sunrpc: convert to lockless lookup of queued server threads

Testing has shown that the pool->sp_lock can be a bottleneck on a busy
server. Every time data is received on a socket, the server must take
that lock in order to dequeue a thread from the sp_threads list.

Address this problem by eliminating the sp_threads list (which contains
threads that are currently idle) and replacing it with a RQ_BUSY flag in
svc_rqst. This allows us to walk the sp_all_threads list under the
rcu_read_lock and find a suitable thread for the xprt by doing a
test_and_set_bit.

Note that we do still have a potential atomicity problem however with
this approach.  We don't want svc_xprt_do_enqueue to set the
rqst->rq_xprt pointer unless a test_and_set_bit of RQ_BUSY returned
zero (which indicates that the thread was idle). But, by the time we
check that, the bit could be flipped by a waking thread.

To address this, we acquire a new per-rqst spinlock (rq_lock) and take
that before doing the test_and_set_bit. If that returns false, then we
can set rq_xprt and drop the spinlock. Then, when the thread wakes up,
it must set the bit under the same spinlock and can trust that if it was
already set then the rq_xprt is also properly set.

With this scheme, the case where we have an idle thread no longer needs
to take the highly contended pool->sp_lock at all, and that removes the
bottleneck.

That still leaves one issue: What of the case where we walk the whole
sp_all_threads list and don't find an idle thread? Because the search is
lockess, it's possible for the queueing to race with a thread that is
going to sleep. To address that, we queue the xprt and then search again.

If we find an idle thread at that point, we can't attach the xprt to it
directly since that might race with a different thread waking up and
finding it.  All we can do is wake the idle thread back up and let it
attempt to find the now-queued xprt.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Tested-by: Chris Worley <chris.worley@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

2 files changed, 128 insertions, 100 deletions

diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index b90d1bca4349..91eaef1844c8 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -476,7 +476,6 @@ __svc_create(struct svc_program *prog, unsigned int bufsize, int npools,
                                 i, serv->sv_name);
 
                 pool->sp_id = i;
-                INIT_LIST_HEAD(&pool->sp_threads);
                 INIT_LIST_HEAD(&pool->sp_sockets);
                 INIT_LIST_HEAD(&pool->sp_all_threads);
                 spin_lock_init(&pool->sp_lock);
@@ -614,12 +613,14 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
                 goto out_enomem;
 
         serv->sv_nrthreads++;
+        __set_bit(RQ_BUSY, &rqstp->rq_flags);
+        spin_lock_init(&rqstp->rq_lock);
+        rqstp->rq_server = serv;
+        rqstp->rq_pool = pool;
         spin_lock_bh(&pool->sp_lock);
         pool->sp_nrthreads++;
         list_add_rcu(&rqstp->rq_all, &pool->sp_all_threads);
         spin_unlock_bh(&pool->sp_lock);
-        rqstp->rq_server = serv;
-        rqstp->rq_pool = pool;
 
         rqstp->rq_argp = kmalloc_node(serv->sv_xdrsize, GFP_KERNEL, node);
         if (!rqstp->rq_argp)
diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index 579ff2249562..ed90d955f733 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -310,25 +310,6 @@ char *svc_print_addr(struct svc_rqst *rqstp, char *buf, size_t len)
 }
 EXPORT_SYMBOL_GPL(svc_print_addr);
 
-/*
- * Queue up an idle server thread.  Must have pool->sp_lock held.
- * Note: this is really a stack rather than a queue, so that we only
- * use as many different threads as we need, and the rest don't pollute
- * the cache.
- */
-static void svc_thread_enqueue(struct svc_pool *pool, struct svc_rqst *rqstp)
-{
-        list_add(&rqstp->rq_list, &pool->sp_threads);
-}
-
-/*
- * Dequeue an nfsd thread.  Must have pool->sp_lock held.
- */
-static void svc_thread_dequeue(struct svc_pool *pool, struct svc_rqst *rqstp)
-{
-        list_del(&rqstp->rq_list);
-}
-
 static bool svc_xprt_has_something_to_do(struct svc_xprt *xprt)
 {
         if (xprt->xpt_flags & ((1<<XPT_CONN)|(1<<XPT_CLOSE)))
@@ -343,6 +324,7 @@ static void svc_xprt_do_enqueue(struct svc_xprt *xprt)
         struct svc_pool *pool;
         struct svc_rqst *rqstp;
         int cpu;
+        bool queued = false;
 
         if (!svc_xprt_has_something_to_do(xprt))
                 return;
@@ -360,37 +342,60 @@ static void svc_xprt_do_enqueue(struct svc_xprt *xprt)
 
         cpu = get_cpu();
         pool = svc_pool_for_cpu(xprt->xpt_server, cpu);
-        spin_lock_bh(&pool->sp_lock);
 
         atomic_long_inc(&pool->sp_stats.packets);
 
