[lttng-dev] [PATCH] urcu: avoid false sharing for rcu_gp_ctr

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Mon May 6 08:44:33 EDT 2013


Since we are bumping the urcu soname version number to 2.0.0 for the
upcoming urcu 0.8 anyway, it's time to merge this patch.

Thanks!

Mathieu

* 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.
> 
> 
> 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
EfficiOS Inc.
http://www.efficios.com



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