| Message ID | 20220628095833.2579903-11-elver@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | perf/hw_breakpoint: Optimize for thousands of tasks | expand |
On Tue, 28 Jun 2022 at 11:59, Marco Elver <elver@google.com> wrote: > > 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@google.com> Reviewed-by: Dmitry Vyukov <dvyukov@google.com> > --- > v2: > * Use percpu-rwsem instead of rwlock. > * Use task_struct::perf_event_mutex. See code comment for reasoning. > ==> Speedup of 2.7x (vs 2.5x in v1). > --- > kernel/events/hw_breakpoint.c | 159 ++++++++++++++++++++++++++++------ > 1 file changed, 132 insertions(+), 27 deletions(-) > > diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c > index 63e39dc836bd..128ba3429223 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; > } > -- > 2.37.0.rc0.161.g10f37bed90-goog >
diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c index 63e39dc836bd..128ba3429223 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; }
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@google.com> --- v2: * Use percpu-rwsem instead of rwlock. * Use task_struct::perf_event_mutex. See code comment for reasoning. ==> Speedup of 2.7x (vs 2.5x in v1). --- kernel/events/hw_breakpoint.c | 159 ++++++++++++++++++++++++++++------ 1 file changed, 132 insertions(+), 27 deletions(-)