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SubjectRe: [PATCH v3 14/14] perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task targets
On Mon, Jul 4, 2022 at 8:07 AM Marco Elver <elver@google.com> wrote:
>
> 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>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
> v3:
> * Fix typo "5 cases" -> "4 cases".
> * Update hw_breakpoint_is_used() to check tsk_pinned_all.
>
> v2:
> * New patch.
> ---
> kernel/events/hw_breakpoint.c | 155 +++++++++++++++++++++++++++-------
> 1 file changed, 124 insertions(+), 31 deletions(-)
>
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index a489f31fe147..7ef0e98d31e2 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -66,6 +66,8 @@ static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
>
> /* Number of pinned CPU breakpoints globally. */
> static struct bp_slots_histogram cpu_pinned[TYPE_MAX];
> +/* Number of pinned CPU-independent task breakpoints. */
> +static struct bp_slots_histogram tsk_pinned_all[TYPE_MAX];
>
> /* Keep track of the breakpoints attached to tasks */
> static struct rhltable task_bps_ht;
> @@ -200,6 +202,8 @@ static __init int init_breakpoint_slots(void)
> for (i = 0; i < TYPE_MAX; i++) {
> if (!bp_slots_histogram_alloc(&cpu_pinned[i], i))
> goto err;
> + if (!bp_slots_histogram_alloc(&tsk_pinned_all[i], i))
> + goto err;
> }
>
> return 0;
> @@ -210,8 +214,10 @@ static __init int init_breakpoint_slots(void)
> if (err_cpu == cpu)
> break;
> }
> - for (i = 0; i < TYPE_MAX; i++)
> + for (i = 0; i < TYPE_MAX; i++) {
> bp_slots_histogram_free(&cpu_pinned[i]);
> + bp_slots_histogram_free(&tsk_pinned_all[i]);
> + }
>
> return -ENOMEM;
> }
> @@ -245,6 +251,26 @@ bp_slots_histogram_max(struct bp_slots_histogram *hist, enum bp_type_idx type)
> return 0;
> }
>
> +static int
> +bp_slots_histogram_max_merge(struct bp_slots_histogram *hist1, struct bp_slots_histogram *hist2,
> + enum bp_type_idx type)
> +{
> + for (int i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
> + const int count1 = atomic_read(&hist1->count[i]);
> + const int count2 = atomic_read(&hist2->count[i]);
> +
> + /* Catch unexpected writers; we want a stable snapshot. */
> + ASSERT_EXCLUSIVE_WRITER(hist1->count[i]);
> + ASSERT_EXCLUSIVE_WRITER(hist2->count[i]);
> + if (count1 + count2 > 0)
> + return i + 1;
> + WARN(count1 < 0, "inconsistent breakpoint slots histogram");
> + WARN(count2 < 0, "inconsistent breakpoint slots histogram");
> + }
> +
> + return 0;
> +}
> +
> #ifndef hw_breakpoint_weight
> static inline int hw_breakpoint_weight(struct perf_event *bp)
> {
> @@ -273,7 +299,7 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
> * toggle_bp_task_slot() to tsk_pinned, and we get a stable snapshot.
> */
> lockdep_assert_held_write(&bp_cpuinfo_sem);
> - return bp_slots_histogram_max(tsk_pinned, type);
> + return bp_slots_histogram_max_merge(tsk_pinned, &tsk_pinned_all[type], type);
> }
>
> /*
> @@ -366,40 +392,22 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
> return pinned_slots;
> }
>
> -/*
> - * Add a pinned breakpoint for the given task in our constraint table
> - */
> -static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
> - enum bp_type_idx type, int weight)
> -{
> - struct bp_slots_histogram *tsk_pinned = &get_bp_info(cpu, type)->tsk_pinned;
> -
> - /*
> - * If bp->hw.target, tsk_pinned is only modified, but not used
> - * otherwise. We can permit concurrent updates as long as there are no
> - * other uses: having acquired bp_cpuinfo_sem as a reader allows
> - * concurrent updates here. Uses of tsk_pinned will require acquiring
> - * bp_cpuinfo_sem as a writer to stabilize tsk_pinned's value.
> - */
> - lockdep_assert_held_read(&bp_cpuinfo_sem);
> - bp_slots_histogram_add(tsk_pinned, task_bp_pinned(cpu, bp, type), weight);
> -}
> -
> /*
> * Add/remove the given breakpoint in our constraint table
> */
> static int
> -toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
> - int weight)
> +toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type, int weight)
> {
> - const struct cpumask *cpumask = cpumask_of_bp(bp);
> - int cpu;
> + int cpu, next_tsk_pinned;
>
> if (!enable)
> weight = -weight;
>
> - /* Pinned counter cpu profiling */
> if (!bp->hw.target) {
> + /*
> + * Update the pinned CPU slots, in per-CPU bp_cpuinfo and in the
> + * global histogram.
