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Message-ID: <CAJpZYjUj1un3pnxQWp1r3fS235WnCHZU94agiV3FCs5Ph8uJWw@mail.gmail.com>
Date: Mon, 24 Feb 2025 10:25:30 -0800
From: Chun-Tse Shao <ctshao@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
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
Thank you for your review, Namhyung.
On Thu, Feb 20, 2025 at 10:46 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> 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'?
I just realized that `update_owner_stat` is unnecessary. The previous
contention duration, `timestamp - otdata->timestamp`, belongs to
`otdata->stack_id` and is already recorded in the code above. Here we
are retrieving the current stack and updating `otdata->stack_id` for
future tracing, but not the previous contention duration. I will
submit a new patchset with this change.
>
>
> > + 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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