* Lai Jiangshan ([email protected]) wrote: > we can define rcu_gp_ctr and registry with aligned attribute, but it is not > reliable way > > We need only this: > unsigned long rcu_gp_ctr __attribute((aligned and padded(don't put other > var next to it except the futex)))
In which situation would this be unreliable ? Thanks, Mathieu > > On Saturday, December 8, 2012, Mathieu Desnoyers wrote: > > > * Lai Jiangshan ([email protected] <javascript:;>) wrote: > > > @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. > > > > Hi Lai, > > > > I agree on the goal: placing registry and rcu_gp_ctr on different > > cache-lines. And yes, it makes sense to put rcu_gp_ctr and rcu_gp_futex > > on the same cache-line. I agree that the next patch is fine too (keeping > > qsbr and other urcu similar). This is indeed what I try to ensure > > myself. > > > > I'm just concerned that this patch seems to break ABI compability for > > liburcu: the read-side, within applications, would have to be > > recompiled. So I guess we should decide if we do this change in a way > > that does not break the ABI (e.g. not introducing a structure), or if we > > choose to bump the library version number. > > > > Thoughts ? > > > > Thanks, > > > > Mathieu > > > > > > > > > > > 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-by: Lai Jiangshan <[email protected] <javascript:;>> > > > --- > > > diff --git a/urcu.c b/urcu.c > > > index 15def09..436d71c 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 973826a..0dd733e 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); > > > > -- > > Mathieu Desnoyers > > Operating System Efficiency R&D Consultant > > EfficiOS Inc. > > http://www.efficios.com > > > > _______________________________________________ > > lttng-dev mailing list > > [email protected] <javascript:;> > > http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev > > -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com _______________________________________________ lttng-dev mailing list [email protected] http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
