Re: [PATCH v2 2/4] perf lock: Retrieve owner callstack in bpf program

Namhyung Kim
2025-01-14T03:35:19Z

On Sun, Jan 12, 2025 at 09:20:15PM -0800, Chun-Tse Shao wrote:
> Tracing owner callstack in `contention_begin()` and `contention_end()`,
> and storing in `owner_lock_stat` bpf map.
>
> Signed-off-by: Chun-Tse Shao <ctshao@google.com>
> ---
> .../perf/util/bpf_skel/lock_contention.bpf.c | 152 +++++++++++++++++-
> 1 file changed, 151 insertions(+), 1 deletion(-)
>
> diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> index 05da19fdab23..3f47fbfa237c 100644
> --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> @@ -7,6 +7,7 @@
> #include <asm-generic/errno-base.h>
>
> #include "lock_data.h"
> +#include <time.h>
>
> /* for collect_lock_syms(). 4096 was rejected by the verifier */
> #define MAX_CPUS 1024
> @@ -178,6 +179,9 @@ int data_fail;
> int task_map_full;
> int data_map_full;
>
> +struct task_struct *bpf_task_from_pid(s32 pid) __ksym;
> +void bpf_task_release(struct task_struct *p) __ksym;

To support old (ancient?) kernels, you can declare them as __weak and
check if one of them is defined and ignore owner stacks on them. Also
you can check them in user space and turn off the option before loading.

> +
> static inline __u64 get_current_cgroup_id(void)
> {
> struct task_struct *task;
> @@ -407,6 +411,60 @@ int contention_begin(u64 *ctx)
> pelem->flags = (__u32)ctx[1];
>
> if (needs_callstack) {
> + u32 i = 0;
> + int owner_pid;
> + unsigned long *entries;
> + struct task_struct *task;
> + cotd *data;
> +
> + if (!lock_owner)
> + goto contention_begin_skip_owner_callstack;

Can be it 'skip_owner'?

> +
> + task = get_lock_owner(pelem->lock, pelem->flags);
> + if (!task)
> + goto contention_begin_skip_owner_callstack;
> +
> + owner_pid = BPF_CORE_READ(task, pid);
> +
> + entries = bpf_map_lookup_elem(&owner_stacks_entries, &i);
> + if (!entries)
> + goto contention_begin_skip_owner_callstack;
> + for (i = 0; i < max_stack; i++)
> + entries[i] = 0x0;
> +
> + task = bpf_task_from_pid(owner_pid);
> + if (task) {
> + bpf_get_task_stack(task, entries,
> + max_stack * sizeof(unsigned long),
> + 0);
> + bpf_task_release(task);
> + }
> +
> + data = bpf_map_lookup_elem(&contention_owner_tracing,
> + &(pelem->lock));

No need for parenthesis.

> +
> + // Contention just happens, or corner case `lock` is owned by
> + // process not `owner_pid`.
> + if (!data || data->pid != owner_pid) {
> + cotd first = {
> + .pid = owner_pid,
> + .timestamp = pelem->timestamp,
> + .count = 1,
> + };
> + bpf_map_update_elem(&contention_owner_tracing,
> + &(pelem->lock), &first, BPF_ANY);
> + bpf_map_update_elem(&contention_owner_stacks,
> + &(pelem->lock), entries, BPF_ANY);

Hmm.. it just discard the old owner data if it comes from a new owner?
Why not save the data into the result for the old lock/callstack?

