Re: [PATCH v2 13/13] perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task targets

[Dmitry Vyukov]

On Tue, 28 Jun 2022 at 11:59, Marco Elver <elver@xxxxxxxxxx> 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
>  |      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
>  |      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@xxxxxxxxxx>

Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>

I don't see any bugs. But the code is quite complex. Does it make
sense to add some asserts to the histogram type? E.g. counters don't
underflow, weight is not negative (e.g. accidentally added -1 returned
from task_bp_pinned()). Not sure if it will be enough to catch all
types of bugs, though.
Could kunit tests check that histograms are all 0's at the end?

I am not just about the current code (which may be correct), but also
future modifications to this code.


> ---
> v2:
> * New patch.
> ---
>  kernel/events/hw_breakpoint.c | 152 +++++++++++++++++++++++++++-------
>  1 file changed, 121 insertions(+), 31 deletions(-)
>
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index b5180a2ccfbf..31b24e42f2b5 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 5 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)
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>