-        if (!list_empty(&pool->sp_threads)) {
-                rqstp = list_entry(pool->sp_threads.next,
-                                   struct svc_rqst,
-                                   rq_list);
-                dprintk("svc: transport %p served by daemon %p\n",
-                        xprt, rqstp);
-                svc_thread_dequeue(pool, rqstp);
-                if (rqstp->rq_xprt)
-                        printk(KERN_ERR
-                                "svc_xprt_enqueue: server %p, rq_xprt=%p!\n",
-                                rqstp, rqstp->rq_xprt);
-                /* Note the order of the following 3 lines:
-                 * We want to assign xprt to rqstp->rq_xprt only _after_
-                 * we've woken up the process, so that we don't race with
-                 * the lockless check in svc_get_next_xprt().
+redo_search:
+        /* find a thread for this xprt */
+        rcu_read_lock();
+        list_for_each_entry_rcu(rqstp, &pool->sp_all_threads, rq_all) {
+                /* Do a lockless check first */
+                if (test_bit(RQ_BUSY, &rqstp->rq_flags))
+                        continue;
+
+                /*
+                 * Once the xprt has been queued, it can only be dequeued by
+                 * the task that intends to service it. All we can do at that
+                 * point is to try to wake this thread back up so that it can
+                 * do so.
                  */
-                svc_xprt_get(xprt);
-                wake_up_process(rqstp->rq_task);
-                rqstp->rq_xprt = xprt;
+                if (!queued) {
+                        spin_lock_bh(&rqstp->rq_lock);
+                        if (test_and_set_bit(RQ_BUSY, &rqstp->rq_flags)) {
+                                /* already busy, move on... */
+                                spin_unlock_bh(&rqstp->rq_lock);
+                                continue;
+                        }
+
+                        /* this one will do */
+                        rqstp->rq_xprt = xprt;
+                        svc_xprt_get(xprt);
+                        spin_unlock_bh(&rqstp->rq_lock);
+                }
+                rcu_read_unlock();
+
                 atomic_long_inc(&pool->sp_stats.threads_woken);
-        } else {
+                wake_up_process(rqstp->rq_task);
+                put_cpu();
+                return;
+        }
+        rcu_read_unlock();
+
+        /*
+         * We didn't find an idle thread to use, so we need to queue the xprt.
+         * Do so and then search again. If we find one, we can't hook this one
+         * up to it directly but we can wake the thread up in the hopes that it
+         * will pick it up once it searches for a xprt to service.
+         */
+        if (!queued) {
+                queued = true;
                 dprintk("svc: transport %p put into queue\n", xprt);
+                spin_lock_bh(&pool->sp_lock);
                 list_add_tail(&xprt->xpt_ready, &pool->sp_sockets);
                 pool->sp_stats.sockets_queued++;
+                spin_unlock_bh(&pool->sp_lock);
+                goto redo_search;
         }
-
-        spin_unlock_bh(&pool->sp_lock);
         put_cpu();
 }
 
@@ -408,21 +413,26 @@ void svc_xprt_enqueue(struct svc_xprt *xprt)
 EXPORT_SYMBOL_GPL(svc_xprt_enqueue);
 
 /*
- * Dequeue the first transport.  Must be called with the pool->sp_lock held.
+ * Dequeue the first transport, if there is one.
  */
 static struct svc_xprt *svc_xprt_dequeue(struct svc_pool *pool)
 {
-        struct svc_xprt *xprt;
+        struct svc_xprt *xprt = NULL;
 
         if (list_empty(&pool->sp_sockets))
                 return NULL;
 
-        xprt = list_entry(pool->sp_sockets.next,
-                          struct svc_xprt, xpt_ready);
-        list_del_init(&xprt->xpt_ready);
+        spin_lock_bh(&pool->sp_lock);
+        if (likely(!list_empty(&pool->sp_sockets))) {
+                xprt = list_first_entry(&pool->sp_sockets,
+                                        struct svc_xprt, xpt_ready);
+                list_del_init(&xprt->xpt_ready);
+                svc_xprt_get(xprt);
 
-        dprintk("svc: transport %p dequeued, inuse=%d\n",
-                xprt, atomic_read(&xprt->xpt_ref.refcount));
+                dprintk("svc: transport %p dequeued, inuse=%d\n",
+                        xprt, atomic_read(&xprt->xpt_ref.refcount));
+        }
+        spin_unlock_bh(&pool->sp_lock);
 
         return xprt;
 }
@@ -497,16 +507,21 @@ void svc_wake_up(struct svc_serv *serv)
 
         pool = &serv->sv_pools[0];
 
-        spin_lock_bh(&pool->sp_lock);
-        if (!list_empty(&pool->sp_threads)) {
-                rqstp = list_entry(pool->sp_threads.next,
-                                   struct svc_rqst,
-                                   rq_list);
+        rcu_read_lock();
+        list_for_each_entry_rcu(rqstp, &pool->sp_all_threads, rq_all) {
+                /* skip any that aren't queued */
+                if (test_bit(RQ_BUSY, &rqstp->rq_flags))
+                        continue;
+                rcu_read_unlock();
                 dprintk("svc: daemon %p woken up.\n", rqstp);
                 wake_up_process(rqstp->rq_task);
-        } else
-                set_bit(SP_TASK_PENDING, &pool->sp_flags);
-        spin_unlock_bh(&pool->sp_lock);
+                return;
+        }
+        rcu_read_unlock();
+
+        /* No free entries available */
+        set_bit(SP_TASK_PENDING, &pool->sp_flags);
+        smp_wmb();
 }
 EXPORT_SYMBOL_GPL(svc_wake_up);
 
