Messages in this thread | | | From | Dmitry Vyukov <> | Date | Thu, 9 Jun 2022 14:30:14 +0200 | Subject | Re: [PATCH 1/8] perf/hw_breakpoint: Optimize list of per-task breakpoints |
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On Thu, 9 Jun 2022 at 13:31, Marco Elver <elver@google.com> wrote: > > On a machine with 256 CPUs, running the recently added perf breakpoint > benchmark results in: > > | $> 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: 236.418 [sec] > | > | 123134.794271 usecs/op > | 7880626.833333 usecs/op/cpu > > The benchmark tests inherited breakpoint perf events across many > threads. > > Looking at a perf profile, we can see that the majority of the time is > spent in various hw_breakpoint.c functions, which execute within the > 'nr_bp_mutex' critical sections which then results in contention on that > mutex as well: > > 37.27% [kernel] [k] osq_lock > 34.92% [kernel] [k] mutex_spin_on_owner > 12.15% [kernel] [k] toggle_bp_slot > 11.90% [kernel] [k] __reserve_bp_slot > > The culprit here is task_bp_pinned(), which has a runtime complexity of > O(#tasks) due to storing all task breakpoints in the same list and > iterating through that list looking for a matching task. Clearly, this > does not scale to thousands of tasks. > > While one option would be to make task_struct a breakpoint list node, > this would only further bloat task_struct for infrequently used data. > > Instead, make use of the "rhashtable" variant "rhltable" which stores > multiple items with the same key in a list. This results in average > runtime complexity of O(1) for task_bp_pinned(). > > With the optimization, the benchmark shows: > > | $> 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: 0.208 [sec] > | > | 108.422396 usecs/op > | 6939.033333 usecs/op/cpu > > On this particular setup that's a speedup of ~1135x. > > Signed-off-by: Marco Elver <elver@google.com> > --- > include/linux/perf_event.h | 3 +- > kernel/events/hw_breakpoint.c | 56 ++++++++++++++++++++++------------- > 2 files changed, 37 insertions(+), 22 deletions(-) > > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h > index 01231f1d976c..e27360436dc6 100644 > --- a/include/linux/perf_event.h > +++ b/include/linux/perf_event.h > @@ -36,6 +36,7 @@ struct perf_guest_info_callbacks { > }; > > #ifdef CONFIG_HAVE_HW_BREAKPOINT > +#include <linux/rhashtable-types.h> > #include <asm/hw_breakpoint.h> > #endif > > @@ -178,7 +179,7 @@ struct hw_perf_event { > * creation and event initalization. > */ > struct arch_hw_breakpoint info; > - struct list_head bp_list; > + struct rhlist_head bp_list; > }; > #endif > struct { /* amd_iommu */ > diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c > index f32320ac02fd..25c94c6e918d 100644 > --- a/kernel/events/hw_breakpoint.c > +++ b/kernel/events/hw_breakpoint.c > @@ -28,7 +28,7 @@ > #include <linux/sched.h> > #include <linux/init.h> > #include <linux/slab.h> > -#include <linux/list.h> > +#include <linux/rhashtable.h> > #include <linux/cpu.h> > #include <linux/smp.h> > #include <linux/bug.h> > @@ -55,7 +55,13 @@ static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type) > } > > /* Keep track of the breakpoints attached to tasks */ > -static LIST_HEAD(bp_task_head); > +static struct rhltable task_bps_ht; > +static const struct rhashtable_params task_bps_ht_params = { > + .head_offset = offsetof(struct hw_perf_event, bp_list), > + .key_offset = offsetof(struct hw_perf_event, target), > + .key_len = sizeof_field(struct hw_perf_event, target), > + .automatic_shrinking = true, > +}; > > static int constraints_initialized; > > @@ -104,17 +110,23 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type) > */ > static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type) > { > - struct task_struct *tsk = bp->hw.target; > + struct rhlist_head *head, *pos; > struct perf_event *iter; > int count = 0; > > - list_for_each_entry(iter, &bp_task_head, hw.bp_list) { > - if (iter->hw.target == tsk && > - find_slot_idx(iter->attr.bp_type) == type && > + rcu_read_lock();
Why do we need rcu_read_lock() here? The patch does not change anything with respect to locking, so all accesses to the container should still be protected by nr_bp_mutex. Similarly for the rcu variant of for_each below.
