4c2-goog