The following commit has been merged into the perf/core branch of tip: Commit-ID: 0912037fec1136d4e4796a3481f4a4ee09a2c325 Gitweb: https://git.kernel.org/tip/0912037fec1136d4e4796a3481f4a4ee09a2c325 Author: Marco Elver <elver@xxxxxxxxxx> AuthorDate: Mon, 29 Aug 2022 14:47:16 +02:00 Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx> CommitterDate: Tue, 30 Aug 2022 10:56:24 +02:00 perf/hw_breakpoint: Reduce contention with large number of tasks While optimizing task_bp_pinned()'s runtime complexity to O(1) on average helps reduce time spent in the critical section, we still suffer due to serializing everything via 'nr_bp_mutex'. Indeed, a profile shows that now contention is the biggest issue: 95.93% [kernel] [k] osq_lock 0.70% [kernel] [k] mutex_spin_on_owner 0.22% [kernel] [k] smp_cfm_core_cond 0.18% [kernel] [k] task_bp_pinned 0.18% [kernel] [k] rhashtable_jhash2 0.15% [kernel] [k] queued_spin_lock_slowpath when running the breakpoint benchmark with (system with 256 CPUs): | $> 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: 0.207 [sec] | | 108.267188 usecs/op | 6929.100000 usecs/op/cpu The main concern for synchronizing the breakpoint constraints data is that a consistent snapshot of the per-CPU and per-task data is observed. The access pattern is as follows: 1. If the target is a task: the task's pinned breakpoints are counted, checked for space, and then appended to; only bp_cpuinfo::cpu_pinned is used to check for conflicts with CPU-only breakpoints; bp_cpuinfo::tsk_pinned are incremented/decremented, but otherwise unused. 2. If the target is a CPU: bp_cpuinfo::cpu_pinned are counted, along with bp_cpuinfo::tsk_pinned; after a successful check, cpu_pinned is incremented. No per-task breakpoints are checked. Since rhltable safely synchronizes insertions/deletions, we can allow concurrency as follows: 1. If the target is a task: independent tasks may update and check the constraints concurrently, but same-task target calls need to be serialized; since bp_cpuinfo::tsk_pinned is only updated, but not checked, these modifications can happen concurrently by switching tsk_pinned to atomic_t. 2. If the target is a CPU: access to the per-CPU constraints needs to be serialized with other CPU-target and task-target callers (to stabilize the bp_cpuinfo::tsk_pinned snapshot). We can allow the above concurrency by introducing a per-CPU constraints data reader-writer lock (bp_cpuinfo_sem), and per-task mutexes (reuses task_struct::perf_event_mutex): 1. If the target is a task: acquires perf_event_mutex, and acquires bp_cpuinfo_sem as a reader. The choice of percpu-rwsem minimizes contention in the presence of many read-lock but few write-lock acquisitions: we assume many orders of magnitude more task target breakpoints creations/destructions than CPU target breakpoints. 2. If the target is a CPU: acquires bp_cpuinfo_sem as a writer. With these changes, contention with thousands of tasks is reduced to the point where waiting on locking no longer dominates the profile: | $> 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: 0.077 [sec] | | 40.201563 usecs/op | 2572.900000 usecs/op/cpu 21.54% [kernel] [k] task_bp_pinned 20.18% [kernel] [k] rhashtable_jhash2 6.81% [kernel] [k] toggle_bp_slot 5.47% [kernel] [k] queued_spin_lock_slowpath 3.75% [kernel] [k] smp_cfm_core_cond 3.48% [kernel] [k] bcmp On this particular setup that's a speedup of 2.7x. We're also getting closer to the theoretical ideal performance through optimizations in hw_breakpoint.c -- constraints accounting disabled: | 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] | | 35.286458 usecs/op | 2258.333333 usecs/op/cpu Which means the current implementation is ~12% slower than the theoretical ideal. For reference, performance without any breakpoints: | $> bench -r 30 breakpoint thread -b 0 -p 64 -t 64 | # Running 'breakpoint/thread' benchmark: | # Created/joined 30 threads with 0 breakpoints and 64 parallelism | Total time: 0.060 [sec] | | 31.365625 usecs/op | 2007.400000 usecs/op/cpu On a system with 256 CPUs, the theoretical ideal is only ~12% slower than no breakpoints at all; the current implementation is ~28% slower. Signed-off-by: Marco Elver <elver@xxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx> Acked-by: Ian Rogers <irogers@xxxxxxxxxx> Link: https://lore.kernel.org/r/20220829124719.675715-12-elver@xxxxxxxxxx --- kernel/events/hw_breakpoint.c | 161 +++++++++++++++++++++++++++------ 1 file changed, 133 insertions(+), 28 deletions(-) diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c index 8b40fca..229c6f4 100644 --- a/kernel/events/hw_breakpoint.c +++ b/kernel/events/hw_breakpoint.c @@ -19,6 +19,7 @@ #include <linux/hw_breakpoint.h> +#include <linux/atomic.h> #include <linux/bug.h> #include <linux/cpu.h> #include <linux/export.h> @@ -28,6 +29,7 @@ #include <linux/kernel.h> #include <linux/mutex.h> #include <linux/notifier.h> +#include <linux/percpu-rwsem.h> #include <linux/percpu.h> #include <linux/rhashtable.h> #include <linux/sched.h> @@ -41,9 +43,9 @@ struct bp_cpuinfo { unsigned int cpu_pinned; /* tsk_pinned[n] is the number of tasks having n+1 breakpoints */ #ifdef hw_breakpoint_slots - unsigned int tsk_pinned[hw_breakpoint_slots(0)]; + atomic_t tsk_pinned[hw_breakpoint_slots(0)]; #else - unsigned int *tsk_pinned; + atomic_t *tsk_pinned; #endif }; @@ -65,8 +67,79 @@ static const struct rhashtable_params task_bps_ht_params = { static bool constraints_initialized __ro_after_init; -/* Serialize accesses to the above constraints */ -static DEFINE_MUTEX(nr_bp_mutex); +/* + * Synchronizes accesses to the per-CPU constraints; the locking rules are: + * + * 1. Atomic updates to bp_cpuinfo::tsk_pinned only require a held read-lock + * (due to bp_slots_histogram::count being atomic, no update are lost). + * + * 2. Holding a write-lock is required for computations that require a + * stable snapshot of all bp_cpuinfo::tsk_pinned. + * + * 3. In all other cases, non-atomic accesses require the appropriately held + * lock (read-lock for read-only accesses; write-lock for reads/writes). + */ +DEFINE_STATIC_PERCPU_RWSEM(bp_cpuinfo_sem); + +/* + * Return mutex to serialize accesses to per-task lists in task_bps_ht. Since + * rhltable synchronizes concurrent insertions/deletions, independent tasks may + * insert/delete concurrently; therefore, a mutex per task is sufficient. + * + * Uses task_struct::perf_event_mutex, to avoid extending task_struct with a + * hw_breakpoint-only mutex, which may be infrequently used. The caveat here is + * that hw_breakpoint may contend with per-task perf event list management. The + * assumption is that perf usecases involving hw_breakpoints are very unlikely + * to result in unnecessary contention. + */ +static inline struct mutex *get_task_bps_mutex(struct perf_event *bp) +{ + struct task_struct *tsk = bp->hw.target; + + return tsk ? &tsk->perf_event_mutex : NULL; +} + +static struct mutex *bp_constraints_lock(struct perf_event *bp) +{ + struct mutex *tsk_mtx = get_task_bps_mutex(bp); + + if (tsk_mtx) { + mutex_lock(tsk_mtx); + percpu_down_read(&bp_cpuinfo_sem); + } else { + percpu_down_write(&bp_cpuinfo_sem); + } + + return tsk_mtx; +} + +static void bp_constraints_unlock(struct mutex *tsk_mtx) +{ + if (tsk_mtx) { + percpu_up_read(&bp_cpuinfo_sem); + mutex_unlock(tsk_mtx); + } else { + percpu_up_write(&bp_cpuinfo_sem); + } +} + +static bool bp_constraints_is_locked(struct perf_event *bp) +{ + struct mutex *tsk_mtx = get_task_bps_mutex(bp); + + return percpu_is_write_locked(&bp_cpuinfo_sem) || + (tsk_mtx ? mutex_is_locked(tsk_mtx) : + percpu_is_read_locked(&bp_cpuinfo_sem)); +} + +static inline void assert_bp_constraints_lock_held(struct perf_event *bp) +{ + struct mutex *tsk_mtx = get_task_bps_mutex(bp); + + if (tsk_mtx) + lockdep_assert_held(tsk_mtx); + lockdep_assert_held(&bp_cpuinfo_sem); +} #ifdef hw_breakpoint_slots /* @@ -97,7 +170,7 @@ static __init int init_breakpoint_slots(void) for (i = 0; i < TYPE_MAX; i++) { struct bp_cpuinfo *info = get_bp_info(cpu, i); - info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(int), GFP_KERNEL); + info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(atomic_t), GFP_KERNEL); if (!info->tsk_pinned) goto err; } @@ -137,11 +210,19 @@ static inline enum bp_type_idx find_slot_idx(u64 bp_type) */ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type) { - unsigned int *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned; + atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned; int i; + /* + * At this point we want to have acquired