Re: [PATCH v2 1/4] perf lock: Add bpf maps for owner stack tracing

Chun-Tse Shao
2025-01-21T22:03:10Z

Hi Namhyung, thanks for your reply!

On Mon, Jan 13, 2025 at 7:05 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> Hello,
>
> On Sun, Jan 12, 2025 at 09:20:14PM -0800, Chun-Tse Shao wrote:
> > Add few bpf maps in order to tracing owner stack.
>
> If you want to split this code as a separate commit, I think you'd
> better explain what these maps do and why you need them.
>
> >
> > Signed-off-by: Chun-Tse Shao <ctshao@google.com>
> > ---
> > tools/perf/util/bpf_lock_contention.c | 17 ++++++--
> > .../perf/util/bpf_skel/lock_contention.bpf.c | 40 +++++++++++++++++--
> > tools/perf/util/bpf_skel/lock_data.h | 6 +++
> > 3 files changed, 56 insertions(+), 7 deletions(-)
> >
> > diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
> > index 41a1ad087895..c9c58f243ceb 100644
> > --- a/tools/perf/util/bpf_lock_contention.c
> > +++ b/tools/perf/util/bpf_lock_contention.c
> > @@ -41,9 +41,20 @@ int lock_contention_prepare(struct lock_contention *con)
> > else
> > bpf_map__set_max_entries(skel->maps.task_data, 1);
> >
> > - if (con->save_callstack)
> > - bpf_map__set_max_entries(skel->maps.stacks, con->map_nr_entries);
> > - else
> > + if (con->save_callstack) {
> > + bpf_map__set_max_entries(skel->maps.stacks,
> > + con->map_nr_entries);
> > + if (con->owner) {
> > + bpf_map__set_value_size(skel->maps.owner_stacks_entries,
> > + con->max_stack * sizeof(u64));
> > + bpf_map__set_value_size(
> > + skel->maps.contention_owner_stacks,
> > + con->max_stack * sizeof(u64));
> > + bpf_map__set_key_size(skel->maps.owner_lock_stat,
> > + con->max_stack * sizeof(u64));
> > + skel->rodata->max_stack = con->max_stack;
> > + }
> > + } else
> > bpf_map__set_max_entries(skel->maps.stacks, 1);
> >
> > if (target__has_cpu(target)) {
> > diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> > index 1069bda5d733..05da19fdab23 100644
> > --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> > +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> > @@ -19,13 +19,37 @@
> > #define LCB_F_PERCPU (1U << 4)
> > #define LCB_F_MUTEX (1U << 5)
> >
>
> Can we rename these shorter and save some typings?

I tend to use longer variable names with full descriptions with some
easy to understand abbreviations. Would a shorter name be preferable
in Linux kernel?

>
> > -/* callstack storage */
> > + /* tmp buffer for owner callstack */
> > struct {
> > - __uint(type, BPF_MAP_TYPE_STACK_TRACE);
> > + __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
> > __uint(key_size, sizeof(__u32));
> > __uint(value_size, sizeof(__u64));
> > + __uint(max_entries, 1);
> > +} owner_stacks_entries SEC(".maps");
>
> I think this can be 'stack_buf'.
>
> > +
> > +/* a map for tracing lock address to owner data */
> > +struct {
> > + __uint(type, BPF_MAP_TYPE_HASH);
> > + __uint(key_size, sizeof(__u64)); // lock address
> > + __uint(value_size, sizeof(cotd));
> > __uint(max_entries, MAX_ENTRIES);
> > -} stacks SEC(".maps");
> > +} contention_owner_tracing SEC(".maps");
>
> owner_data.
>
> > +
> > +/* a map for tracing lock address to owner stacktrace */
> > +struct {
> > + __uint(type, BPF_MAP_TYPE_HASH);
> > + __uint(key_size, sizeof(__u64)); // lock address
> > + __uint(value_size, sizeof(__u64)); // straktrace
>
> Typo.
>
> > + __uint(max_entries, MAX_ENTRIES);
> > +} contention_owner_stacks SEC(".maps");
>
> owner_stack.
>
> > +
> > +/* owner callstack to contention data storage */
> > +struct {
> > + __uint(type, BPF_MAP_TYPE_HASH);
> > + __uint(key_size, sizeof(__u64));
> > + __uint(value_size, sizeof(struct contention_data));
> > + __uint(max_entries, MAX_ENTRIES);
> > +} owner_lock_stat SEC(".maps");
>
> owner_stat. What do you think?
>
> By the way, I got an idea to implement stackid map in BPF using hash
> map. For owner stack, you can use the stacktrace as a key and make a
> value an unique integer. Then the return value can be used as a stack
> id (like from bpf_get_stackid) for the owner_data and owner_stat.
>
> Something like:
>
> s32 get_stack_id(struct owner_stack *owner_stack, u64 stacktrace[])
> {
> s32 *id, new_id;
> static s32 id_gen = 1;
>
> id = bpf_map_lookup_elem(owner_stack, stacktrace);
> if (id)
> return *id;
>
> new_id = __sync_fetch_and_add(&id_gen, 1);
> bpf_map_update_elem(owner_stack, stacktrace, &new_id, BPF_NOEXIST);
>
> id = bpf_map_lookup_elem(owner_stack, stacktrace);
> if (id)
> return *id;
>
> return -1;
> }
>
> Later, in user space, you can traverse the owner_stack map to build
> reverse mapping from id to stacktrace.

