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We can still see that a majority of the time is spent hashing task pointers: ... 16.98% [kernel] [k] rhashtable_jhash2 ... Doing the bookkeeping in toggle_bp_slots() is currently O(#cpus), calling task_bp_pinned() for each CPU, even if task_bp_pinned() is CPU-independent. The reason for this is to update the per-CPU 'tsk_pinned' histogram. To optimize the CPU-independent case to O(1), keep a separate CPU-independent 'tsk_pinned_all' histogram. The major source of complexity are transitions between "all CPU-independent task breakpoints" and "mixed CPU-independent and CPU-dependent task breakpoints". The code comments list all cases that require handling. After this optimization: | $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512 | # Running 'breakpoint/thread' benchmark: | # Created/joined 100 threads with 4 breakpoints and 128 parallelism | Total time: 1.758 [sec] | | 34.336621 usecs/op | 4395.087500 usecs/op/cpu 38.08% [kernel] [k] queued_spin_lock_slowpath 10.81% [kernel] [k] smp_cfm_core_cond 3.01% [kernel] [k] update_sg_lb_stats 2.58% [kernel] [k] osq_lock 2.57% [kernel] [k] llist_reverse_order 1.45% [kernel] [k] find_next_bit 1.21% [kernel] [k] flush_tlb_func_common 1.01% [kernel] [k] arch_install_hw_breakpoint Showing that the time spent hashing keys has become insignificant. With the given benchmark parameters, that's an improvement of 12% compared with the old O(#cpus) version. And finally, using the less aggressive parameters from the preceding changes, we now observe: | $> 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 Which is an improvement of 12% compared to without the histogram optimizations (baseline is 40 usecs/op). This is now on par with the theoretical ideal (constraints disabled), and only 12% slower than no breakpoints at all. Signed-off-by: Marco Elver <elver@google.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Dmitry Vyukov <dvyukov@google.com> Acked-by: Ian Rogers <irogers@google.com> Link: https://lore.kernel.org/r/20220829124719.675715-15-elver@google.com |
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.. | ||
callchain.c | ||
core.c | ||
hw_breakpoint_test.c | ||
hw_breakpoint.c | ||
internal.h | ||
Makefile | ||
ring_buffer.c | ||
uprobes.c |