[PATCH RFC 2/8] SUNRPC: Execute initial async RPC states in caller's context

[Chuck Lever <cel@kernel.org>]

Each async RPC task is dispatched to the rpciod workqueue before its
first FSM state runs, though the initial states (call_start through
call_allocate) typically complete without blocking. Under high
concurrency this adds a workqueue enqueue, dequeue, and context
switch per RPC and contributes to rpciod pool lock contention.

Have rpc_execute() call __rpc_execute() directly for async tasks as
well. The initial states then run in the caller's context, and
__rpc_execute() returns to the caller when the task first sleeps on
a wait queue. Later wake-ups still dispatch to rpciod via
rpc_make_runnable().

Commit d6a1ed08c6ac ("SUNRPC: Reduce asynchronous RPC task stack
usage") moved async dispatch onto rpciod to bound caller stack
depth. Inline execution preserves that bound because __rpc_execute()
still runs each state from the same stack frame.

Signed-off-by: Chuck Lever <cel@kernel.org>
---
 net/sunrpc/sched.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
index 016f16ca5779..9b2800f6bfc7 100644
--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -1003,8 +1003,9 @@ static void __rpc_execute(struct rpc_task *task)
 	current_restore_flags(pflags, PF_MEMALLOC);
 }
 
-/*
- * User-visible entry point to the scheduler.
+/**
+ * rpc_execute - Consumer entry point to the RPC scheduler
+ * @task: RPC task to be scheduled
  *
  * This may be called recursively if e.g. an async NFS task updates
  * the attributes and finds that dirty pages must be flushed.
@@ -1014,15 +1015,12 @@ static void __rpc_execute(struct rpc_task *task)
  */
 void rpc_execute(struct rpc_task *task)
 {
-	bool is_async = RPC_IS_ASYNC(task);
+	unsigned int pflags = memalloc_nofs_save();
 
 	rpc_set_active(task);
-	rpc_make_runnable(rpciod_workqueue, task);
-	if (!is_async) {
-		unsigned int pflags = memalloc_nofs_save();
-		__rpc_execute(task);
-		memalloc_nofs_restore(pflags);
-	}
+	rpc_test_and_set_running(task);
+	__rpc_execute(task);
+	memalloc_nofs_restore(pflags);
 }
 
 static void rpc_async_schedule(struct work_struct *work)

-- 
2.55.0