[lttng-dev] [PATCH] urcu: avoid false sharing for rcu_gp_ctr
Lai Jiangshan
laijs at cn.fujitsu.com
Fri Dec 7 02:06:29 EST 2012
@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);
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