x86: Add NMI duration tracepoints

This patch has been invaluable in my adventures finding
issues in the perf NMI handler.  I'm as big a fan of
printk() as anybody is, but using printk() in NMIs is
deadly when they're happening frequently.

Even hacking in trace_printk() ended up eating enough
CPU to throw off some of the measurements I was making.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: paulus@samba.org
Cc: acme@ghostprotocols.net
Cc: Dave Hansen <dave@sr71.net>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

3 files changed, 87 insertions, 2 deletions

diff --git a/Documentation/trace/events-nmi.txt b/Documentation/trace/events-nmi.txt
new file mode 100644
index 000000000000..c03c8c89f08d
--- /dev/null
+++ b/Documentation/trace/events-nmi.txt
@@ -0,0 +1,43 @@
+NMI Trace Events
+
+These events normally show up here:
+
+	/sys/kernel/debug/tracing/events/nmi
+
+--
+
+nmi_handler:
+
+You might want to use this tracepoint if you suspect that your
+NMI handlers are hogging large amounts of CPU time.  The kernel
+will warn if it sees long-running handlers:
+
+	INFO: NMI handler took too long to run: 9.207 msecs
+
+and this tracepoint will allow you to drill down and get some
+more details.
+
+Let's say you suspect that perf_event_nmi_handler() is causing
+you some problems and you only want to trace that handler
+specifically.  You need to find its address:
+
+	$ grep perf_event_nmi_handler /proc/kallsyms
+	ffffffff81625600 t perf_event_nmi_handler
+
+Let's also say you are only interested in when that function is
+really hogging a lot of CPU time, like a millisecond at a time.
+Note that the kernel's output is in milliseconds, but the input
+to the filter is in nanoseconds!  You can filter on 'delta_ns':
+
+cd /sys/kernel/debug/tracing/events/nmi/nmi_handler
+echo 'handler==0xffffffff81625600 && delta_ns>1000000' > filter
+echo 1 > enable
+
+Your output would then look like:
+
+$ cat /sys/kernel/debug/tracing/trace_pipe
+<idle>-0     [000] d.h3   505.397558: nmi_handler: perf_event_nmi_handler() delta_ns: 3236765 handled: 1
+<idle>-0     [000] d.h3   505.805893: nmi_handler: perf_event_nmi_handler() delta_ns: 3174234 handled: 1
+<idle>-0     [000] d.h3   506.158206: nmi_handler: perf_event_nmi_handler() delta_ns: 3084642 handled: 1
+<idle>-0     [000] d.h3   506.334346: nmi_handler: perf_event_nmi_handler() delta_ns: 3080351 handled: 1
+
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index e9bae4c2f2dd..0920212e6159 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -30,6 +30,9 @@
 #include <asm/nmi.h>
 #include <asm/x86_init.h>
 
+#define CREATE_TRACE_POINTS
+#include <trace/events/nmi.h>
+
 struct nmi_desc {
 	spinlock_t lock;
 	struct list_head head;
@@ -108,11 +111,13 @@ static int __kprobes nmi_handle(unsigned int type, struct pt_regs *regs, bool b2
 	 */
 	list_for_each_entry_rcu(a, &desc->head, list) {
 		u64 before, delta, whole_msecs;
-		int decimal_msecs;
+		int decimal_msecs, thishandled;
 
 		before = local_clock();
-		handled += a->handler(type, regs);
+		thishandled = a->handler(type, regs);
+		handled += thishandled;
 		delta = local_clock() - before;
+		trace_nmi_handler(a->handler, (int)delta, thishandled);
 
 		if (delta < nmi_longest_ns)
 			continue;
diff --git a/include/trace/events/nmi.h b/include/trace/events/nmi.h
new file mode 100644
index 000000000000..da3ee96b8d03
--- /dev/null
+++ b/include/trace/events/nmi.h
@@ -0,0 +1,37 @@
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM nmi
+
+#if !defined(_TRACE_NMI_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_NMI_H
+
+#include <linux/ktime.h>
+#include <linux/tracepoint.h>
+
+TRACE_EVENT(nmi_handler,
+
+	TP_PROTO(void *handler, s64 delta_ns, int handled),
+
+	TP_ARGS(handler, delta_ns, handled),
+
+	TP_STRUCT__entry(
+		__field(	void *,		handler	)
+		__field(	s64,		delta_ns)
+		__field(	int,		handled	)
+	),
+
+	TP_fast_assign(
+		__entry->handler = handler;
+		__entry->delta_ns = delta_ns;
+		__entry->handled = handled;
+	),
+
+	TP_printk("%ps() delta_ns: %lld handled: %d",
+		__entry->handler,
+		__entry->delta_ns,
+		__entry->handled)
+);
+
+#endif /* _TRACE_NMI_H */
+
+/* This part ust be outside protection */
+#include <trace/define_trace.h>