[lttng-dev] [PATCH] urcu: avoid false sharing for rcu_gp_ctr
Mathieu Desnoyers
mathieu.desnoyers at efficios.com
Fri Dec 7 11:32:34 EST 2012
* Lai Jiangshan (laijs at cn.fujitsu.com) 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 at 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 at 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 at 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 at 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 at 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 <laijs at cn.fujitsu.com>
> ---
> 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
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