Boqun Feng's patch series and LPC talk gave me a few ideas I wanted to try. I figured we could improve the concept of reference counters by adding a hazard-pointer protected fast-path to them. This API combines hazard pointers and reference counters. It uses hazard pointers as fast-paths, and falls back to reference counters either explicitly when the reader expects to hold the object for a long time, or when no hazard pointer slots are available. This prototype is implemented in userspace within the liburcu project, and depends on librseq for per-cpu data structure allocation, indexing, and updates. - The top of this liburcu feature branch can be found at: https://github.com/compudj/userspace-rcu-dev/tree/hpref (it's implemented within liburcu for convenience, but it does not actually use RCU). - librseq can be found at: https://git.kernel.org/pub/scm/libs/librseq/librseq.git/ This leverages the fact that both synchronization mechanisms aim to guarantee existence of objects, and those existence guarantees can be chained. Each mechanism achieves its purpose in a different way with different tradeoffs. The hazard pointers are faster to read and scale better than reference counters, but they consume more memory than a per-object reference counter. The fall-back to reference counter allows bounding the number of hazard pointer slots to a fixed size for the entire system: nr_cpus * N, where N=8 as it fills a single 64 bytes cache line on 64-bit architectures. Porting it to the Linux kernel should be straightforward. We might want to pick heavily contented reference counts such as the mm_struct mm_count field as a starting point to see if it provides significant performance gains. The hpref read-side performs well even compared to RCU in my benchmarks: Results: CPU(s): 16 On-line CPU(s) list: 0-15 Vendor ID: AuthenticAMD Model name: AMD Ryzen 7 PRO 6850U with Radeon Graphics 8 readers, 1 writer, 10s test_urcu_mb (smp_mb) nr_reads 829829784 nr_writes 18057836 nr_ops 847887620 test_hpref_benchmark (smp_mb) nr_reads 2040376076 nr_writes 2993246 nr_ops 2043369322 test_hpref_benchmark (barrier/membarrier) nr_reads 10609164704 nr_writes 2208432 nr_ops 10611373136 test_urcu_bp (barrier/membarrier) nr_reads 20242102863 nr_writes 599484 nr_ops 20242702347 test_urcu (barrier/membarrier) nr_reads 20714490759 nr_writes 782045 nr_ops 20715272804 test_urcu_qsbr nr_reads 40774708959 nr_writes 3512904 nr_ops 40778221863 References: [1]: M. M. Michael, "Hazard pointers: safe memory reclamation for lock-free objects," in IEEE Transactions on Parallel and Distributed Systems, vol. 15, no. 6, pp. 491-504, June 2004 Link: https://lore.kernel.org/lkml/j3scdl5iymjlxavomgc6u5ndg3svhab6ga23dr36o4f5mt333w@7xslvq6b6hmv/ Link: https://lpc.events/event/18/contributions/1731/ Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> Change-Id: I6369064a0e1a1f9632394df31ff41c76905d17e3 Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx> Cc: Will Deacon <will@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Boqun Feng <boqun.feng@xxxxxxxxx> Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> Cc: John Stultz <jstultz@xxxxxxxxxx> To: Neeraj Upadhyay <Neeraj.Upadhyay@xxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Boqun Feng <boqun.feng@xxxxxxxxx> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx> Cc: Uladzislau Rezki <urezki@xxxxxxxxx> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx> Cc: Zqiang <qiang.zhang1211@xxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Waiman Long <longman@xxxxxxxxxx> Cc: Mark Rutland <mark.rutland@xxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Vlastimil Babka <vbabka@xxxxxxx> Cc: maged.michael@xxxxxxxxx Cc: Mateusz Guzik <mjguzik@xxxxxxxxx> Cc: rcu@xxxxxxxxxxxxxxx Cc: linux-mm@xxxxxxxxx Cc: lkmm@xxxxxxxxxxxxxxx --- include/urcu/hpref.h | 222 +++++++++++++++++++++++++++++++++++++++++++ src/Makefile.am | 6 +- src/hpref.c | 78 +++++++++++++++ 3 files changed, 305 insertions(+), 1 deletion(-) create mode 100644 include/urcu/hpref.h create mode 100644 src/hpref.c diff --git a/include/urcu/hpref.h b/include/urcu/hpref.h new file mode 100644 index 00000000..300f7d4e --- /dev/null +++ b/include/urcu/hpref.h @@ -0,0 +1,222 @@ +// SPDX-FileCopyrightText: 2024 Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> +// +// SPDX-License-Identifier: LGPL-2.1-or-later + +#ifndef _URCU_HPREF_H +#define _URCU_HPREF_H + +/* + * HPREF: Hazard