Messages in this thread | | | Date | Thu, 9 Jun 2022 13:30:38 +0200 | Subject | [PATCH 0/8] perf/hw_breakpoint: Optimize for thousands of tasks | From | Marco Elver <> |
| |
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
| |