> + */
> struct bp_cpuinfo *info = get_bp_info(bp->cpu, type);
>
> lockdep_assert_held_write(&bp_cpuinfo_sem);
> @@ -408,9 +416,91 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
> return 0;
> }
>
> - /* Pinned counter task profiling */
> - for_each_cpu(cpu, cpumask)
> - toggle_bp_task_slot(bp, cpu, type, weight);
> + /*
> + * If bp->hw.target, tsk_pinned is only modified, but not used
> + * otherwise. We can permit concurrent updates as long as there are no
> + * other uses: having acquired bp_cpuinfo_sem as a reader allows
> + * concurrent updates here. Uses of tsk_pinned will require acquiring
> + * bp_cpuinfo_sem as a writer to stabilize tsk_pinned's value.
> + */
> + lockdep_assert_held_read(&bp_cpuinfo_sem);
> +
> + /*
> + * Update the pinned task slots, in per-CPU bp_cpuinfo and in the global
> + * histogram. We need to take care of 4 cases:
> + *
> + * 1. This breakpoint targets all CPUs (cpu < 0), and there may only
> + * exist other task breakpoints targeting all CPUs. In this case we
> + * can simply update the global slots histogram.
> + *
> + * 2. This breakpoint targets a specific CPU (cpu >= 0), but there may
> + * only exist other task breakpoints targeting all CPUs.
> + *
> + * a. On enable: remove the existing breakpoints from the global
> + * slots histogram and use the per-CPU histogram.
> + *
> + * b. On disable: re-insert the existing breakpoints into the global
> + * slots histogram and remove from per-CPU histogram.
> + *
> + * 3. Some other existing task breakpoints target specific CPUs. Only
> + * update the per-CPU slots histogram.
> + */
> +
> + if (!enable) {
> + /*
> + * Remove before updating histograms so we can determine if this
> + * was the last task breakpoint for a specific CPU.
> + */
> + int ret = rhltable_remove(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
> +
> + if (ret)
> + return ret;
> + }
> + /*
> + * Note: If !enable, next_tsk_pinned will not count the to-be-removed breakpoint.
> + */
> + next_tsk_pinned = task_bp_pinned(-1, bp, type);
> +
> + if (next_tsk_pinned >= 0) {
> + if (bp->cpu < 0) { /* Case 1: fast path */
> + if (!enable)
> + next_tsk_pinned += hw_breakpoint_weight(bp);
> + bp_slots_histogram_add(&tsk_pinned_all[type], next_tsk_pinned, weight);
> + } else if (enable) { /* Case 2.a: slow path */
> + /* Add existing to per-CPU histograms. */
> + for_each_possible_cpu(cpu) {
> + bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
> + 0, next_tsk_pinned);
> + }
> + /* Add this first CPU-pinned task breakpoint. */
> + bp_slots_histogram_add(&get_bp_info(bp->cpu, type)->tsk_pinned,
> + next_tsk_pinned, weight);
> + /* Rebalance global task pinned histogram. */
> + bp_slots_histogram_add(&tsk_pinned_all[type], next_tsk_pinned,
> + -next_tsk_pinned);
> + } else { /* Case 2.b: slow path */
> + /* Remove this last CPU-pinned task breakpoint. */
> + bp_slots_histogram_add(&get_bp_info(bp->cpu, type)->tsk_pinned,
> + next_tsk_pinned + hw_breakpoint_weight(bp), weight);
> + /* Remove all from per-CPU histograms. */
> + for_each_possible_cpu(cpu) {
> + bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
> + next_tsk_pinned, -next_tsk_pinned);
> + }
> + /* Rebalance global task pinned histogram. */
> + bp_slots_histogram_add(&tsk_pinned_all[type], 0, next_tsk_pinned);
> + }
> + } else { /* Case 3: slow path */
> + const struct cpumask *cpumask = cpumask_of_bp(bp);
> +
> + for_each_cpu(cpu, cpumask) {
> + next_tsk_pinned = task_bp_pinned(cpu, bp, type);
> + if (!enable)
> + next_tsk_pinned += hw_breakpoint_weight(bp);
> + bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
> + next_tsk_pinned, weight);
> + }
> + }
>
> /*
> * Readers want a stable snapshot of the per-task breakpoint list.
> @@ -419,8 +509,8 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
>
> if (enable)
> return rhltable_insert(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
> - else
> - return rhltable_remove(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
> +
> + return 0;
> }
>
> __weak int arch_reserve_bp_slot(struct perf_event *bp)
> @@ -850,6 +940,9 @@ bool hw_breakpoint_is_used(void)
> */
> if (WARN_ON(atomic_read(&cpu_pinned[type].count[slot])))
> return true;
> +
> + if (atomic_read(&tsk_pinned_all[type].count[slot]))
> + return true;
> }
> }
>
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

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