pointers with reference counters + * + * This API combines hazard pointers and reference counters. + * It uses hazard pointers as fast-paths, and fall-back to reference + * counters either explicitly when the reader expects to hold the object + * for a long time, or when no hazard pointer slots are available. + * + * This leverages the fact that both synchronization mechanisms aim to + * guarantee existence of objects, and those existence guarantees can be + * chained. Each mechanism achieves its purpose in a different way with + * different tradeoffs. The hazard pointers are faster to read and scale + * better than reference counters, but they consume more memory than a + * per-object reference counter. + * + * The fall-back to reference counter allows bounding the number of + * hazard pointer slots to a fixed size for the entire system: + * nr_cpus * N, where N=8 as it fills a single 64 bytes cache line on + * 64-bit architectures. + * + * References: + * + * [1]: M. M. Michael, "Hazard pointers: safe memory reclamation for + * lock-free objects," in IEEE Transactions on Parallel and + * Distributed Systems, vol. 15, no. 6, pp. 491-504, June 2004 + */ + +#include <stdlib.h> +#include <unistd.h> +#include <poll.h> +#include <stdbool.h> +#include <rseq/mempool.h> /* Per-CPU memory */ +#include <rseq/rseq.h> + +#include <urcu/ref.h> +#include <urcu/uatomic.h> +#include <urcu/compiler.h> + +struct hpref_node { + struct urcu_ref refcount; + void (*release)(struct hpref_node *node); +}; + +struct hpref_slot { + /* Use rseq to set from reader only if zero. */ + struct hpref_node *node; +}; + +#define NR_PERCPU_SLOTS_BITS 3 +#define HPREF_NR_PERCPU_SLOTS (1U << NR_PERCPU_SLOTS_BITS) +/* + * The emergency slot is only used for short critical sections + * (would be preempt off in when porting this code to the kernel): only + * to ensure we have a free slot for taking a reference count as + * fallback. + */ +#define HPREF_EMERGENCY_SLOT (HPREF_NR_PERCPU_SLOTS - 1) + +struct hpref_percpu_slots { + struct hpref_slot slots[HPREF_NR_PERCPU_SLOTS]; + unsigned int current_slot; +}; + +enum hpref_type { + HPREF_TYPE_HP, + HPREF_TYPE_REF, +}; + +struct hpref_ctx { + struct hpref_slot *slot; + struct hpref_node *hp; + enum hpref_type type; +}; + +#ifdef __cplusplus +extern "C" { +#endif + +extern struct hpref_percpu_slots *hpref_percpu_slots; + +void hpref_release(struct urcu_ref *ref); + +/* + * hpref_synchronize: Wait for any reader possessing a hazard pointer to + * @node to clear its hazard pointer slot. + */ +void hpref_synchronize(struct hpref_node *node); + +/* + * hpref_synchronize_put: Wait for any reader possessing a hazard + * pointer to clear its slot and put reference + * count. + */ +void hpref_synchronize_put(struct hpref_node *node); + +static inline +void hpref_node_init(struct hpref_node *node, + void (*release)(struct hpref_node *node)) +{ + urcu_ref_init(&node->refcount); + node->release = release; +} + +/* + * hpref_promote_hp_to_ref: Promote hazard pointer to reference count. + */ +static inline +void hpref_promote_hp_to_ref(struct hpref_ctx *ctx) +{ + if (ctx->type == HPREF_TYPE_REF) + return; + urcu_ref_get(&ctx->hp->refcount); + uatomic_store(&ctx->slot->node, NULL, CMM_RELEASE); + ctx->slot = NULL; + ctx->type = HPREF_TYPE_REF; +} + +/* + * hpref_hp_get: Obtain a reference to a stable object, protected either + * by hazard pointer (fast-path) or using reference + * counter as fall-back. + */ +static inline +bool hpref_hp_get(struct hpref_node **node_p, struct hpref_ctx *ctx) +{ + int cpu = rseq_current_cpu_raw(); + struct hpref_percpu_slots *cpu_slots = rseq_percpu_ptr(hpref_percpu_slots, cpu); + struct hpref_slot *slot = &cpu_slots->slots[cpu_slots->current_slot]; + bool use_refcount = false; + struct hpref_node *node, *node2; + unsigned int next_slot; + +retry: + node = uatomic_load(node_p, CMM_RELAXED); + if (!node) + return false; + /* Use rseq to try setting current slot hp. Store B. */ + if (rseq_load_cbne_store__ptr(RSEQ_MO_RELAXED, RSEQ_PERCPU_CPU_ID, + (intptr_t *) &slot->node, (intptr_t) NULL, + (intptr_t) node, cpu)) { + slot = &cpu_slots->slots[HPREF_EMERGENCY_SLOT]; + use_refcount = true; + /* + * This may busy-wait for another reader using the + * emergency slot to transition to refcount. + */ + caa_cpu_relax(); + goto retry; + } + /* Memory ordering: Store B before Load A. */ + cmm_smp_mb(); + node2 = uatomic_load(node_p, CMM_RELAXED); /* Load