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Message-ID: <Z7ghWIq8wXCJ2Y8T@google.com>
Date: Thu, 20 Feb 2025 22:46:48 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Chun-Tse Shao <ctshao@...gle.com>
Cc: linux-kernel@...r.kernel.org, peterz@...radead.org, mingo@...hat.com,
acme@...nel.org, mark.rutland@....com,
alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
irogers@...gle.com, adrian.hunter@...el.com,
kan.liang@...ux.intel.com, nick.forrington@....com,
linux-perf-users@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v6 2/4] perf lock: Retrieve owner callstack in bpf program
On Wed, Feb 19, 2025 at 01:40:01PM -0800, Chun-Tse Shao wrote:
> This implements per-callstack aggregation of lock owners in addition to
> per-thread. The owner callstack is captured using `bpf_get_task_stack()`
> at `contention_begin()` and it also adds a custom stackid function for the
> owner stacks to be compared easily.
>
> The owner info is kept in a hash map using lock addr as a key to handle
> multiple waiters for the same lock. At `contention_end()`, it updates the
> owner lock stat based on the info that was saved at `contention_begin()`.
> If there are more waiters, it'd update the owner pid to itself as
> `contention_end()` means it gets the lock now. But it also needs to check
> the return value of the lock function in case task was killed by a signal
> or something.
>
> Signed-off-by: Chun-Tse Shao <ctshao@...gle.com>
> ---
> .../perf/util/bpf_skel/lock_contention.bpf.c | 218 +++++++++++++++++-
> 1 file changed, 209 insertions(+), 9 deletions(-)
>
> diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> index 23fe9cc980ae..e8b113d5802a 100644
> --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> @@ -197,6 +197,9 @@ int data_fail;
> int task_map_full;
> int data_map_full;
>
> +struct task_struct *bpf_task_from_pid(s32 pid) __ksym __weak;
> +void bpf_task_release(struct task_struct *p) __ksym __weak;
> +
> static inline __u64 get_current_cgroup_id(void)
> {
> struct task_struct *task;
> @@ -420,6 +423,61 @@ static inline struct tstamp_data *get_tstamp_elem(__u32 flags)
> return pelem;
> }
>
> +static inline s32 get_owner_stack_id(u64 *stacktrace)
> +{
> + s32 *id, new_id;
> + static s64 id_gen = 1;
> +
> + id = bpf_map_lookup_elem(&owner_stacks, stacktrace);
> + if (id)
> + return *id;
> +
> + new_id = (s32)__sync_fetch_and_add(&id_gen, 1);
> +
> + bpf_map_update_elem(&owner_stacks, stacktrace, &new_id, BPF_NOEXIST);
> +
> + id = bpf_map_lookup_elem(&owner_stacks, stacktrace);
> + if (id)
> + return *id;
> +
> + return -1;
> +}
> +
> +static inline void update_contention_data(struct contention_data *data, u64 duration, u32 count)
> +{
> + __sync_fetch_and_add(&data->total_time, duration);
> + __sync_fetch_and_add(&data->count, count);
> +
> + /* FIXME: need atomic operations */
> + if (data->max_time < duration)
> + data->max_time = duration;
> + if (data->min_time > duration)
> + data->min_time = duration;
> +}
> +
> +static inline void update_owner_stat(u32 id, u64 duration, u32 flags)
> +{
> + struct contention_key key = {
> + .stack_id = id,
> + .pid = 0,
> + .lock_addr_or_cgroup = 0,
> + };
> + struct contention_data *data = bpf_map_lookup_elem(&owner_stat, &key);
> +
> + if (!data) {
> + struct contention_data first = {
> + .total_time = duration,
> + .max_time = duration,
> + .min_time = duration,
> + .count = 1,
> + .flags = flags,
> + };
> + bpf_map_update_elem(&owner_stat, &key, &first, BPF_NOEXIST);
> + } else {
> + update_contention_data(data, duration, 1);
> + }
> +}
> +
> SEC("tp_btf/contention_begin")
> int contention_begin(u64 *ctx)
> {
> @@ -437,6 +495,72 @@ int contention_begin(u64 *ctx)
> pelem->flags = (__u32)ctx[1];
>
> if (needs_callstack) {
> + u32 i = 0;
> + u32 id = 0;
> + int owner_pid;
> + u64 *buf;
> + struct task_struct *task;
> + struct owner_tracing_data *otdata;
> +
> + if (!lock_owner)
> + goto skip_owner;
> +
> + task = get_lock_owner(pelem->lock, pelem->flags);
> + if (!task)
> + goto skip_owner;
> +
> + owner_pid = BPF_CORE_READ(task, pid);
> +
> + buf = bpf_map_lookup_elem(&stack_buf, &i);
> + if (!buf)
> + goto skip_owner;
> + for (i = 0; i < max_stack; i++)
> + buf[i] = 0x0;
> +
> + if (!bpf_task_from_pid)
> + goto skip_owner;
> +
> + task = bpf_task_from_pid(owner_pid);
> + if (!task)
> + goto skip_owner;
> +
> + bpf_get_task_stack(task, buf, max_stack * sizeof(unsigned long), 0);
> + bpf_task_release(task);
> +
> + otdata = bpf_map_lookup_elem(&owner_data, &pelem->lock);
> + id = get_owner_stack_id(buf);
> +
> + /*
> + * Contention just happens, or corner case `lock` is owned by process not
> + * `owner_pid`. For the corner case we treat it as unexpected internal error and
> + * just ignore the precvious tracing record.
