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

* [PATCH] perf lock contention: Do not use BPF task local storage
@ 2022-11-18 19:01 Namhyung Kim
  2022-11-21 17:32 ` Martin KaFai Lau
  0 siblings, 1 reply; 8+ messages in thread
From: Namhyung Kim @ 2022-11-18 19:01 UTC (permalink / raw)
  To: Arnaldo Carvalho de Melo, Jiri Olsa
  Cc: Ingo Molnar, Peter Zijlstra, LKML, Ian Rogers, Adrian Hunter,
	linux-perf-users, Song Liu, bpf, Blake Jones, Chris Li

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


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