@@ -617,22 +632,47 @@ static int svc_alloc_arg(struct svc_rqst *rqstp)
         return 0;
 }
 
+static bool
+rqst_should_sleep(struct svc_rqst *rqstp)
+{
+        struct svc_pool         *pool = rqstp->rq_pool;
+
+        /* did someone call svc_wake_up? */
+        if (test_and_clear_bit(SP_TASK_PENDING, &pool->sp_flags))
+                return false;
+
+        /* was a socket queued? */
+        if (!list_empty(&pool->sp_sockets))
+                return false;
+
+        /* are we shutting down? */
+        if (signalled() || kthread_should_stop())
+                return false;
+
+        /* are we freezing? */
+        if (freezing(current))
+                return false;
+
+        return true;
+}
+
 static struct svc_xprt *svc_get_next_xprt(struct svc_rqst *rqstp, long timeout)
 {
         struct svc_xprt *xprt;
         struct svc_pool         *pool = rqstp->rq_pool;
         long                    time_left = 0;
 
+        /* rq_xprt should be clear on entry */
+        WARN_ON_ONCE(rqstp->rq_xprt);
+
         /* Normally we will wait up to 5 seconds for any required
          * cache information to be provided.
          */
         rqstp->rq_chandle.thread_wait = 5*HZ;
 
-        spin_lock_bh(&pool->sp_lock);
         xprt = svc_xprt_dequeue(pool);
         if (xprt) {
                 rqstp->rq_xprt = xprt;
-                svc_xprt_get(xprt);
 
                 /* As there is a shortage of threads and this request
                  * had to be queued, don't allow the thread to wait so
@@ -640,51 +680,38 @@ static struct svc_xprt *svc_get_next_xprt(struct svc_rqst *rqstp, long timeout)
                  */
                 rqstp->rq_chandle.thread_wait = 1*HZ;
                 clear_bit(SP_TASK_PENDING, &pool->sp_flags);
-        } else {
-                if (test_and_clear_bit(SP_TASK_PENDING, &pool->sp_flags)) {
-                        xprt = ERR_PTR(-EAGAIN);
-                        goto out;
-                }
-                /*
-                 * We have to be able to interrupt this wait
-                 * to bring down the daemons ...
-                 */
-                set_current_state(TASK_INTERRUPTIBLE);
+                return xprt;
+        }
 
-                /* No data pending. Go to sleep */
-                svc_thread_enqueue(pool, rqstp);
-                spin_unlock_bh(&pool->sp_lock);
+        /*
+         * We have to be able to interrupt this wait
+         * to bring down the daemons ...
+         */
+        set_current_state(TASK_INTERRUPTIBLE);
+        clear_bit(RQ_BUSY, &rqstp->rq_flags);
+        smp_mb();
+
+        if (likely(rqst_should_sleep(rqstp)))
+                time_left = schedule_timeout(timeout);
+        else
+                __set_current_state(TASK_RUNNING);
 
-                if (!(signalled() || kthread_should_stop())) {
-                        time_left = schedule_timeout(timeout);
-                        __set_current_state(TASK_RUNNING);
+        try_to_freeze();
 
-                        try_to_freeze();
+        spin_lock_bh(&rqstp->rq_lock);
+        set_bit(RQ_BUSY, &rqstp->rq_flags);
+        spin_unlock_bh(&rqstp->rq_lock);
 
-                        xprt = rqstp->rq_xprt;
-                        if (xprt != NULL)
-                                return xprt;
-                } else
-                        __set_current_state(TASK_RUNNING);
+        xprt = rqstp->rq_xprt;
+        if (xprt != NULL)
+                return xprt;
 
-                spin_lock_bh(&pool->sp_lock);
-                if (!time_left)
-                        atomic_long_inc(&pool->sp_stats.threads_timedout);
+        if (!time_left)
+                atomic_long_inc(&pool->sp_stats.threads_timedout);
 
-                xprt = rqstp->rq_xprt;
-                if (!xprt) {
-                        svc_thread_dequeue(pool, rqstp);
-                        spin_unlock_bh(&pool->sp_lock);
-                        dprintk("svc: server %p, no data yet\n", rqstp);
-                        if (signalled() || kthread_should_stop())
-                                return ERR_PTR(-EINTR);
-                        else
-                                return ERR_PTR(-EAGAIN);
-                }
-        }
-out:
-        spin_unlock_bh(&pool->sp_lock);
-        return xprt;
+        if (signalled() || kthread_should_stop())
+                return ERR_PTR(-EINTR);
+        return ERR_PTR(-EAGAIN);
 }
 
 static void svc_add_new_temp_xprt(struct svc_serv *serv, struct svc_xprt *newxpt)