A */ + if (node != node2) { + uatomic_store(&slot->node, NULL, CMM_RELAXED); + if (!node2) + return false; + goto retry; + } + ctx->type = HPREF_TYPE_HP; + ctx->hp = node; + ctx->slot = slot; + if (use_refcount) { + hpref_promote_hp_to_ref(ctx); + return true; + } + /* + * Increment current slot (racy increment is OK because it is + * just a position hint). Skip the emergency slot. + */ + next_slot = uatomic_load(&cpu_slots->current_slot, CMM_RELAXED) + 1; + if (next_slot >= HPREF_EMERGENCY_SLOT) + next_slot = 0; + uatomic_store(&cpu_slots->current_slot, next_slot, CMM_RELAXED); + return true; +} + +static inline +void hpref_put(struct hpref_ctx *ctx) +{ + if (ctx->type == HPREF_TYPE_REF) { + urcu_ref_put(&ctx->hp->refcount, hpref_release); + } else { + /* Release HP. */ + uatomic_store(&ctx->slot->node, NULL, CMM_RELEASE); + } + ctx->hp = NULL; +} + +/* + * hpref_set_pointer: Store pointer @node to @ptr, with RCU publication + * guarantees. + */ +static inline +void hpref_set_pointer(struct hpref_node **ptr, struct hpref_node *node) +{ + if (__builtin_constant_p(node) && node == NULL) + uatomic_store(ptr, NULL, CMM_RELAXED); + else + uatomic_store(ptr, node, CMM_RELEASE); +} + +/* + * Return the content of the hpref context hazard pointer field. + */ +static inline +struct hpref_node *hpref_ctx_pointer(struct hpref_ctx *ctx) +{ + return ctx->hp; +} + +#ifdef __cplusplus +} +#endif + +#endif /* _URCU_HPREF_H */ diff --git a/src/Makefile.am b/src/Makefile.am index b555c818..7312c9f7 100644 --- a/src/Makefile.am +++ b/src/Makefile.am @@ -19,7 +19,8 @@ RCULFHASH = rculfhash.c rculfhash-mm-order.c rculfhash-mm-chunk.c \ lib_LTLIBRARIES = liburcu-common.la \ liburcu.la liburcu-qsbr.la \ liburcu-mb.la liburcu-bp.la \ - liburcu-memb.la liburcu-cds.la + liburcu-memb.la liburcu-cds.la \ + liburcu-hpref.la # # liburcu-common contains wait-free queues (needed by call_rcu) as well @@ -50,6 +51,9 @@ liburcu_cds_la_SOURCES = rculfqueue.c rculfstack.c lfstack.c \ workqueue.c workqueue.h $(RCULFHASH) $(COMPAT) liburcu_cds_la_LIBADD = liburcu-common.la +liburcu_hpref_la_SOURCES = hpref.c +liburcu_hpref_la_LIBADD = -lrseq + pkgconfigdir = $(libdir)/pkgconfig pkgconfig_DATA = liburcu-cds.pc liburcu.pc liburcu-bp.pc liburcu-qsbr.pc \ liburcu-mb.pc liburcu-memb.pc diff --git a/src/hpref.c b/src/hpref.c new file mode 100644 index 00000000..f63530f5 --- /dev/null +++ b/src/hpref.c @@ -0,0 +1,78 @@ +// SPDX-FileCopyrightText: 2024 Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> +// +// SPDX-License-Identifier: LGPL-2.1-or-later + +/* + * HPREF: Hazard pointers with reference counters + */ + +#define _LGPL_SOURCE +#include <urcu/hpref.h> +#include <rseq/mempool.h> /* Per-CPU memory */ + +static struct rseq_mempool *mempool; +struct hpref_percpu_slots *hpref_percpu_slots; + +void hpref_release(struct urcu_ref *ref) +{ + struct hpref_node *node = caa_container_of(ref, struct hpref_node, refcount); + + node->release(node); +} + +/* + * hpref_synchronize: Wait for any reader possessing a hazard pointer to + * @node to clear its hazard pointer slot. + */ +void hpref_synchronize(struct hpref_node *node) +{ + int nr_cpus = rseq_get_max_nr_cpus(), cpu; + + /* Memory ordering: Store A before Load B. */ + cmm_smp_mb(); + /* Scan all CPUs slots. */ + for (cpu = 0; cpu < nr_cpus; cpu++) { + struct hpref_percpu_slots *cpu_slots = rseq_percpu_ptr(hpref_percpu_slots, cpu); + struct hpref_slot *slot; + unsigned int i; + + for (i = 0; i < HPREF_NR_PERCPU_SLOTS; i++) { + slot = &cpu_slots->slots[i]; + /* Busy-wait if node is found. */ + while (uatomic_load(&slot->node, CMM_ACQUIRE) == node) /* Load B */ + caa_cpu_relax(); + } + } +} + +/* + * hpref_synchronize_put: Wait for any reader possessing a hazard + * pointer to clear its slot and put reference + * count. + */ +void hpref_synchronize_put(struct hpref_node *node) +{ + if (!node) + return; + hpref_synchronize(node); + urcu_ref_put(&node->refcount, hpref_release); +} + +static __attribute__((constructor)) +void hpref_init(void) +{ + mempool = rseq_mempool_create("hpref", sizeof(struct hpref_percpu_slots), NULL); + if (!mempool) + abort(); + hpref_percpu_slots = rseq_mempool_percpu_zmalloc(mempool); + if (!hpref_percpu_slots) + abort(); +} + +static __attribute__((destructor)) +void hpref_exit(void) +{ + rseq_mempool_percpu_free(hpref_percpu_slots); + if (rseq_mempool_destroy(mempool)) + abort(); +} -- 2.39.2
