FAILED: patch "[PATCH] SUNRPC in case of backlog, hand free slots directly to" failed to apply to 4.19-stable tree

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From e877a88d1f069edced4160792f42c2a8e2dba942 Mon Sep 17 00:00:00 2001
From: NeilBrown <[email protected]>
Date: Mon, 17 May 2021 09:59:10 +1000
Subject: [PATCH] SUNRPC in case of backlog, hand free slots directly to
waiting task

If sunrpc.tcp_max_slot_table_entries is small and there are tasks on the backlog queue, then when a request completes it is freed and the first task on the queue is woken. The expectation is that it will wake and claim that request. However if it was a sync task and the waiting process was killed at just that moment, it will wake and NOT claim the request.

As long as TASK_CONGESTED remains set, requests can only be claimed by tasks woken from the backlog, and they are woken only as requests are freed, so when a task doesn't claim a request, no other task can ever get that request until TASK_CONGESTED is cleared. Each time this happens the number of available requests is decreased by one.

With a sufficiently high workload and sufficiently low setting of max_slot (16 in the case where this was seen), TASK_CONGESTED can remain set for an extended period, and the above scenario (of a process being killed just as its task was woken) can repeat until no requests can be allocated. Then traffic stops.

This patch addresses the problem by introducing a positive handover of a request from a completing task to a backlog task - the request is never freed when there is a backlog.

When a task is woken it might not already have a request attached in which case it is *not* freed (as with current code) but is initialised (if needed) and used. If it isn't used it will eventually be freed by rpc_exit_task(). xprt_release() is enhanced to be able to correctly release an uninitialised request.

index f555d335e910..42623d6b8f0e 100644
@@ -1677,13 +1677,6 @@ call_reserveresult(struct rpc_task *task)
 		return;
 	}
 
-	/*
-	 * Even though there was an error, we may have acquired
-	 * a request slot somehow.  Make sure not to leak it.
-	 */
-	if (task->tk_rqstp)
-		xprt_release(task);
-
 	switch (status) {
 	case -ENOMEM:
 		rpc_delay(task, HZ >> 2);