[ovs-dev] [urcu-fixes 4/4] ovs-rcu: Log a helpful warning when ovsrcu_synchronize() stalls.

[Ben Pfaff]

This made it easier for me to find a thread that was causing stalls.

Signed-off-by: Ben Pfaff <b...@nicira.com>
---
 lib/ovs-rcu.c    |   24 +++++++++++++++++++++++-
 lib/ovs-thread.c |    4 +++-
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/lib/ovs-rcu.c b/lib/ovs-rcu.c
index c1ac61a..8a12564 100644
--- a/lib/ovs-rcu.c
+++ b/lib/ovs-rcu.c
@@ -21,6 +21,10 @@
 #include "ovs-thread.h"
 #include "poll-loop.h"
 #include "seq.h"
+#include "timeval.h"
+#include "vlog.h"
+
+VLOG_DEFINE_THIS_MODULE(ovs_rcu);
 
 struct ovsrcu_cb {
     void (*function)(void *aux);
@@ -34,11 +38,12 @@ struct ovsrcu_cbset {
 };
 
 struct ovsrcu_perthread {
-    struct list list_node;           /* In global list. */
+    struct list list_node;      /* In global list. */
 
     struct ovs_mutex mutex;
     uint64_t seqno;
     struct ovsrcu_cbset *cbset;
+    char name[16];              /* This thread's name. */
 };
 
 static struct seq *global_seqno;
@@ -70,6 +75,8 @@ ovsrcu_perthread_get(void)
         ovs_mutex_init(&perthread->mutex);
         perthread->seqno = seq_read(global_seqno);
         perthread->cbset = NULL;
+        ovs_strlcpy(perthread->name, get_subprogram_name(),
+                    sizeof perthread->name);
 
         ovs_mutex_lock(&ovsrcu_threads_mutex);
         list_push_back(&ovsrcu_threads, &perthread->list_node);
@@ -144,7 +151,9 @@ ovsrcu_is_quiescent(void)
 static void
 ovsrcu_synchronize(void)
 {
+    unsigned int warning_threshold = 1000;
     uint64_t target_seqno;
+    long long int start;
 
     if (single_threaded()) {
         return;
@@ -152,15 +161,20 @@ ovsrcu_synchronize(void)
 
     target_seqno = seq_read(global_seqno);
     ovsrcu_quiesce_start();
+    start = time_msec();
 
     for (;;) {
         uint64_t cur_seqno = seq_read(global_seqno);
         struct ovsrcu_perthread *perthread;
+        char stalled_thread[16];
+        unsigned int elapsed;
         bool done = true;
 
         ovs_mutex_lock(&ovsrcu_threads_mutex);
         LIST_FOR_EACH (perthread, list_node, &ovsrcu_threads) {
             if (perthread->seqno <= target_seqno) {
+                ovs_strlcpy(stalled_thread, perthread->name,
+                            sizeof stalled_thread);
                 done = false;
                 break;
             }
@@ -171,6 +185,14 @@ ovsrcu_synchronize(void)
             break;
         }
 
+        elapsed = time_msec() - start;
+        if (elapsed >= warning_threshold) {
+            VLOG_WARN("blocked %u ms waiting for %s to quiesce",
+                      elapsed, stalled_thread);
+            warning_threshold *= 2;
+        }
+        poll_timer_wait_until(start + warning_threshold);
+
         seq_wait(global_seqno, cur_seqno);
         poll_block();
     }
diff --git a/lib/ovs-thread.c b/lib/ovs-thread.c
index d835b39..19edf8f 100644
--- a/lib/ovs-thread.c
+++ b/lib/ovs-thread.c
@@ -274,9 +274,11 @@ ovsthread_wrapper(void *aux_)
     aux = *auxp;
     free(auxp);
 
+    /* The order of the following calls is important, because
+     * ovsrcu_quiesce_end() saves a copy of the thread name. */
     set_subprogram_name("%s%u", aux.name, id);
-
     ovsrcu_quiesce_end();
+
     return aux.start(aux.arg);
 }
 
-- 
1.7.10.4

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