[PATCH 0/8] perf/hw_breakpoint: Optimize for thousands of tasks

[Marco Elver <elver@google.com>]

The hw_breakpoint subsystem's code has seen little change in over 10
years. In that time, systems with >100s of CPUs have become common,
along with improvements to the perf subsystem: using breakpoints on
thousands of concurrent tasks should be a supported usecase.

The breakpoint constraints accounting algorithm is the major bottleneck
in doing so:

  1. task_bp_pinned() has been O(#tasks), and called twice for each CPU.

  2. Everything is serialized on a global mutex, 'nr_bp_mutex'.

This series first optimizes task_bp_pinned() to only take O(1) on
average, and then reworks synchronization to allow concurrency when
checking and updating breakpoint constraints for tasks. Along the way,
smaller micro-optimizations and cleanups are done as they seemed obvious
when staring at the code (but likely insignificant).

The result is (on a system with 256 CPUs) that we go from:

 | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
	 	[ ^ more aggressive benchmark parameters took too long ]
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
 |      Total time: 236.418 [sec]
 |
 |   123134.794271 usecs/op
 |  7880626.833333 usecs/op/cpu

... to -- with all optimizations:

 | $> 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: 0.071 [sec]
 |
 |       37.134896 usecs/op
 |     2376.633333 usecs/op/cpu

On the used test system, that's an effective speedup of ~3315x per op.

Which is close to the theoretical ideal performance through
optimizations in hw_breakpoint.c -- for reference, constraints
accounting disabled:

 | 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]
 |
 |       35.286458 usecs/op
 |     2258.333333 usecs/op/cpu

At this point, the current implementation is only ~5% slower than the
theoretical ideal. However, given constraints accounting cannot
realistically be disabled, this is likely as far as we can push it.

Marco Elver (8):
  perf/hw_breakpoint: Optimize list of per-task breakpoints
  perf/hw_breakpoint: Mark data __ro_after_init
  perf/hw_breakpoint: Optimize constant number of breakpoint slots
  perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable
  perf/hw_breakpoint: Remove useless code related to flexible
    breakpoints
  perf/hw_breakpoint: Reduce contention with large number of tasks
  perf/hw_breakpoint: Optimize task_bp_pinned() if CPU-independent
  perf/hw_breakpoint: Clean up headers

 arch/sh/include/asm/hw_breakpoint.h  |   5 +-
 arch/x86/include/asm/hw_breakpoint.h |   5 +-
 include/linux/hw_breakpoint.h        |   1 -
 include/linux/perf_event.h           |   3 +-
 kernel/events/hw_breakpoint.c        | 374 +++++++++++++++++++--------
 5 files changed, 276 insertions(+), 112 deletions(-)

-- 
2.36.1.255.ge46751e96f-goog