SUNRPC: Add trace event when alloc_pages_bulk() makes no progress

This is an operational low memory situation that needs to be
flagged. The new tracepoint records a timestamp and the nfsd thread
that failed to allocate pages.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

include/trace/events/sunrpc.h

@@ -1859,6 +1859,24 @@ TRACE_EVENT(svc_wake_up,
 	TP_printk("pid=%d", __entry->pid)
 );
 
+TRACE_EVENT(svc_alloc_arg_err,
+	TP_PROTO(
+		unsigned int pages
+	),
+
+	TP_ARGS(pages),
+
+	TP_STRUCT__entry(
+		__field(unsigned int, pages)
+	),
+
+	TP_fast_assign(
+		__entry->pages = pages;
+	),
+
+	TP_printk("pages=%u", __entry->pages)
+);
+
 TRACE_EVENT(svc_handle_xprt,
 	TP_PROTO(struct svc_xprt *xprt, int len),
 

net/sunrpc/svc_xprt.c

@@ -687,6 +687,7 @@ static int svc_alloc_arg(struct svc_rqst *rqstp)
 			set_current_state(TASK_RUNNING);
 			return -EINTR;
 		}
+		trace_svc_alloc_arg_err(pages);
 		schedule_timeout(msecs_to_jiffies(500));
 	}
 	rqstp->rq_page_end = &rqstp->rq_pages[pages];