[PATCH 3/3][RFC] tracing: Add config to permanently have trace_printk() use percpu

[Steven Rostedt]

From: Steven Rostedt <srostedt@xxxxxxxxxx>

For those that do a lot of developing, and use trace_printk() often.
There is no need to have a switch to change between percpu buffers and
a locked protected buffer. Just have the percpu buffers permanently
allocated via a config option and keep a fast and non-intrusive
tracing mechanism that can handle tracing in NMIs, but sacrifices
memory overhead. But developers don't mind that.

Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 kernel/trace/Kconfig |   18 ++++++++++++++++++
 kernel/trace/trace.c |   41 ++++++++++++++++++++++++++++++++++++-----
 2 files changed, 54 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 2ad39e5..18bd377 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -223,6 +223,24 @@ config SCHED_TRACER
 	  This tracer tracks the latency of the highest priority task
 	  to be scheduled in, starting from the point it has woken up.
 
+config TRACE_PRINTK_PERCPU
+	bool "Allocate trace_printk() per_cpu buffers"
+	depends on TRACING
+	 help
+	  By default trace_printk() uses a singe 1KB buffer protected
+	  with a spinlock. It also may lose trace data from NMIs, if an
+	  NMI happens on a CPU that a trace is being recorded on.
+
+	  By default, per_cpu buffers can be allocated either by
+	  setting trace_printk_percpu on the kernel command line,
+	  or by enabling the trace option trace_printk_percpu.
+
+	  For developers that want it always set, by enabling this
+	  config, per_cpu buffers will be permanently set, removing
+	  any logic to check between single and per cpu buffers
+	  adding less impact to tracing with the added cost of more
+	  memory overhead.
+
 config ENABLE_DEFAULT_TRACERS
 	bool "Trace process context switches and events"
 	depends on !GENERIC_TRACER
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index c59ab4c..b245a4e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -338,7 +338,11 @@ static DECLARE_WAIT_QUEUE_HEAD(trace_wait);
 /* trace_flags holds trace_options default values */
 unsigned long trace_flags = TRACE_ITER_PRINT_PARENT | TRACE_ITER_PRINTK |
 	TRACE_ITER_ANNOTATE | TRACE_ITER_CONTEXT_INFO | TRACE_ITER_SLEEP_TIME |
-	TRACE_ITER_GRAPH_TIME | TRACE_ITER_RECORD_CMD | TRACE_ITER_OVERWRITE;
+	TRACE_ITER_GRAPH_TIME | TRACE_ITER_RECORD_CMD | TRACE_ITER_OVERWRITE
+#ifdef CONFIG_TRACE_PRINTK_PERCPU
+	| TRACE_ITER_PRINTK_PERCPU
+#endif
+	;
 
 static int trace_stop_count;
 static DEFINE_SPINLOCK(tracing_start_lock);
@@ -1450,15 +1454,32 @@ static void __trace_userstack(struct trace_array *tr, unsigned long flags)
 
 #endif /* CONFIG_STACKTRACE */
 
-static char trace_buf[TRACE_BUF_SIZE];
-static arch_spinlock_t trace_buf_lock =
-	(arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED;
-
 /* created for use with alloc_percpu */
 struct trace_buffer_struct {
 	char buffer[TRACE_BUF_SIZE];
 };
 
+#ifdef CONFIG_TRACE_PRINTK_PERCPU
+static DEFINE_PER_CPU(struct trace_buffer_struct, trace_buffers);
+static DEFINE_PER_CPU(struct trace_buffer_struct, trace_nmi_buffers);
+
+static inline char *get_trace_buf(struct trace_array *tr, int *put)
+{
+	if (in_nmi())
+		return __get_cpu_var(trace_nmi_buffers).buffer;
+	else
+		return __get_cpu_var(trace_buffers).buffer;
+}
+
+static inline void put_trace_buf(struct trace_array *tr, int put)
+{
+}
+
+#else
+static char trace_buf[TRACE_BUF_SIZE];
+static arch_spinlock_t trace_buf_lock =
+	(arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED;
+
 static struct trace_buffer_struct *trace_percpu_buffer;
 static struct trace_buffer_struct *trace_percpu_nmi_buffer;
 
@@ -1526,6 +1547,7 @@ static void put_trace_buf(struct trace_array *tr, int put)
 	arch_spin_unlock(&trace_buf_lock);
 	atomic_dec_return(&data->disabled);
 }
+#endif /* CONFIG_TRACE_PRINTK_PERCPU */
 
 /**
  * trace_vbprintk - write binary msg to tracing buffer
@@ -2708,6 +2730,7 @@ static int set_tracer_option(struct tracer *trace, char *cmp, int neg)
 	return -EINVAL;
 }
 
+#ifndef CONFIG_TRACE_PRINTK_PERCPU
 static void alloc_percpu_trace_buffer(void)
 {
 	struct trace_buffer_struct *buffers;
@@ -2752,6 +2775,14 @@ static void free_percpu_trace_buffer(void)
 	free_percpu(buffers);
 	free_percpu(nmi_buffers);
 }
+#else
+static void alloc_percpu_trace_buffer(void) { }
+static void free_percpu_trace_buffer(void)
+{
+	/* trace_printk() buffers are permanently on */
+	trace_flags |= TRACE_ITER_PRINTK_PERCPU;
+}
+#endif
 
 static void set_tracer_flags(unsigned int mask, int enabled)
 {
-- 
1.7.6.3

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