tracing: Add a proc file to stop tracing and free buffer

The proc file entry buffer_size_kb is used to set the size of tracing
buffer. The memory to expand the buffer size is kernel memory. Consider
a use case where tracing is handled by a user space utility, which acts
as a gate keeper for tracing requests. In an OOM condition, tracing is
considered a low priority task and if the utility gets killed the ring
buffer memory cannot be released back to the kernel.

This patch adds a proc file called "free_buffer" whose purpose is to
stop tracing and free up the ring buffer when it is closed.

The user space process can then set the desired size in buffer_size_kb
file and open the fd to the "free_buffer" file. Under OOM condition, if
the process gets killed, the kernel closes the file descriptor. The
release handler stops the tracing and releases the kernel memory
automatically.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Michael Rubin <mrubin@google.com>
Cc: David Sharp <dhsharp@google.com>
Signed-off-by: Vaibhav Nagarnaik <vnagarnaik@google.com>
Link: http://lkml.kernel.org/r/1308012717-11148-1-git-send-email-vnagarnaik@google.com
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 73 insertions, 35 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 61fda6b6f1ab..9c557ae6a21e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2768,7 +2768,7 @@ int tracer_init(struct tracer *t, struct trace_array *tr)
         return t->init(tr);
 }
 
-static int tracing_resize_ring_buffer(unsigned long size)
+static int __tracing_resize_ring_buffer(unsigned long size)
 {
         int ret;
 
@@ -2820,6 +2820,41 @@ static int tracing_resize_ring_buffer(unsigned long size)
         return ret;
 }
 
+static ssize_t tracing_resize_ring_buffer(unsigned long size)
+{
+        int cpu, ret = size;
+
+        mutex_lock(&trace_types_lock);
+
+        tracing_stop();
+
+        /* disable all cpu buffers */
+        for_each_tracing_cpu(cpu) {
+                if (global_trace.data[cpu])
+                        atomic_inc(&global_trace.data[cpu]->disabled);
+                if (max_tr.data[cpu])
+                        atomic_inc(&max_tr.data[cpu]->disabled);
+        }
+
+        if (size != global_trace.entries)
+                ret = __tracing_resize_ring_buffer(size);
+
+        if (ret < 0)
+                ret = -ENOMEM;
+
+        for_each_tracing_cpu(cpu) {
+                if (global_trace.data[cpu])
+                        atomic_dec(&global_trace.data[cpu]->disabled);
+                if (max_tr.data[cpu])
+                        atomic_dec(&max_tr.data[cpu]->disabled);
+        }
+
+        tracing_start();
+        mutex_unlock(&trace_types_lock);
+
+        return ret;
+}
+
 
 /**
  * tracing_update_buffers - used by tracing facility to expand ring buffers
@@ -2837,7 +2872,7 @@ int tracing_update_buffers(void)
 
         mutex_lock(&trace_types_lock);
         if (!ring_buffer_expanded)
-                ret = tracing_resize_ring_buffer(trace_buf_size);
+                ret = __tracing_resize_ring_buffer(trace_buf_size);
         mutex_unlock(&trace_types_lock);
 
         return ret;
@@ -2861,7 +2896,7 @@ static int tracing_set_tracer(const char *buf)
         mutex_lock(&trace_types_lock);
 
         if (!ring_buffer_expanded) {
-                ret = tracing_resize_ring_buffer(trace_buf_size);
+                ret = __tracing_resize_ring_buffer(trace_buf_size);
                 if (ret < 0)
                         goto out;
                 ret = 0;
@@ -3436,7 +3471,7 @@ tracing_entries_write(struct file *filp, const char __user *ubuf,
 {
         unsigned long val;
         char buf[64];
-        int ret, cpu;
+        int ret;
 
         if (cnt >= sizeof(buf))
                 return -EINVAL;
@@ -3454,48 +3489,43 @@ tracing_entries_write(struct file *filp, const char __user *ubuf,
         if (!val)
                 return -EINVAL;
 
-        mutex_lock(&trace_types_lock);
-
-        tracing_stop();
-
-        /* disable all cpu buffers */
-        for_each_tracing_cpu(cpu) {
-                if (global_trace.data[cpu])
-                        atomic_inc(&global_trace.data[cpu]->disabled);
-                if (max_tr.data[cpu])
-                        atomic_inc(&max_tr.data[cpu]->disabled);
-        }
-
         /* value is in KB */
         val <<= 10;
 
-        if (val != global_trace.entries) {
-                ret = tracing_resize_ring_buffer(val);
-                if (ret < 0) {
-                        cnt = ret;
-                        goto out;
-                }
-        }
+        ret = tracing_resize_ring_buffer(val);
+        if (ret < 0)
+                return ret;
 
         *ppos += cnt;
 
-        /* If check pages failed, return ENOMEM */
-        if (tracing_disabled)
-                cnt = -ENOMEM;
- out:
-        for_each_tracing_cpu(cpu) {
-                if (global_trace.data[cpu])
-                        atomic_dec(&global_trace.data[cpu]->disabled);
-                if (max_tr.data[cpu])
-                        atomic_dec(&max_tr.data[cpu]->disabled);
-        }
+        return cnt;
+}
 
-        tracing_start();
-        mutex_unlock(&trace_types_lock);
+static ssize_t
+tracing_free_buffer_write(struct file *filp, const char __user *ubuf,
+                          size_t cnt, loff_t *ppos)
+{
+        /*
+         * There is no need to read what the user has written, this function
+         * is just to make sure that there is no error when "echo" is used
+         */
+
+        *ppos += cnt;
 
         return cnt;
 }
 
+static int
+tracing_free_buffer_release(struct inode *inode, struct file *filp)
+{
+        /* disable tracing */
+        tracing_off();
+        /* resize the ring buffer to 0 */
+        tracing_resize_ring_buffer(0);
+
+        return 0;
+}
+
 static int mark_printk(const char *fmt, ...)
 {
         int ret;
@@ -3641,6 +3671,11 @@ static const struct file_operations tracing_entries_fops = {
         .llseek         = generic_file_llseek,
 };
 
+static const struct file_operations tracing_free_buffer_fops = {
+        .write          = tracing_free_buffer_write,
+        .release        = tracing_free_buffer_release,
+};
+
 static const struct file_operations tracing_mark_fops = {
         .open           = tracing_open_generic,
         .write          = tracing_mark_write,
@@ -4365,6 +4400,9 @@ static __init int tracer_init_debugfs(void)
         trace_create_file("buffer_size_kb", 0644, d_tracer,
                         &global_trace, &tracing_entries_fops);
 
+        trace_create_file("free_buffer", 0644, d_tracer,
+                        &global_trace, &tracing_free_buffer_fops);
+
         trace_create_file("trace_marker", 0220, d_tracer,
                         NULL, &tracing_mark_fops);