Add 2 benchmarks:

1. Performance of thread creation/exiting in presence of breakpoints.
2. Performance of breakpoint modification in presence of threads.

The benchmarks capture use cases that we are interested in:
using inheritable breakpoints in large highly-threaded applications.

The benchmarks show significant slowdown imposed by breakpoints
(even when they don't fire).

Testing on Intel 8173M with 112 HW threads show:

  perf bench --repeat=56 breakpoint thread --breakpoints=0 --parallelism=56 --threads=20
        78.675000 usecs/op
  perf bench --repeat=56 breakpoint thread --breakpoints=4 --parallelism=56 --threads=20
     12967.135714 usecs/op

That's 165x slowdown due to presence of the breakpoints.

  perf bench --repeat=20000 breakpoint enable --passive=0 --active=0
         1.433250 usecs/op
  perf bench --repeat=20000 breakpoint enable --passive=224 --active=0
       585.318400 usecs/op
  perf bench --repeat=20000 breakpoint enable --passive=0 --active=111
       635.953000 usecs/op

That's 408x and 444x slowdown due to presence of threads.

Profiles show some overhead in toggle_bp_slot,
but also very high contention:

    90.83%  breakpoint-thre  [kernel.kallsyms]  [k] osq_lock
     4.69%  breakpoint-thre  [kernel.kallsyms]  [k] mutex_spin_on_owner
     2.06%  breakpoint-thre  [kernel.kallsyms]  [k] __reserve_bp_slot
     2.04%  breakpoint-thre  [kernel.kallsyms]  [k] toggle_bp_slot

    79.01%  breakpoint-enab  [kernel.kallsyms]  [k] smp_call_function_single
     9.94%  breakpoint-enab  [kernel.kallsyms]  [k] llist_add_batch
     5.70%  breakpoint-enab  [kernel.kallsyms]  [k] _raw_spin_lock_irq
     1.84%  breakpoint-enab  [kernel.kallsyms]  [k] event_function_call
     1.12%  breakpoint-enab  [kernel.kallsyms]  [k] send_call_function_single_ipi
     0.37%  breakpoint-enab  [kernel.kallsyms]  [k] generic_exec_single
     0.24%  breakpoint-enab  [kernel.kallsyms]  [k] __perf_event_disable
     0.20%  breakpoint-enab  [kernel.kallsyms]  [k] _perf_event_enable
     0.18%  breakpoint-enab  [kernel.kallsyms]  [k] toggle_bp_slot

Committer notes:

Fixup struct init for older compilers:

   3    32.90 alpine:3.5                    : FAIL clang version 3.8.1 (tags/RELEASE_381/final)
    bench/breakpoint.c:49:34: error: missing field 'size' initializer [-Werror,-Wmissing-field-initializers]
            struct perf_event_attr attr = {0};
                                            ^
    1 error generated.
   7    37.31 alpine:3.9                    : FAIL gcc version 8.3.0 (Alpine 8.3.0)
    bench/breakpoint.c:49:34: error: missing field 'size' initializer [-Werror,-Wmissing-field-initializers]
            struct perf_event_attr attr = {0};
                                            ^
    1 error generated.
Signed-off-by: Dmitriy Vyukov <dvyukov@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Marco Elver <elver@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220505155745.1690906-1-dvyukov@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>