timer: relax barrier for status update

Volatile has no ordering semantics. The rte_timer structure defines
timer status as a volatile variable and uses the rte_r/wmb barrier
to guarantee inter-thread visibility.

This patch optimized the volatile operation with c11 atomic operations
and one-way barrier to save the performance penalty. According to the
timer_perf_autotest benchmarking results, this patch can uplift 10%~16%
timer appending performance, 3%~20% timer resetting performance and 45%
timer callbacks scheduling performance on aarch64 and no loss in
performance for x86.

Suggested-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Signed-off-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
Acked-by: Erik Gabriel Carrillo <erik.g.carrillo@intel.com>

diff --git a/lib/librte_timer/rte_timer.c b/lib/librte_timer/rte_timer.c
index 269e921..6d19ce4 100644 (file)
--- a/lib/librte_timer/rte_timer.c
+++ b/lib/librte_timer/rte_timer.c
@@ -10,7 +10,6 @@
 #include <assert.h>
 #include <sys/queue.h>
 
-#include <rte_atomic.h>
 #include <rte_common.h>
 #include <rte_cycles.h>
 #include <rte_eal_memconfig.h>
@@ -218,7 +217,7 @@ rte_timer_init(struct rte_timer *tim)
 
        status.state = RTE_TIMER_STOP;
        status.owner = RTE_TIMER_NO_OWNER;
-       tim->status.u32 = status.u32;
+       __atomic_store_n(&tim->status.u32, status.u32, __ATOMIC_RELAXED);
 }
 
 /*
@@ -239,9 +238,9 @@ timer_set_config_state(struct rte_timer *tim,
 
        /* wait that the timer is in correct status before update,
         * and mark it as being configured */
-       while (success == 0) {
-               prev_status.u32 = tim->status.u32;
+       prev_status.u32 = __atomic_load_n(&tim->status.u32, __ATOMIC_RELAXED);
 
+       while (success == 0) {
                /* timer is running on another core
                 * or ready to run on local core, exit
                 */
@@ -258,9 +257,15 @@ timer_set_config_state(struct rte_timer *tim,
                 * mark it atomically as being configured */
                status.state = RTE_TIMER_CONFIG;
                status.owner = (int16_t)lcore_id;
-               success = rte_atomic32_cmpset(&tim->status.u32,
-                                             prev_status.u32,
-                                             status.u32);
+               /* CONFIG states are acting as locked states. If the
+                * timer is in CONFIG state, the state cannot be changed
+                * by other threads. So, we should use ACQUIRE here.
+                */
+               success = __atomic_compare_exchange_n(&tim->status.u32,
+                                             &prev_status.u32,
+                                             status.u32, 0,
+                                             __ATOMIC_ACQUIRE,
+                                             __ATOMIC_RELAXED);
        }
 
        ret_prev_status->u32 = prev_status.u32;
@@ -279,20 +284,27 @@ timer_set_running_state(struct rte_timer *tim)
 
        /* wait that the timer is in correct status before update,
         * and mark it as running */
-       while (success == 0) {
-               prev_status.u32 = tim->status.u32;
+       prev_status.u32 = __atomic_load_n(&tim->status.u32, __ATOMIC_RELAXED);
 
+       while (success == 0) {
                /* timer is not pending anymore */
                if (prev_status.state != RTE_TIMER_PENDING)
                        return -1;
 
-               /* here, we know that timer is stopped or pending,
-                * mark it atomically as being configured */
+               /* we know that the timer will be pending at this point
+                * mark it atomically as being running
+                */
                status.state = RTE_TIMER_RUNNING;
                status.owner = (int16_t)lcore_id;
-               success = rte_atomic32_cmpset(&tim->status.u32,
-                                             prev_status.u32,
-                                             status.u32);
+               /* RUNNING states are acting as locked states. If the
+                * timer is in RUNNING state, the state cannot be changed
+                * by other threads. So, we should use ACQUIRE here.
+                */
+               success = __atomic_compare_exchange_n(&tim->status.u32,
+                                             &prev_status.u32,
+                                             status.u32, 0,
+                                             __ATOMIC_ACQUIRE,
+                                             __ATOMIC_RELAXED);
        }
 
        return 0;
@@ -520,10 +532,12 @@ __rte_timer_reset(struct rte_timer *tim, uint64_t expire,
 
