We can still see that a majority of the time is spent hashing task pointers:

    ...
    16.98%  [kernel]       [k] rhashtable_jhash2
    ...

Doing the bookkeeping in toggle_bp_slots() is currently O(#cpus),
calling task_bp_pinned() for each CPU, even if task_bp_pinned() is
CPU-independent. The reason for this is to update the per-CPU
'tsk_pinned' histogram.

To optimize the CPU-independent case to O(1), keep a separate
CPU-independent 'tsk_pinned_all' histogram.

The major source of complexity are transitions between "all
CPU-independent task breakpoints" and "mixed CPU-independent and
CPU-dependent task breakpoints". The code comments list all cases that
require handling.

After this optimization:

 | $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 100 threads with 4 breakpoints and 128 parallelism
 |      Total time: 1.758 [sec]
 |
 |       34.336621 usecs/op
 |     4395.087500 usecs/op/cpu

    38.08%  [kernel]       [k] queued_spin_lock_slowpath
    10.81%  [kernel]       [k] smp_cfm_core_cond
     3.01%  [kernel]       [k] update_sg_lb_stats
     2.58%  [kernel]       [k] osq_lock
     2.57%  [kernel]       [k] llist_reverse_order
     1.45%  [kernel]       [k] find_next_bit
     1.21%  [kernel]       [k] flush_tlb_func_common
     1.01%  [kernel]       [k] arch_install_hw_breakpoint

Showing that the time spent hashing keys has become insignificant.

With the given benchmark parameters, that's an improvement of 12%
compared with the old O(#cpus) version.

And finally, using the less aggressive parameters from the preceding
changes, we now observe:

 | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
 |      Total time: 0.067 [sec]
 |
 |       35.292187 usecs/op
 |     2258.700000 usecs/op/cpu

Which is an improvement of 12% compared to without the histogram
optimizations (baseline is 40 usecs/op). This is now on par with the
theoretical ideal (constraints disabled), and only 12% slower than no
breakpoints at all.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20220829124719.675715-15-elver@google.com