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

Dmitry Vyukov
2022-06-09T12:28:44Z

On Thu, 9 Jun 2022 at 13:30, Marco Elver <elver@google.com> wrote:
>
> 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.

Awesome!

> 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