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

Marco Elver
2022-06-09T11:31:02Z

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(-)

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2.36.1.255.ge46751e96f-goog