I wonder if stack_id is necessary here. So far I have three bpf maps.
2 bpf maps for tracing owner stack on given lock address in bpf program:
key: lock_address, value: a struct for tracing owner pid, count of
waiters and contention begin timestamp.
key: lock_address, value: owner stack, which is variable length so I
have to put it in a separate bpf map.

1 bpf map for reporting owner stack in user mode:
key: owner stack, value: struct lock_stat.

With stackid I think there will still be 3 bpf maps, one for
lock_address to owner's info with stackid, one for stackid to stack,
and one for contention_key (has stackid inside) to lock_stat. I think
it is just another way to implement and does not simplify the
implementation. WDYT?

>
> >
> > /* maintain timestamp at the beginning of contention */
> > struct {
> > @@ -43,6 +67,14 @@ struct {
> > __uint(max_entries, 1);
> > } tstamp_cpu SEC(".maps");
> >
> > +/* callstack storage */
> > +struct {
> > + __uint(type, BPF_MAP_TYPE_STACK_TRACE);
> > + __uint(key_size, sizeof(__u32));
> > + __uint(value_size, sizeof(__u64));
> > + __uint(max_entries, MAX_ENTRIES);
> > +} stacks SEC(".maps");
> > +
> > /* actual lock contention statistics */
> > struct {
> > __uint(type, BPF_MAP_TYPE_HASH);
> > @@ -126,6 +158,7 @@ const volatile int needs_callstack;
> > const volatile int stack_skip;
> > const volatile int lock_owner;
> > const volatile int use_cgroup_v2;
> > +const volatile int max_stack;
> >
> > /* determine the key of lock stat */
> > const volatile int aggr_mode;
> > @@ -436,7 +469,6 @@ int contention_end(u64 *ctx)
> > return 0;
> > need_delete = true;
> > }
> > -
> > duration = bpf_ktime_get_ns() - pelem->timestamp;
> > if ((__s64)duration < 0) {
> > __sync_fetch_and_add(&time_fail, 1);
> > diff --git a/tools/perf/util/bpf_skel/lock_data.h b/tools/perf/util/bpf_skel/lock_data.h
> > index de12892f992f..1ef0bca9860e 100644
> > --- a/tools/perf/util/bpf_skel/lock_data.h
> > +++ b/tools/perf/util/bpf_skel/lock_data.h
> > @@ -3,6 +3,12 @@
> > #ifndef UTIL_BPF_SKEL_LOCK_DATA_H
> > #define UTIL_BPF_SKEL_LOCK_DATA_H
> >
> > +typedef struct contention_owner_tracing_data {
> > + u32 pid; // Who has the lock.
> > + u64 timestamp; // The time while the owner acquires lock and contention is going on.
> > + u32 count; // How many waiters for this lock.
>
> Switching the order of timestamp and count would remove padding.

Thanks for the nit!

>
> > +} cotd;
>
> Usually we don't use typedef to remove the struct tag.
>
> Thanks,
> Namhyung
>
> > +
> > struct tstamp_data {
> > u64 timestamp;
> > u64 lock;
> > --
> > 2.47.1.688.g23fc6f90ad-goog
> >