        /* update state: as we are in CONFIG state, only us can modify
         * the state so we don't need to use cmpset() here */
-       rte_wmb();
        status.state = RTE_TIMER_PENDING;
        status.owner = (int16_t)tim_lcore;
-       tim->status.u32 = status.u32;
+       /* The "RELEASE" ordering guarantees the memory operations above
+        * the status update are observed before the update by all threads
+        */
+       __atomic_store_n(&tim->status.u32, status.u32, __ATOMIC_RELEASE);
 
        if (tim_lcore != lcore_id || !local_is_locked)
                rte_spinlock_unlock(&priv_timer[tim_lcore].list_lock);
@@ -600,10 +614,12 @@ __rte_timer_stop(struct rte_timer *tim, int local_is_locked,
        }
 
        /* mark timer as stopped */
-       rte_wmb();
        status.state = RTE_TIMER_STOP;
        status.owner = RTE_TIMER_NO_OWNER;
-       tim->status.u32 = status.u32;
+       /* The "RELEASE" ordering guarantees the memory operations above
+        * the status update are observed before the update by all threads
+        */
+       __atomic_store_n(&tim->status.u32, status.u32, __ATOMIC_RELEASE);
 
        return 0;
 }
@@ -637,7 +653,8 @@ rte_timer_stop_sync(struct rte_timer *tim)
 int
 rte_timer_pending(struct rte_timer *tim)
 {
-       return tim->status.state == RTE_TIMER_PENDING;
+       return __atomic_load_n(&tim->status.state,
+                               __ATOMIC_RELAXED) == RTE_TIMER_PENDING;
 }
 
 /* must be called periodically, run all timer that expired */
@@ -739,8 +756,12 @@ __rte_timer_manage(struct rte_timer_data *timer_data)
                        /* remove from done list and mark timer as stopped */
                        status.state = RTE_TIMER_STOP;
                        status.owner = RTE_TIMER_NO_OWNER;
-                       rte_wmb();
-                       tim->status.u32 = status.u32;
+                       /* The "RELEASE" ordering guarantees the memory
+                        * operations above the status update are observed
+                        * before the update by all threads
+                        */
+                       __atomic_store_n(&tim->status.u32, status.u32,
+                               __ATOMIC_RELEASE);
                }
                else {
                        /* keep it in list and mark timer as pending */
@@ -748,8 +769,12 @@ __rte_timer_manage(struct rte_timer_data *timer_data)
                        status.state = RTE_TIMER_PENDING;
                        __TIMER_STAT_ADD(priv_timer, pending, 1);
                        status.owner = (int16_t)lcore_id;
-                       rte_wmb();
-                       tim->status.u32 = status.u32;
+                       /* The "RELEASE" ordering guarantees the memory
+                        * operations above the status update are observed
+                        * before the update by all threads
+                        */
+                       __atomic_store_n(&tim->status.u32, status.u32,
+                               __ATOMIC_RELEASE);
                        __rte_timer_reset(tim, tim->expire + tim->period,
                                tim->period, lcore_id, tim->f, tim->arg, 1,
                                timer_data);
@@ -919,8 +944,12 @@ rte_timer_alt_manage(uint32_t timer_data_id,
                        /* remove from done list and mark timer as stopped */
                        status.state = RTE_TIMER_STOP;
                        status.owner = RTE_TIMER_NO_OWNER;
-                       rte_wmb();
-                       tim->status.u32 = status.u32;
+                       /* The "RELEASE" ordering guarantees the memory
+                        * operations above the status update are observed
+                        * before the update by all threads
+                        */
+                       __atomic_store_n(&tim->status.u32, status.u32,
+                               __ATOMIC_RELEASE);
                } else {
                        /* keep it in list and mark timer as pending */
                        rte_spinlock_lock(
@@ -928,8 +957,12 @@ rte_timer_alt_manage(uint32_t timer_data_id,
                        status.state = RTE_TIMER_PENDING;
                        __TIMER_STAT_ADD(data->priv_timer, pending, 1);
                        status.owner = (int16_t)this_lcore;
-                       rte_wmb();
-                       tim->status.u32 = status.u32;
+                       /* The "RELEASE" ordering guarantees the memory
+                        * operations above the status update are observed
+                        * before the update by all threads
+                        */
+                       __atomic_store_n(&tim->status.u32, status.u32,
+                               __ATOMIC_RELEASE);
                        __rte_timer_reset(tim, tim->expire + tim->period,
                                tim->period, this_lcore, tim->f, tim->arg, 1,
                                data);