[PATCH 5/6] SUNRPC: add list of idle threads

[NeilBrown <neilb@suse.de>]

Rather than searching a list of threads to find an idle one, having a
list of idle threads allows an idle thread to be found immediately.

This adds some spin_lock calls which is not ideal, but as the hold-time
is tiny it is still faster than searching a list.  A future patch will
remove them using llist.h.  This involves some subtlety and so is left
to a separate patch.

This removes the need for the RQ_BUSY flag.  The rqst is "busy"
precisely when it is not on the "idle" list.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 include/linux/sunrpc/svc.h    | 25 ++++++++++++++++++++++++-
 include/trace/events/sunrpc.h |  1 -
 net/sunrpc/svc.c              | 13 ++++++++-----
 net/sunrpc/svc_xprt.c         | 15 +++++++++++----
 4 files changed, 43 insertions(+), 11 deletions(-)

diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index 1ac6f74781aa..8b93af92dd53 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -37,6 +37,7 @@ struct svc_pool {
 	struct list_head	sp_sockets;	/* pending sockets */
 	unsigned int		sp_nrthreads;	/* # of threads in pool */
 	struct list_head	sp_all_threads;	/* all server threads */
+	struct list_head	sp_idle_threads; /* idle server threads */
 
 	/* statistics on pool operation */
 	struct percpu_counter	sp_messages_arrived;
@@ -186,6 +187,7 @@ extern u32 svc_max_payload(const struct svc_rqst *rqstp);
  */
 struct svc_rqst {
 	struct list_head	rq_all;		/* all threads list */
+	struct list_head	rq_idle;	/* On the idle list */
 	struct rcu_head		rq_rcu_head;	/* for RCU deferred kfree */
 	struct svc_xprt *	rq_xprt;	/* transport ptr */
 
@@ -262,10 +264,31 @@ enum {
 	RQ_SPLICE_OK,		/* turned off in gss privacy to prevent
 				 * encrypting page cache pages */
 	RQ_VICTIM,		/* Have agreed to shut down */
-	RQ_BUSY,		/* request is busy */
 	RQ_DATA,		/* request has data */
 };
 
+/**
+ * svc_thread_set_busy - mark a thread as busy
+ * @rqstp: the thread which is now busy
+ *
+ * If rq_idle is "empty", the thread must be busy.
+ */
+static inline void svc_thread_set_busy(struct svc_rqst *rqstp)
+{
+	INIT_LIST_HEAD(&rqstp->rq_idle);
+}
+
+/**
+ * svc_thread_busy - check if a thread as busy
+ * @rqstp: the thread which might be busy
+ *
+ * If rq_idle is "empty", the thread must be busy.
+ */
+static inline bool svc_thread_busy(struct svc_rqst *rqstp)
+{
+	return list_empty(&rqstp->rq_idle);
+}
+
 #define SVC_NET(rqst) (rqst->rq_xprt ? rqst->rq_xprt->xpt_net : rqst->rq_bc_net)
 
 /*
diff --git a/include/trace/events/sunrpc.h b/include/trace/events/sunrpc.h
index 6beb38c1dcb5..337c90787fb1 100644
--- a/include/trace/events/sunrpc.h
+++ b/include/trace/events/sunrpc.h
@@ -1677,7 +1677,6 @@ DEFINE_SVCXDRBUF_EVENT(sendto);
 	svc_rqst_flag(DROPME)						\
 	svc_rqst_flag(SPLICE_OK)					\
 	svc_rqst_flag(VICTIM)						\
-	svc_rqst_flag(BUSY)						\
 	svc_rqst_flag_end(DATA)
 
 #undef svc_rqst_flag
diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index 1233d72714b9..dce433dea1bd 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -641,7 +641,7 @@ svc_rqst_alloc(struct svc_serv *serv, struct svc_pool *pool, int node)
 
 	folio_batch_init(&rqstp->rq_fbatch);
 
-	__set_bit(RQ_BUSY, &rqstp->rq_flags);
+	svc_thread_set_busy(rqstp);
 	rqstp->rq_server = serv;
 	rqstp->rq_pool = pool;
 
@@ -702,10 +702,13 @@ void svc_pool_wake_idle_thread(struct svc_pool *pool)
 	struct svc_rqst	*rqstp;
 
 	rcu_read_lock();
-	list_for_each_entry_rcu(rqstp, &pool->sp_all_threads, rq_all) {
-		if (test_and_set_bit(RQ_BUSY, &rqstp->rq_flags))
-			continue;
-
+	spin_lock_bh(&pool->sp_lock);
+	rqstp = list_first_entry_or_null(&pool->sp_idle_threads,
+					 struct svc_rqst, rq_idle);
+	if (rqstp)
+		list_del_init(&rqstp->rq_idle);
+	spin_unlock_bh(&pool->sp_lock);
+	if (rqstp) {
 		WRITE_ONCE(rqstp->rq_qtime, ktime_get());
 		wake_up_process(rqstp->rq_task);
 		rcu_read_unlock();
diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index 0a300ae6a7ed..e44efcc21b63 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -737,8 +737,9 @@ static void svc_rqst_wait_for_work(struct svc_rqst *rqstp)
 		set_current_state(TASK_IDLE);
 		smp_mb__before_atomic();
 		clear_bit(SP_CONGESTED, &pool->sp_flags);
-		clear_bit(RQ_BUSY, &rqstp->rq_flags);
-		smp_mb__after_atomic();
+		spin_lock_bh(&pool->sp_lock);
+		list_add(&rqstp->rq_idle, &pool->sp_idle_threads);
+		spin_unlock_bh(&pool->sp_lock);
 
 		/* Need to check should_sleep() again after
 		 * setting task state in case a wakeup happened
@@ -751,8 +752,14 @@ static void svc_rqst_wait_for_work(struct svc_rqst *rqstp)
 			cond_resched();
 		}
 
-		set_bit(RQ_BUSY, &rqstp->rq_flags);
-		smp_mb__after_atomic();
+		/* We *must* be removed from the list before we can continue.
+		 * If we were woken, this is already done
+		 */
+		if (!svc_thread_busy(rqstp)) {
+			spin_lock_bh(&pool->sp_lock);
+			list_del_init(&rqstp->rq_idle);
+			spin_unlock_bh(&pool->sp_lock);
+		}
 	} else
 		cond_resched();
 	try_to_freeze();
-- 
2.40.1