the bp_cpuinfo_sem as a + * writer to ensure that there are no concurrent writers in + * toggle_bp_task_slot() to tsk_pinned, and we get a stable snapshot. + */ + lockdep_assert_held_write(&bp_cpuinfo_sem); + for (i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) { - if (tsk_pinned[i] > 0) + ASSERT_EXCLUSIVE_WRITER(tsk_pinned[i]); /* Catch unexpected writers. */ + if (atomic_read(&tsk_pinned[i]) > 0) return i + 1; } @@ -158,6 +239,11 @@ static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type) struct perf_event *iter; int count = 0; + /* + * We need a stable snapshot of the per-task breakpoint list. + */ + assert_bp_constraints_lock_held(bp); + rcu_read_lock(); head = rhltable_lookup(&task_bps_ht, &bp->hw.target, task_bps_ht_params); if (!head) @@ -214,16 +300,25 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type) static void toggle_bp_task_slot(struct perf_event *bp, int cpu, enum bp_type_idx type, int weight) { - unsigned int *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned; + atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned; int old_idx, new_idx; + /* + * 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); + old_idx = task_bp_pinned(cpu, bp, type) - 1; new_idx = old_idx + weight; if (old_idx >= 0) - tsk_pinned[old_idx]--; + atomic_dec(&tsk_pinned[old_idx]); if (new_idx >= 0) - tsk_pinned[new_idx]++; + atomic_inc(&tsk_pinned[new_idx]); } /* @@ -241,6 +336,7 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type, /* Pinned counter cpu profiling */ if (!bp->hw.target) { + lockdep_assert_held_write(&bp_cpuinfo_sem); get_bp_info(bp->cpu, type)->cpu_pinned += weight; return 0; } @@ -249,6 +345,11 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type, for_each_cpu(cpu, cpumask) toggle_bp_task_slot(bp, cpu, type, weight); + /* + * Readers want a stable snapshot of the per-task breakpoint list. + */ + assert_bp_constraints_lock_held(bp); + if (enable) return rhltable_insert(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params); else @@ -354,14 +455,10 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type) int reserve_bp_slot(struct perf_event *bp) { - int ret; - - mutex_lock(&nr_bp_mutex); - - ret = __reserve_bp_slot(bp, bp->attr.bp_type); - - mutex_unlock(&nr_bp_mutex); + struct mutex *mtx = bp_constraints_lock(bp); + int ret = __reserve_bp_slot(bp, bp->attr.bp_type); + bp_constraints_unlock(mtx); return ret; } @@ -379,12 +476,11 @@ static void __release_bp_slot(struct perf_event *bp, u64 bp_type) void release_bp_slot(struct perf_event *bp) { - mutex_lock(&nr_bp_mutex); + struct mutex *mtx = bp_constraints_lock(bp); arch_unregister_hw_breakpoint(bp); __release_bp_slot(bp, bp->attr.bp_type); - - mutex_unlock(&nr_bp_mutex); + bp_constraints_unlock(mtx); } static int __modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type) @@ -411,11 +507,10 @@ static int __modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type) static int modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type) { - int ret; + struct mutex *mtx = bp_constraints_lock(bp); + int ret = __modify_bp_slot(bp, old_type, new_type); - mutex_lock(&nr_bp_mutex); - ret = __modify_bp_slot(bp, old_type, new_type); - mutex_unlock(&nr_bp_mutex); + bp_constraints_unlock(mtx); return ret; } @@ -426,18 +521,28 @@ static int modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type) */ int dbg_reserve_bp_slot(struct perf_event *bp) { - if (mutex_is_locked(&nr_bp_mutex)) + int ret; + + if (bp_constraints_is_locked(bp)) return -1; - return __reserve_bp_slot(bp, bp->attr.bp_type); + /* Locks aren't held; disable lockdep assert checking. */ + lockdep_off(); + ret = __reserve_bp_slot(bp, bp->attr.bp_type); + lockdep_on(); + + return ret; } int dbg_release_bp_slot(struct perf_event *bp) { - if (mutex_is_locked(&nr_bp_mutex)) + if (bp_constraints_is_locked(bp)) return -1; + /* Locks aren't held; disable lockdep assert checking. */ + lockdep_off(); __release_bp_slot(bp, bp->attr.bp_type); + lockdep_on(); return 0; } @@ -663,7 +768,7 @@ bool hw_breakpoint_is_used(void) return true; for (int slot = 0; slot < hw_breakpoint_slots_cached(type); ++slot) { - if (info->tsk_pinned[slot]) + if (atomic_read(&info->tsk_pinned[slot])) return true; } }
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