sunrpc: add a Kconfig option to redirect dfprintk() output to trace buffer

We have a lot of old dprintk() call sites that aren't going anywhere
anytime soon. At the same time, turning them up is a serious burden on
the host due to the console locking overhead.

Add a new Kconfig option that redirects dfprintk() output to the trace
buffer. This is more efficient than logging to the console and allows
for proper interleaving of dprintk and static tracepoint events.

Since using trace_printk() causes scary warnings to pop at boot time,
this new option defaults to "n".

Signed-off-by: Jeff Layton <[e-mail]>
Reviewed-by: Chuck Lever <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Signed-off-by: Anna Schumaker <[e-mail]>

include/linux/sunrpc/debug.h

···
 #if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
 # define ifdebug(fac)		if (unlikely(rpc_debug & RPCDBG_##fac))
 
+# if IS_ENABLED(CONFIG_SUNRPC_DEBUG_TRACE)
+#  define __sunrpc_printk(fmt, ...)	trace_printk(fmt, ##__VA_ARGS__)
+# else
+#  define __sunrpc_printk(fmt, ...)	printk(KERN_DEFAULT fmt, ##__VA_ARGS__)
+# endif
+
 # define dfprintk(fac, fmt, ...)					\
 do {									\
 	ifdebug(fac)							\
-		printk(KERN_DEFAULT fmt, ##__VA_ARGS__);				\
+		__sunrpc_printk(fmt, ##__VA_ARGS__);			\
 } while (0)
 
 # define dfprintk_rcu(fac, fmt, ...)					\
 do {									\
 	ifdebug(fac) {							\
 		rcu_read_lock();					\
-		printk(KERN_DEFAULT fmt, ##__VA_ARGS__);				\
+		__sunrpc_printk(fmt, ##__VA_ARGS__);			\
 		rcu_read_unlock();					\
 	}								\
 } while (0)

net/sunrpc/Kconfig

···
 
 	  If unsure, say Y.
 
+config SUNRPC_DEBUG_TRACE
+	bool "RPC: Send dfprintk() output to the trace buffer"
+	depends on SUNRPC_DEBUG && TRACING
+	default n
+	help
+          dprintk() output can be voluminous, which can overwhelm the
+          kernel's logging facility as it must be sent to the console.
+          This option causes dprintk() output to go to the trace buffer
+          instead of the kernel log.
+
+          This will cause warnings about trace_printk() being used to be
+          logged at boot time, so say N unless you are debugging a problem
+          with sunrpc-based clients or services.
+
 config SUNRPC_XPRT_RDMA
 	tristate "RPC-over-RDMA transport"
 	depends on SUNRPC && INFINIBAND && INFINIBAND_ADDR_TRANS