From: Lai Jiangshan Date: Mon, 6 May 2013 12:42:27 +0000 (-0400) Subject: urcu: avoid false sharing for rcu_gp_ctr X-Git-Tag: v0.8.0~90 X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=ed1b099ef8acd1396dbde7c7150d1a5eb16ce8a6;p=urcu.git urcu: avoid false sharing for rcu_gp_ctr @rcu_gp_ctr and @registry share the same cache line, it causes false sharing and slowdown both of the read site and update site. Fix: Use different cache line for them. Although rcu_gp_futex is updated less than rcu_gp_ctr, but they always be accessed at almost the same time, so we also move rcu_gp_futex to the cacheline of rcu_gp_ctr to reduce the cacheline-usage or cache-missing of read site. test: (4X6=24 CPUs) Before patch: [root@localhost userspace-rcu]# ./tests/test_urcu_mb 20 1 20 SUMMARY ./tests/test_urcu_mb testdur 20 nr_readers 20 rdur 0 wdur 0 nr_writers 1 wdelay 0 nr_reads 2100285330 nr_writes 3390219 nr_ops 2103675549 [root@localhost userspace-rcu]# ./tests/test_urcu_mb 20 1 20 SUMMARY ./tests/test_urcu_mb testdur 20 nr_readers 20 rdur 0 wdur 0 nr_writers 1 wdelay 0 nr_reads 1619868562 nr_writes 3529478 nr_ops 1623398040 [root@localhost userspace-rcu]# ./tests/test_urcu_mb 20 1 20 SUMMARY ./tests/test_urcu_mb testdur 20 nr_readers 20 rdur 0 wdur 0 nr_writers 1 wdelay 0 nr_reads 1949067038 nr_writes 3469334 nr_ops 1952536372 after patch: [root@localhost userspace-rcu]# ./tests/test_urcu_mb 20 1 20 SUMMARY ./tests/test_urcu_mb testdur 20 nr_readers 20 rdur 0 wdur 0 nr_writers 1 wdelay 0 nr_reads 3380191848 nr_writes 4903248 nr_ops 3385095096 [root@localhost userspace-rcu]# ./tests/test_urcu_mb 20 1 20 SUMMARY ./tests/test_urcu_mb testdur 20 nr_readers 20 rdur 0 wdur 0 nr_writers 1 wdelay 0 nr_reads 3397637486 nr_writes 4129809 nr_ops 3401767295 Singed-off-by: Lai Jiangshan Signed-off-by: Mathieu Desnoyers --- diff --git a/urcu.c b/urcu.c index a78c02b..da6839f 100644 --- a/urcu.c +++ b/urcu.c @@ -83,16 +83,7 @@ void __attribute__((destructor)) rcu_exit(void); #endif static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER; - -int32_t rcu_gp_futex; - -/* - * Global grace period counter. - * Contains the current RCU_GP_CTR_PHASE. - * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path. - * Written to only by writer with mutex taken. Read by both writer and readers. - */ -unsigned long rcu_gp_ctr = RCU_GP_COUNT; +struct urcu_gp rcu_gp = { .ctr = RCU_GP_COUNT }; /* * Written to only by each individual reader. Read by both the reader and the @@ -217,8 +208,8 @@ static void wait_gp(void) { /* Read reader_gp before read futex */ smp_mb_master(RCU_MB_GROUP); - if (uatomic_read(&rcu_gp_futex) == -1) - futex_async(&rcu_gp_futex, FUTEX_WAIT, -1, + if (uatomic_read(&rcu_gp.futex) == -1) + futex_async(&rcu_gp.futex, FUTEX_WAIT, -1, NULL, NULL, 0); } @@ -232,12 +223,12 @@ static void wait_for_readers(struct cds_list_head *input_readers, /* * Wait for each thread URCU_TLS(rcu_reader).ctr to either * indicate quiescence (not nested), or observe the current - * rcu_gp_ctr value. + * rcu_gp.ctr value. */ for (;;) { wait_loops++; if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) { - uatomic_dec(&rcu_gp_futex); + uatomic_dec(&rcu_gp.futex); /* Write futex before read reader_gp */ smp_mb_master(RCU_MB_GROUP); } @@ -270,7 +261,7 @@ static void wait_for_readers(struct cds_list_head *input_readers, if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) { /* Read reader_gp before write futex */ smp_mb_master(RCU_MB_GROUP); - uatomic_set(&rcu_gp_futex, 0); + uatomic_set(&rcu_gp.futex, 0); } break; } else { @@ -289,7 +280,7 @@ static void wait_for_readers(struct cds_list_head *input_readers, if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) { /* Read reader_gp before write futex */ smp_mb_master(RCU_MB_GROUP); - uatomic_set(&rcu_gp_futex, 0); + uatomic_set(&rcu_gp.futex, 0); } break; } else { @@ -357,10 +348,10 @@ void synchronize_rcu(void) /* * Must finish waiting for quiescent state for original parity before - * committing next rcu_gp_ctr update to memory. Failure to do so could + * committing next rcu_gp.ctr update to memory. Failure to do so could * result in the writer waiting forever while new readers are always * accessing data (no progress). Enforce compiler-order of load - * URCU_TLS(rcu_reader).ctr before store to rcu_gp_ctr. + * URCU_TLS(rcu_reader).ctr before store to rcu_gp.ctr. */ cmm_barrier(); @@ -372,13 +363,13 @@ void synchronize_rcu(void) cmm_smp_mb(); /* Switch parity: 0 -> 1, 1 -> 0 */ - CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE); + CMM_STORE_SHARED(rcu_gp.ctr, rcu_gp.ctr ^ RCU_GP_CTR_PHASE); /* - * Must commit rcu_gp_ctr update to memory before waiting for quiescent + * Must commit rcu_gp.ctr update to memory before waiting for quiescent * state. Failure to do so could result in the writer waiting forever * while new readers are always accessing data (no progress). Enforce - * compiler-order of store to rcu_gp_ctr before load rcu_reader ctr. + * compiler-order of store to rcu_gp.ctr before load rcu_reader ctr. */ cmm_barrier(); diff --git a/urcu/static/urcu.h b/urcu/static/urcu.h index 6c8b926..bde1459 100644 --- a/urcu/static/urcu.h +++ b/urcu/static/urcu.h @@ -213,12 +213,20 @@ static inline void smp_mb_slave(int group) #define RCU_GP_CTR_PHASE (1UL << (sizeof(unsigned long) << 2)) #define RCU_GP_CTR_NEST_MASK (RCU_GP_CTR_PHASE - 1) -/* - * Global quiescent period counter with low-order bits unused. - * Using a int rather than a char to eliminate false register dependencies - * causing stalls on some architectures. - */ -extern unsigned long rcu_gp_ctr; +struct urcu_gp { + /* + * Global grace period counter. + * Contains the current RCU_GP_CTR_PHASE. + * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path. + * Written to only by writer with mutex taken. + * Read by both writer and readers. + */ + unsigned long ctr; + + int32_t futex; +} __attribute__((aligned(CAA_CACHE_LINE_SIZE))); + +extern struct urcu_gp rcu_gp; struct rcu_reader { /* Data used by both reader and synchronize_rcu() */ @@ -231,16 +239,14 @@ struct rcu_reader { extern DECLARE_URCU_TLS(struct rcu_reader, rcu_reader); -extern int32_t rcu_gp_futex; - /* * Wake-up waiting synchronize_rcu(). Called from many concurrent threads. */ static inline void wake_up_gp(void) { - if (caa_unlikely(uatomic_read(&rcu_gp_futex) == -1)) { - uatomic_set(&rcu_gp_futex, 0); - futex_async(&rcu_gp_futex, FUTEX_WAKE, 1, + if (caa_unlikely(uatomic_read(&rcu_gp.futex) == -1)) { + uatomic_set(&rcu_gp.futex, 0); + futex_async(&rcu_gp.futex, FUTEX_WAKE, 1, NULL, NULL, 0); } } @@ -256,13 +262,13 @@ static inline enum rcu_state rcu_reader_state(unsigned long *ctr) v = CMM_LOAD_SHARED(*ctr); if (!(v & RCU_GP_CTR_NEST_MASK)) return RCU_READER_INACTIVE; - if (!((v ^ rcu_gp_ctr) & RCU_GP_CTR_PHASE)) + if (!((v ^ rcu_gp.ctr) & RCU_GP_CTR_PHASE)) return RCU_READER_ACTIVE_CURRENT; return RCU_READER_ACTIVE_OLD; } /* - * Helper for _rcu_read_lock(). The format of rcu_gp_ctr (as well as + * Helper for _rcu_read_lock(). The format of rcu_gp.ctr (as well as * the per-thread rcu_reader.ctr) has the upper bits containing a count of * _rcu_read_lock() nesting, and a lower-order bit that contains either zero * or RCU_GP_CTR_PHASE. The smp_mb_slave() ensures that the accesses in @@ -271,7 +277,7 @@ static inline enum rcu_state rcu_reader_state(unsigned long *ctr) static inline void _rcu_read_lock_update(unsigned long tmp) { if (caa_likely(!(tmp & RCU_GP_CTR_NEST_MASK))) { - _CMM_STORE_SHARED(URCU_TLS(rcu_reader).ctr, _CMM_LOAD_SHARED(rcu_gp_ctr)); + _CMM_STORE_SHARED(URCU_TLS(rcu_reader).ctr, _CMM_LOAD_SHARED(rcu_gp.ctr)); smp_mb_slave(RCU_MB_GROUP); } else _CMM_STORE_SHARED(URCU_TLS(rcu_reader).ctr, tmp + RCU_GP_COUNT);