Otherwise the change looks good to me.
> + head = rhltable_lookup(&task_bps_ht, &bp->hw.target, task_bps_ht_params); > + if (!head) > + goto out; > + > + rhl_for_each_entry_rcu(iter, pos, head, hw.bp_list) { > + if (find_slot_idx(iter->attr.bp_type) == type && > (iter->cpu < 0 || cpu == iter->cpu)) > count += hw_breakpoint_weight(iter); > } > > +out: > + rcu_read_unlock(); > return count; > } > > @@ -187,7 +199,7 @@ static void toggle_bp_task_slot(struct perf_event *bp, int cpu, > /* > * Add/remove the given breakpoint in our constraint table > */ > -static void > +static int > toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type, > int weight) > { > @@ -200,7 +212,7 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type, > /* Pinned counter cpu profiling */ > if (!bp->hw.target) { > get_bp_info(bp->cpu, type)->cpu_pinned += weight; > - return; > + return 0; > } > > /* Pinned counter task profiling */ > @@ -208,9 +220,9 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type, > toggle_bp_task_slot(bp, cpu, type, weight); > > if (enable) > - list_add_tail(&bp->hw.bp_list, &bp_task_head); > + return rhltable_insert(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params); > else > - list_del(&bp->hw.bp_list); > + return rhltable_remove(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params); > } > > __weak int arch_reserve_bp_slot(struct perf_event *bp) > @@ -308,9 +320,7 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type) > if (ret) > return ret; > > - toggle_bp_slot(bp, true, type, weight); > - > - return 0; > + return toggle_bp_slot(bp, true, type, weight); > } > > int reserve_bp_slot(struct perf_event *bp) > @@ -335,7 +345,7 @@ static void __release_bp_slot(struct perf_event *bp, u64 bp_type) > > type = find_slot_idx(bp_type); > weight = hw_breakpoint_weight(bp); > - toggle_bp_slot(bp, false, type, weight); > + WARN_ON(toggle_bp_slot(bp, false, type, weight)); > } > > void release_bp_slot(struct perf_event *bp) > @@ -679,7 +689,7 @@ static struct pmu perf_breakpoint = { > int __init init_hw_breakpoint(void) > { > int cpu, err_cpu; > - int i; > + int i, ret; > > for (i = 0; i < TYPE_MAX; i++) > nr_slots[i] = hw_breakpoint_slots(i); > @@ -690,18 +700,24 @@ int __init init_hw_breakpoint(void) > > info->tsk_pinned = kcalloc(nr_slots[i], sizeof(int), > GFP_KERNEL); > - if (!info->tsk_pinned) > - goto err_alloc; > + if (!info->tsk_pinned) { > + ret = -ENOMEM; > + goto err; > + } > } > } > > + ret = rhltable_init(&task_bps_ht, &task_bps_ht_params); > + if (ret) > + goto err; > + > constraints_initialized = 1; > > perf_pmu_register(&perf_breakpoint, "breakpoint", PERF_TYPE_BREAKPOINT); > > return register_die_notifier(&hw_breakpoint_exceptions_nb); > > - err_alloc: > +err: > for_each_possible_cpu(err_cpu) { > for (i = 0; i < TYPE_MAX; i++) > kfree(get_bp_info(err_cpu, i)->tsk_pinned); > @@ -709,7 +725,5 @@ int __init init_hw_breakpoint(void) > break; > } > > - return -ENOMEM; > + return ret; > } > - > - > -- > 2.36.1.255.ge46751e96f-goog >
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