[dpdk-dev] [PATCH] Do RCU synchronization at fixed interval in PMD main loop.

[Nitin Katiyar <nitin.katiyar@ericsson.com>]

Each PMD updates the global sequence number for RCU synchronization
purpose with other OVS threads. This is done at every 1025th iteration
in PMD main loop.

If the PMD thread is responsible for polling large number of queues
that are carrying traffic, it spends a lot of time processing packets
and this results in significant delay in performing the housekeeping
activities.

If the OVS main thread is waiting to synchronize with the PMD threads
and if those threads delay performing housekeeping activities for
more than 3 sec then LACP processing will be impacted and it will lead
to LACP flaps. Similarly, other controls protocols run by OVS main
thread are impacted.

For e.g. a PMD thread polling 200 ports/queues with average of 1600
processing cycles per packet with batch size of 32 may take 10240000
(200 * 1600 * 32) cycles per iteration. In system with 2.0 GHz CPU
it means more than 5 ms per iteration. So, for 1024 iterations to
complete it would be more than 5 seconds.

This gets worse when there are PMD threads which are less loaded.
It reduces possibility of getting mutex lock in ovsrcu_try_quiesce()
by heavily loaded PMD and next attempt to quiesce would be after 1024
iterations.

With this patch, PMD RCU synchronization will be performed after fixed
interval instead after a fixed number of iterations. This will ensure
that even if the packet processing load is high the RCU synchronization
will not be delayed long.

Signed-off-by: Anju Thomas <anju.thomas@ericsson.com>
Signed-off-by: Nitin Katiyar <nitin.katiyar@ericsson.com>
---
 lib/dpif-netdev-perf.c | 16 ----------------
 lib/dpif-netdev-perf.h | 17 +++++++++++++++++
 lib/dpif-netdev.c      | 27 +++++++++++++++++++++++++++
 3 files changed, 44 insertions(+), 16 deletions(-)

diff --git a/lib/dpif-netdev-perf.c b/lib/dpif-netdev-perf.c
index e7ed49e..c888e5d 100644
--- a/lib/dpif-netdev-perf.c
+++ b/lib/dpif-netdev-perf.c
@@ -43,22 +43,6 @@ uint64_t iter_cycle_threshold;
 
 static struct vlog_rate_limit latency_rl = VLOG_RATE_LIMIT_INIT(600, 600);
 
-#ifdef DPDK_NETDEV
-static uint64_t
-get_tsc_hz(void)
-{
-    return rte_get_tsc_hz();
-}
-#else
-/* This function is only invoked from PMD threads which depend on DPDK.
- * A dummy function is sufficient when building without DPDK_NETDEV. */
-static uint64_t
-get_tsc_hz(void)
-{
-    return 1;
-}
-#endif
-
 /* Histogram functions. */
 
 static void
diff --git a/lib/dpif-netdev-perf.h b/lib/dpif-netdev-perf.h
index 244813f..3f2ee1c 100644
--- a/lib/dpif-netdev-perf.h
+++ b/lib/dpif-netdev-perf.h
@@ -187,6 +187,23 @@ struct pmd_perf_stats {
     char *log_reason;
 };
 
+#ifdef DPDK_NETDEV
+static inline uint64_t
+get_tsc_hz(void)
+{
+    return rte_get_tsc_hz();
+}
+#else
+/* This function is only invoked from PMD threads which depend on DPDK.
+ * A dummy function is sufficient when building without DPDK_NETDEV. */
+static inline uint64_t
+get_tsc_hz(void)
+{
+    return 1;
+}
+#endif
+
+
 #ifdef __linux__
 static inline uint64_t
 rdtsc_syscall(struct pmd_perf_stats *s)
diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
index d0a1c58..c3d6835 100644
--- a/lib/dpif-netdev.c
+++ b/lib/dpif-netdev.c
@@ -751,6 +751,9 @@ struct dp_netdev_pmd_thread {
 
     /* Set to true if the pmd thread needs to be reloaded. */
     bool need_reload;
+
+    /* Last time (in tsc) when PMD was last quiesced */
+    uint64_t last_rcu_quiesced;
 };
 
 /* Interface to netdev-based datapath. */
@@ -5445,6 +5448,7 @@ pmd_thread_main(void *f_)
     int poll_cnt;
     int i;
     int process_packets = 0;
+    uint64_t rcu_quiesce_interval = 0;
 
     poll_list = NULL;
 
@@ -5486,6 +5490,13 @@ reload:
     pmd->intrvl_tsc_prev = 0;
     atomic_store_relaxed(&pmd->intrvl_cycles, 0);
     cycles_counter_update(s);
+
+    if (get_tsc_hz() > 1) {
+        /* Calculate ~10 ms interval. */
+        rcu_quiesce_interval = get_tsc_hz() / 100;
+        pmd->last_rcu_quiesced = cycles_counter_get(s);
+    }
+
     /* Protect pmd stats from external clearing while polling. */
     ovs_mutex_lock(&pmd->perf_stats.stats_mutex);
     for (;;) {
@@ -5493,6 +5504,19 @@ reload:
 
         pmd_perf_start_iteration(s);
 
+        /* Do RCU synchronization at fixed interval instead of doing it
+         * at fixed number of iterations. This ensures that synchronization
+         * would not be delayed long even at high load of packet
+         * processing. */
+
+        if (rcu_quiesce_interval &&
+            ((cycles_counter_get(s) - pmd->last_rcu_quiesced) >
+             rcu_quiesce_interval)) {
+            if (!ovsrcu_try_quiesce()) {
+                pmd->last_rcu_quiesced = cycles_counter_get(s);
+            }
+        }
+
         for (i = 0; i < poll_cnt; i++) {
 
             if (!poll_list[i].rxq_enabled) {
@@ -5527,6 +5551,9 @@ reload:
             dp_netdev_pmd_try_optimize(pmd, poll_list, poll_cnt);
             if (!ovsrcu_try_quiesce()) {
                 emc_cache_slow_sweep(&((pmd->flow_cache).emc_cache));
+                if (rcu_quiesce_interval) {
+                    pmd->last_rcu_quiesced = cycles_counter_get(s);
+                }
             }
 
             for (i = 0; i < poll_cnt; i++) {
-- 
1.9.1