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

[Namhyung Kim <namhyung@kernel.org>]

On Tue, Jan 21, 2025 at 02:35:46PM -0800, Chun-Tse Shao wrote:
> On Mon, Jan 13, 2025 at 7:35 PM Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > 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?
> 
> There are two conditions which enter this if statement:
> 1. (!data) contention just started, `&pelem->lock` entry in
> `contetion_owner_tracing` is empty.
> 2. (data->pid != owner_pid) Some internal errors so `data->pid` is
> misaligned with `owner_pid`. In this case the timestamp would be
> incorrect so I prefer to drop it.
> WDYT?

Ok, now I think that it should see contention_end from the earlier
waiter for the second case so it should be rare.  Probably ok to drop
it for now.

> 
> >
> >
> > > +             }
> > > +             // 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.
> 
> I will reindent it with `ColumnLimit=100`. I was using 80 since it was
> predefined in `.clang-format`, looks outdated but no one updated it..

Probably you can send a patch. :)

But it still holds the same, please try not to indent a lot (and user
shorter names).

Thanks,
Namhyung

> 
> >
> >   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
> > >