Re: [PATCH v2 10/13] perf/hw_breakpoint: Reduce contention with large number of tasks

Dmitry Vyukov
2022-06-28T14:45:18Z

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
>