| Message ID | 20220704150514.48816-1-elver@google.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | perf/hw_breakpoint: Optimize for thousands of tasks | expand |
On Mon, 4 Jul 2022 at 17:05, 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. [...] > Marco Elver (14): > perf/hw_breakpoint: Add KUnit test for constraints accounting > perf/hw_breakpoint: Provide hw_breakpoint_is_used() and use in test > perf/hw_breakpoint: Clean up headers > 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 > powerpc/hw_breakpoint: Avoid relying on caller synchronization > locking/percpu-rwsem: Add percpu_is_write_locked() and > percpu_is_read_locked() > perf/hw_breakpoint: Reduce contention with large number of tasks > perf/hw_breakpoint: Introduce bp_slots_histogram > perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent > task targets > perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task > targets [...] This is ready from our side, and given the silence, assume it's ready to pick up and/or have a maintainer take a look. Since this is mostly kernel/events, would -tip/perf/core be appropriate? Thanks, -- Marco
On Tue, Jul 12, 2022 at 6:41 AM Marco Elver <elver@google.com> wrote: > > On Mon, 4 Jul 2022 at 17:05, 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. > [...] > > Marco Elver (14): > > perf/hw_breakpoint: Add KUnit test for constraints accounting > > perf/hw_breakpoint: Provide hw_breakpoint_is_used() and use in test > > perf/hw_breakpoint: Clean up headers > > 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 > > powerpc/hw_breakpoint: Avoid relying on caller synchronization > > locking/percpu-rwsem: Add percpu_is_write_locked() and > > percpu_is_read_locked() > > perf/hw_breakpoint: Reduce contention with large number of tasks > > perf/hw_breakpoint: Introduce bp_slots_histogram > > perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent > > task targets > > perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task > > targets > [...] > > This is ready from our side, and given the silence, assume it's ready > to pick up and/or have a maintainer take a look. Since this is mostly > kernel/events, would -tip/perf/core be appropriate? These are awesome improvements, I've added my acked-by to every change. I hope we can pull these changes, as you say, into tip.git perf/core and get them into 5.20. Thanks, Ian > Thanks, > -- Marco
On Wed, 20 Jul 2022 at 17:47, Ian Rogers <irogers@google.com> wrote: > On Tue, Jul 12, 2022 at 6:41 AM Marco Elver <elver@google.com> wrote: > > On Mon, 4 Jul 2022 at 17:05, 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. > > [...] > > > Marco Elver (14): > > > perf/hw_breakpoint: Add KUnit test for constraints accounting > > > perf/hw_breakpoint: Provide hw_breakpoint_is_used() and use in test > > > perf/hw_breakpoint: Clean up headers > > > 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 > > > powerpc/hw_breakpoint: Avoid relying on caller synchronization > > > locking/percpu-rwsem: Add percpu_is_write_locked() and > > > percpu_is_read_locked() > > > perf/hw_breakpoint: Reduce contention with large number of tasks > > > perf/hw_breakpoint: Introduce bp_slots_histogram > > > perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent > > > task targets > > > perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task > > > targets > > [...] > > > > This is ready from our side, and given the silence, assume it's ready > > to pick up and/or have a maintainer take a look. Since this is mostly > > kernel/events, would -tip/perf/core be appropriate? > > These are awesome improvements, I've added my acked-by to every > change. I hope we can pull these changes, as you say, into tip.git > perf/core and get them into 5.20. These still apply cleanly to 6.0-rc1 and the test passes, but let me know if I shall send a rebased version. Thanks -- Marco
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. toggle_bp_slot() and fetch_bp_busy_slots() are O(#cpus * #tasks): Both iterate through all CPUs and call task_bp_pinned(), which is O(#tasks). 2. Everything is serialized on a global mutex, 'nr_bp_mutex'. The series progresses with the simpler optimizations and finishes with the more complex optimizations: 1. We first optimize task_bp_pinned() to only take O(1) on average. 2. Rework synchronization to allow concurrency when checking and updating breakpoint constraints for tasks. 3. Eliminate the O(#cpus) loops in the CPU-independent case. 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 the following 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: 0.067 [sec] | | 35.292187 usecs/op | 2258.700000 usecs/op/cpu On the used test system, that's an effective speedup of ~3490x per op. Which is on par with the theoretical ideal performance through optimizations in hw_breakpoint.c (constraints accounting disabled), and only 12% slower than no breakpoints at all. Changelog --------- v3: * Fix typos. * Introduce hw_breakpoint_is_used() for the test. * Add WARN_ON in bp_blots_histogram_add(). * Don't use raw_smp_processor_id() in test. * Apply Acked-by/Reviewed-by given in v2 for mostly unchanged patches. v2: https://lkml.kernel.org/r/20220628095833.2579903-1-elver@google.com * Add KUnit test suite. * Remove struct bp_busy_slots and simplify functions. * Add "powerpc/hw_breakpoint: Avoid relying on caller synchronization". * Add "locking/percpu-rwsem: Add percpu_is_write_locked() and percpu_is_read_locked()". * Use percpu-rwsem instead of rwlock. * Use task_struct::perf_event_mutex instead of sharded mutex. * Drop v1 "perf/hw_breakpoint: Optimize task_bp_pinned() if CPU-independent". * Add "perf/hw_breakpoint: Introduce bp_slots_histogram". * Add "perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent task targets". * Add "perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task targets". * Apply Acked-by/Reviewed-by given in v1 for unchanged patches. ==> Speedup of ~3490x (vs. ~3315x in v1). v1: https://lore.kernel.org/all/20220609113046.780504-1-elver@google.com/ Marco Elver (14): perf/hw_breakpoint: Add KUnit test for constraints accounting perf/hw_breakpoint: Provide hw_breakpoint_is_used() and use in test perf/hw_breakpoint: Clean up headers 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 powerpc/hw_breakpoint: Avoid relying on caller synchronization locking/percpu-rwsem: Add percpu_is_write_locked() and percpu_is_read_locked() perf/hw_breakpoint: Reduce contention with large number of tasks perf/hw_breakpoint: Introduce bp_slots_histogram perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent task targets perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task targets arch/powerpc/kernel/hw_breakpoint.c | 53 ++- arch/sh/include/asm/hw_breakpoint.h | 5 +- arch/x86/include/asm/hw_breakpoint.h | 5 +- include/linux/hw_breakpoint.h | 4 +- include/linux/percpu-rwsem.h | 6 + include/linux/perf_event.h | 3 +- kernel/events/Makefile | 1 + kernel/events/hw_breakpoint.c | 638 ++++++++++++++++++++------- kernel/events/hw_breakpoint_test.c | 333 ++++++++++++++ kernel/locking/percpu-rwsem.c | 6 + lib/Kconfig.debug | 10 + 11 files changed, 885 insertions(+), 179 deletions(-) create mode 100644 kernel/events/hw_breakpoint_test.c