Currently lock contention timestamp is maintained in a hash map keyed by
pid.  That means it needs to get and release a map element (which is
proctected by spinlock!) on each contention begin and end pair.  This
can impact on performance if there are a lot of contention (usually from
spinlocks).

It used to go with task local storage but it had an issue on memory
allocation in some critical paths.  Although it's addressed in recent
kernels IIUC, the tool should support old kernels too.  So it cannot
simply switch to the task local storage at least for now.

As spinlocks create lots of contention and they disabled preemption
during the spinning, it can use per-cpu array to keep the timestamp to
avoid overhead in hashmap update and delete.

In contention_begin, it's easy to check the lock types since it can see
the flags.  But contention_end cannot see it.  So let's try to per-cpu
array first (unconditionally) if it has an active element (lock != 0).
Then it should be used and per-task tstamp map should not be used until
the per-cpu array element is cleared which means nested spinlock
contention (if any) was finished and it nows see (the outer) lock.
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Hao Luo <haoluo@google.com>
Cc: Song Liu <song@kernel.org>
Cc: bpf@vger.kernel.org
Link: https://lore.kernel.org/r/20231020204741.1869520-3-namhyung@kernel.org