> + }
> + // Contention is going on and new waiter joins.
> + else {
> + __sync_fetch_and_add(&data->count, 1);
> + // TODO: Since for owner the callstack would change at
> + // different time, We should check and report if the
> + // callstack is different with the recorded one in
> + // `contention_owner_stacks`.
> + }
> +contention_begin_skip_owner_callstack:
> pelem->stack_id = bpf_get_stackid(ctx, &stacks,
> BPF_F_FAST_STACK_CMP | stack_skip);
> if (pelem->stack_id < 0)
> @@ -443,6 +501,7 @@ 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;
>
> @@ -469,12 +528,103 @@ int contention_end(u64 *ctx)
> return 0;
> 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) {
> + u64 owner_contention_time;
> + unsigned long *owner_stack;
> + struct contention_data *cdata;
> + cotd *otdata;
> +
> + otdata = bpf_map_lookup_elem(&contention_owner_tracing,
> + &(pelem->lock));
> + owner_stack = bpf_map_lookup_elem(&contention_owner_stacks,
> + &(pelem->lock));
> + if (!otdata || !owner_stack)
> + goto contention_end_skip_owner_callstack;
> +
> + owner_contention_time = timestamp - otdata->timestamp;
> +
> + // Update `owner_lock_stat` if `owner_stack` is
> + // available.
> + if (owner_stack[0] != 0x0) {
> + cdata = bpf_map_lookup_elem(&owner_lock_stat,
> + owner_stack);
> + if (!cdata) {
> + struct contention_data first = {
> + .total_time = owner_contention_time,
> + .max_time = owner_contention_time,
> + .min_time = owner_contention_time,
> + .count = 1,
> + .flags = pelem->flags,
> + };
> + bpf_map_update_elem(&owner_lock_stat,
> + owner_stack, &first,
> + BPF_ANY);
> + } else {
> + __sync_fetch_and_add(&cdata->total_time,
> + owner_contention_time);
> + __sync_fetch_and_add(&cdata->count, 1);
> +
> + /* FIXME: need atomic operations */
> + if (cdata->max_time < owner_contention_time)
> + cdata->max_time = owner_contention_time;
> + if (cdata->min_time > owner_contention_time)
> + cdata->min_time = owner_contention_time;
> + }
> + }
> +
> + // No contention is going on, delete `lock` in
> + // `contention_owner_tracing` and
> + // `contention_owner_stacks`
> + if (otdata->count <= 1) {
> + bpf_map_delete_elem(&contention_owner_tracing,
> + &(pelem->lock));
> + bpf_map_delete_elem(&contention_owner_stacks,
> + &(pelem->lock));
> + }
> + // Contention is still going on, with a new owner
> + // (current task). `otdata` should be updated accordingly.
> + else {
> + (otdata->count)--;

No need for parenthesis, and it needs to be atomic dec.

> +
> + // If ctx[1] is not 0, the current task terminates lock
> + // waiting without acquiring it. Owner is not changed.

Please add a comment that ctx[1] has the return code of the lock
function. Maybe it's better to use a local variable.

Also I think you need to say about the normal case too. Returing 0
means the waiter now gets the lock and becomes a new owner. So it needs
to update the owner information.

> + if (ctx[1] == 0) {
> + u32 i = 0;
> + unsigned long *entries = bpf_map_lookup_elem(
> + &owner_stacks_entries, &i);
> + if (entries) {
> + for (i = 0; i < (u32)max_stack; i++)
> + entries[i] = 0x0;
> +
> + bpf_get_task_stack(
> + bpf_get_current_task_btf(),

Same as bpf_get_stack(), right?

> + entries,
> + max_stack *
> + sizeof(unsigned long),
> + 0);
> + bpf_map_update_elem(
> + &contention_owner_stacks,
> + &(pelem->lock), entries,
> + BPF_ANY);

Please factor out the code if it indents too much. Or you can use goto
or something to reduce the indentation level.

if (ret != 0)
goto skip_update;

...

if (entries == NULL)
goto skip_stack;

...

Thanks,
Namhyung

> + }
> +
> + otdata->pid = pid;
> + otdata->timestamp = timestamp;
> + }
> +
> + bpf_map_update_elem(&contention_owner_tracing,
> + &(pelem->lock), otdata, BPF_ANY);
> + }
> + }
> +contention_end_skip_owner_callstack:
> +
> switch (aggr_mode) {
> case LOCK_AGGR_CALLER:
> key.stack_id = pelem->stack_id;
> --
> 2.47.1.688.g23fc6f90ad-goog
>