> + */
> + if (!otdata || otdata->pid != owner_pid) {
> + struct owner_tracing_data first = {
> + .pid = owner_pid,
> + .timestamp = pelem->timestamp,
> + .count = 1,
> + .stack_id = id,
> + };
> + bpf_map_update_elem(&owner_data, &pelem->lock, &first, BPF_ANY);
> + }
> + /* Contention is ongoing and new waiter joins */
> + else {
> + __sync_fetch_and_add(&otdata->count, 1);
> +
> + /*
> + * The owner is the same, but stacktrace might be changed. In this case we
> + * store/update `owner_stat` based on current owner stack id.
> + */
> + if (id != otdata->stack_id) {
> + update_owner_stat(id, pelem->timestamp - otdata->timestamp,
> + pelem->flags);
> +
> + otdata->timestamp = pelem->timestamp;
> + otdata->stack_id = id;
> + }
> + }
> +skip_owner:
> pelem->stack_id = bpf_get_stackid(ctx, &stacks,
> BPF_F_FAST_STACK_CMP | stack_skip);
> if (pelem->stack_id < 0)
> @@ -473,6 +597,7 @@ int contention_end(u64 *ctx)
> struct tstamp_data *pelem;
> struct contention_key key = {};
> struct contention_data *data;
> + __u64 timestamp;
> __u64 duration;
> bool need_delete = false;
>
> @@ -500,12 +625,94 @@ int contention_end(u64 *ctx)
> need_delete = true;
> }
>
> - duration = bpf_ktime_get_ns() - pelem->timestamp;
> + timestamp = bpf_ktime_get_ns();
> + duration = timestamp - pelem->timestamp;
> if ((__s64)duration < 0) {
> __sync_fetch_and_add(&time_fail, 1);
> goto out;
> }
>
> + if (needs_callstack && lock_owner) {
> + struct owner_tracing_data *otdata = bpf_map_lookup_elem(&owner_data, &pelem->lock);
> +
> + if (!otdata)
> + goto skip_owner;
> +
> + /* Update `owner_stat` */
> + update_owner_stat(otdata->stack_id, timestamp - otdata->timestamp, pelem->flags);
> +
> + /* No contention is occurring, delete `lock` entry in `owner_data` */
> + if (otdata->count <= 1)
> + bpf_map_delete_elem(&owner_data, &pelem->lock);
> + /*
> + * Contention is still ongoing, with a new owner (current task). `owner_data`
> + * should be updated accordingly.
> + */
> + else {
> + u32 i = 0;
> + s32 ret = (s32)ctx[1];
> + u64 *buf;
> +
> + __sync_fetch_and_add(&otdata->count, -1);
> +
> + buf = bpf_map_lookup_elem(&stack_buf, &i);
> + if (!buf)
> + goto skip_owner;
> + for (i = 0; i < (u32)max_stack; i++)
> + buf[i] = 0x0;
> +
> + /*
> + * `ret` has the return code of the lock function.
> + * If `ret` is negative, the current task terminates lock waiting without
> + * acquiring it. Owner is not changed, but we still need to update the owner
> + * stack.
> + */
> + if (ret < 0) {
> + s32 id = 0;
> + struct task_struct *task;
> +
> + if (!bpf_task_from_pid)
> + goto skip_owner;
> +
> + task = bpf_task_from_pid(otdata->pid);
> + if (!task)
> + goto skip_owner;
> +
> + bpf_get_task_stack(task, buf,
> + max_stack * sizeof(unsigned long), 0);
> + bpf_task_release(task);
> +
> + id = get_owner_stack_id(buf);
> +
> + /*
> + * If owner stack is changed, update `owner_data` and `owner_stat`
> + * accordingly.
> + */
> + if (id != otdata->stack_id) {
> + update_owner_stat(id, pelem->timestamp - otdata->timestamp,
Shouldn't it be 'timestamp' instead of 'pelem->timestamp'?
> + pelem->flags);
> +
> + otdata->timestamp = pelem->timestamp;
Ditto.
Thanks,
Namhyung
> + otdata->stack_id = id;
> + }
> + }
> + /*
> + * Otherwise, update tracing data with the current task, which is the new
> + * owner.
> + */
> + else {
> + otdata->pid = pid;
> + otdata->timestamp = timestamp;
> + /*
> + * We don't want to retrieve callstack here, since it is where the
> + * current task acquires the lock and provides no additional
> + * information. We simply assign -1 to invalidate it.
> + */
> + otdata->stack_id = -1;
> + }
> + }
> + }
> +skip_owner:
> switch (aggr_mode) {
> case LOCK_AGGR_CALLER:
> key.stack_id = pelem->stack_id;
> @@ -589,14 +796,7 @@ int contention_end(u64 *ctx)
> }
>
> found:
> - __sync_fetch_and_add(&data->total_time, duration);
> - __sync_fetch_and_add(&data->count, 1);
> -
> - /* FIXME: need atomic operations */
> - if (data->max_time < duration)
> - data->max_time = duration;
> - if (data->min_time > duration)
> - data->min_time = duration;
> + update_contention_data(data, duration, 1);
>
> out:
> pelem->lock = 0;
> --
> 2.48.1.601.g30ceb7b040-goog
>
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