From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Mon, 29 Jun 2015 22:45:07 +0000 (-0400)
Subject: Fix: call_rcu_thread() affinity failure
X-Git-Tag: v0.7.15~5
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=5a631bd917d9173deda3fa06cf2ec22ab9db3194;p=userspace-rcu.git

Fix: call_rcu_thread() affinity failure

Make call_rcu_thread() affine itself more persistently

Currently, URCU simply fails if a call_rcu_thread() fails to affine
itself. This is problematic when execution is constrained by cgroup
and hotunplugged CPUs. This commit therefore makes call_rcu_thread()
retry setting its affinity every 256 grace periods, but only if it
detects that it migrated to a different CPU. Since sched_getcpu() is
cheap on many architectures, this check is less costly than going
through a system call.

Reported-by: Michael Jeanson <mjeanson@efficios.com>
Suggested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/urcu-call-rcu-impl.h b/urcu-call-rcu-impl.h
index 51ee91f..e70789a 100644
--- a/urcu-call-rcu-impl.h
+++ b/urcu-call-rcu-impl.h
@@ -43,6 +43,9 @@
 #include "urcu/tls-compat.h"
 #include "urcu-die.h"
 
+#define SET_AFFINITY_CHECK_PERIOD		(1U << 8)	/* 256 */
+#define SET_AFFINITY_CHECK_PERIOD_MASK		(SET_AFFINITY_CHECK_PERIOD - 1)
+
 /* Data structure that identifies a call_rcu thread. */
 
 struct call_rcu_data {
@@ -52,6 +55,7 @@ struct call_rcu_data {
 	unsigned long qlen; /* maintained for debugging. */
 	pthread_t tid;
 	int cpu_affinity;
+	unsigned long gp_count;
 	struct cds_list_head list;
 } __attribute__((aligned(CAA_CACHE_LINE_SIZE)));
 
@@ -182,22 +186,42 @@ static void call_rcu_unlock(pthread_mutex_t *pmp)
 		urcu_die(ret);
 }
 
+/*
+ * Periodically retry setting CPU affinity if we migrate.
+ * Losing affinity can be caused by CPU hotunplug/hotplug, or by
+ * cpuset(7).
+ */
 #if HAVE_SCHED_SETAFFINITY
 static
 int set_thread_cpu_affinity(struct call_rcu_data *crdp)
 {
 	cpu_set_t mask;
+	int ret;
 
 	if (crdp->cpu_affinity < 0)
 		return 0;
+	if (++crdp->gp_count & SET_AFFINITY_CHECK_PERIOD_MASK)
+		return 0;
+	if (urcu_sched_getcpu() == crdp->cpu_affinity)
+		return 0;
 
 	CPU_ZERO(&mask);
 	CPU_SET(crdp->cpu_affinity, &mask);
 #if SCHED_SETAFFINITY_ARGS == 2
-	return sched_setaffinity(0, &mask);
+	ret = sched_setaffinity(0, &mask);
 #else
-	return sched_setaffinity(0, sizeof(mask), &mask);
+	ret = sched_setaffinity(0, sizeof(mask), &mask);
 #endif
+	/*
+	 * EINVAL is fine: can be caused by hotunplugged CPUs, or by
+	 * cpuset(7). This is why we should always retry if we detect
+	 * migration.
+	 */
+	if (ret && errno == EINVAL) {
+		ret = 0;
+		errno = 0;
+	}
+	return ret;
 }
 #else
 static
@@ -237,10 +261,8 @@ static void *call_rcu_thread(void *arg)
 	struct call_rcu_data *crdp = (struct call_rcu_data *)arg;
 	struct rcu_head *rhp;
 	int rt = !!(uatomic_read(&crdp->flags) & URCU_CALL_RCU_RT);
-	int ret;
 
-	ret = set_thread_cpu_affinity(crdp);
-	if (ret)
+	if (set_thread_cpu_affinity(crdp))
 		urcu_die(errno);
 
 	/*
@@ -255,6 +277,9 @@ static void *call_rcu_thread(void *arg)
 		cmm_smp_mb();
 	}
 	for (;;) {
+		if (set_thread_cpu_affinity(crdp))
+			urcu_die(errno);
+
 		if (uatomic_read(&crdp->flags) & URCU_CALL_RCU_PAUSE) {
 			/*
 			 * Pause requested. Become quiescent: remove
@@ -352,6 +377,7 @@ static void call_rcu_data_init(struct call_rcu_data **crdpp,
 	crdp->flags = flags;
 	cds_list_add(&crdp->list, &call_rcu_data_list);
 	crdp->cpu_affinity = cpu_affinity;
+	crdp->gp_count = 0;
 	cmm_smp_mb();  /* Structure initialized before pointer is planted. */
 	*crdpp = crdp;
 	ret = pthread_create(&crdp->tid, NULL, call_rcu_thread, crdp);