[for-next][PATCH 07/12] tracing: Choose static tp_printk buffer by explicit nesting count

From: Steven Rostedt
Date: Sun Jul 03 2016 - 16:36:26 EST


From: Andy Lutomirski <luto@xxxxxxxxxx>

Currently, the trace_printk code chooses which static buffer to use based
on what type of atomic context (NMI, IRQ, etc) it's in. Simplify the
code and make it more robust: simply count the nesting depth and choose
a buffer based on the current nesting depth.

The new code will only drop an event if we nest more than 4 deep,
and the old code was guaranteed to malfunction if that happened.

Link: http://lkml.kernel.org/r/07ab03aecfba25fcce8f9a211b14c9c5e2865c58.1464289095.git.luto@xxxxxxxxxx

Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
kernel/trace/trace.c | 83 +++++++++++++++-------------------------------------
1 file changed, 24 insertions(+), 59 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index aa240551fc5d..45e6747589c6 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2339,83 +2339,41 @@ static void __trace_userstack(struct trace_array *tr, unsigned long flags)

/* created for use with alloc_percpu */
struct trace_buffer_struct {
- char buffer[TRACE_BUF_SIZE];
+ int nesting;
+ char buffer[4][TRACE_BUF_SIZE];
};

static struct trace_buffer_struct *trace_percpu_buffer;
-static struct trace_buffer_struct *trace_percpu_sirq_buffer;
-static struct trace_buffer_struct *trace_percpu_irq_buffer;
-static struct trace_buffer_struct *trace_percpu_nmi_buffer;

/*
- * The buffer used is dependent on the context. There is a per cpu
- * buffer for normal context, softirq contex, hard irq context and
- * for NMI context. Thise allows for lockless recording.
- *
- * Note, if the buffers failed to be allocated, then this returns NULL
+ * Thise allows for lockless recording. If we're nested too deeply, then
+ * this returns NULL.
*/
static char *get_trace_buf(void)
{
- struct trace_buffer_struct *percpu_buffer;
-
- /*
- * If we have allocated per cpu buffers, then we do not
- * need to do any locking.
- */
- if (in_nmi())
- percpu_buffer = trace_percpu_nmi_buffer;
- else if (in_irq())
- percpu_buffer = trace_percpu_irq_buffer;
- else if (in_softirq())
- percpu_buffer = trace_percpu_sirq_buffer;
- else
- percpu_buffer = trace_percpu_buffer;
+ struct trace_buffer_struct *buffer = this_cpu_ptr(trace_percpu_buffer);

- if (!percpu_buffer)
+ if (!buffer || buffer->nesting >= 4)
return NULL;

- return this_cpu_ptr(&percpu_buffer->buffer[0]);
+ return &buffer->buffer[buffer->nesting++][0];
+}
+
+static void put_trace_buf(void)
+{
+ this_cpu_dec(trace_percpu_buffer->nesting);
}

static int alloc_percpu_trace_buffer(void)
{
struct trace_buffer_struct *buffers;
- struct trace_buffer_struct *sirq_buffers;
- struct trace_buffer_struct *irq_buffers;
- struct trace_buffer_struct *nmi_buffers;

buffers = alloc_percpu(struct trace_buffer_struct);
- if (!buffers)
- goto err_warn;
-
- sirq_buffers = alloc_percpu(struct trace_buffer_struct);
- if (!sirq_buffers)
- goto err_sirq;
-
- irq_buffers = alloc_percpu(struct trace_buffer_struct);
- if (!irq_buffers)
- goto err_irq;
-
- nmi_buffers = alloc_percpu(struct trace_buffer_struct);
- if (!nmi_buffers)
- goto err_nmi;
+ if (WARN(!buffers, "Could not allocate percpu trace_printk buffer"))
+ return -ENOMEM;

trace_percpu_buffer = buffers;
- trace_percpu_sirq_buffer = sirq_buffers;
- trace_percpu_irq_buffer = irq_buffers;
- trace_percpu_nmi_buffer = nmi_buffers;
-
return 0;
-
- err_nmi:
- free_percpu(irq_buffers);
- err_irq:
- free_percpu(sirq_buffers);
- err_sirq:
- free_percpu(buffers);
- err_warn:
- WARN(1, "Could not allocate percpu trace_printk buffer");
- return -ENOMEM;
}

static int buffers_allocated;
@@ -2506,7 +2464,7 @@ int trace_vbprintk(unsigned long ip, const char *fmt, va_list args)
tbuffer = get_trace_buf();
if (!tbuffer) {
len = 0;
- goto out;
+ goto out_nobuffer;
}

len = vbin_printf((u32 *)tbuffer, TRACE_BUF_SIZE/sizeof(int), fmt, args);
@@ -2532,6 +2490,9 @@ int trace_vbprintk(unsigned long ip, const char *fmt, va_list args)
}

out:
+ put_trace_buf();
+
+out_nobuffer:
preempt_enable_notrace();
unpause_graph_tracing();

@@ -2563,7 +2524,7 @@ __trace_array_vprintk(struct ring_buffer *buffer,
tbuffer = get_trace_buf();
if (!tbuffer) {
len = 0;
- goto out;
+ goto out_nobuffer;
}

len = vscnprintf(tbuffer, TRACE_BUF_SIZE, fmt, args);
@@ -2582,7 +2543,11 @@ __trace_array_vprintk(struct ring_buffer *buffer,
__buffer_unlock_commit(buffer, event);
ftrace_trace_stack(&global_trace, buffer, flags, 6, pc, NULL);
}
- out:
+
+out:
+ put_trace_buf();
+
+out_nobuffer:
preempt_enable_notrace();
unpause_graph_tracing();

--
2.8.1