perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent task targets

Running the perf benchmark with (note: more aggressive parameters vs.
preceding changes, but same 256 CPUs host):

 | $> 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: 1.989 [sec]
 |
 |       38.854160 usecs/op
 |     4973.332500 usecs/op/cpu

    20.43%  [kernel]       [k] queued_spin_lock_slowpath
    18.75%  [kernel]       [k] osq_lock
    16.98%  [kernel]       [k] rhashtable_jhash2
     8.34%  [kernel]       [k] task_bp_pinned
     4.23%  [kernel]       [k] smp_cfm_core_cond
     3.65%  [kernel]       [k] bcmp
     2.83%  [kernel]       [k] toggle_bp_slot
     1.87%  [kernel]       [k] find_next_bit
     1.49%  [kernel]       [k] __reserve_bp_slot

We can see that a majority of the time is now spent hashing task
pointers to index into task_bps_ht in task_bp_pinned().

Obtaining the max_bp_pinned_slots() for CPU-independent task targets
currently is O(#cpus), and calls task_bp_pinned() for each CPU, even if
the result of task_bp_pinned() is CPU-independent.

The loop in max_bp_pinned_slots() wants to compute the maximum slots
across all CPUs. If task_bp_pinned() is CPU-independent, we can do so by
obtaining the max slots across all CPUs and adding task_bp_pinned().

To do so in O(1), use a bp_slots_histogram for CPU-pinned slots.

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: 1.930 [sec]
 |
 |       37.697832 usecs/op
 |     4825.322500 usecs/op/cpu

    19.13%  [kernel]       [k] queued_spin_lock_slowpath
    18.21%  [kernel]       [k] rhashtable_jhash2
    15.46%  [kernel]       [k] osq_lock
     6.27%  [kernel]       [k] toggle_bp_slot
     5.91%  [kernel]       [k] task_bp_pinned
     5.05%  [kernel]       [k] smp_cfm_core_cond
     1.78%  [kernel]       [k] update_sg_lb_stats
     1.36%  [kernel]       [k] llist_reverse_order
     1.34%  [kernel]       [k] find_next_bit
     1.19%  [kernel]       [k] bcmp

Suggesting that time spent in task_bp_pinned() has been reduced.
However, we're still hashing too much, which will be addressed in the
subsequent change.

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/20220829124719.675715-14-elver@google.com

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