relay: add buffer-only functionality, allowing for early kernel tracing

relay_open() can now handle NULL base_filename, to register a buffer-only channel. Using a new function, relay_late_setup_files(), one can assign files after creating the channel, thus allowing for doing early tracing in the kernel, before VFS is up.

Signed-off-by Eduard - Gabriel Munteanu <[email protected]>

---
 Documentation/filesystems/relay.txt |   11 ++++
 include/linux/relay.h               |    4 ++
 kernel/relay.c                      |  102 ++++++++++++++++++++++++++---------
 3 files changed, 91 insertions(+), 26 deletions(-)

Documentation/filesystems/relay.txt +11 -0

index 094f2d2..b59c228 100644
@@ -161,6 +161,7 @@ TBD(curr. line MT:/API/)
     relay_close(chan)
     relay_flush(chan)
     relay_reset(chan)
+    relay_late_setup_files(chan, base_filename, parent)
 
   channel management typically called on instigation of userspace:
 

@@ -294,6 +295,16 @@ user-defined data with a channel, and is immediately available
 (including in create_buf_file()) via chan->private_data or
 buf->chan->private_data.
 
+Buffer-only channels
+--------------------
+
+These channels have no files associated and can be created with
+relay_open(NULL, NULL, ...). Such channels are useful in scenarios such
+as when doing early tracing in the kernel, before the VFS is up. In these
+cases, one may open a channel a buffer-only channel and then call
+relay_late_setup_files() when the kernel is ready to handle files, to expose
+the buffered data to the userspace.
+
 Channel 'modes'
 ---------------
 

include/linux/relay.h +4 -0

index 6cd8c44..ca9ee3f 100644
@@ -68,6 +68,7 @@ struct rchan
 	int is_global;			/* One global buffer ? */
 	struct list_head list;		/* for channel list */
 	struct dentry *parent;		/* parent dentry passed to open */
+	int has_base_filename;		/* has a filename associated? */
 	char base_filename[NAME_MAX];	/* saved base filename */
 };
 

@@ -169,6 +170,9 @@ struct rchan *relay_open(const char *base_filename,
 			 size_t n_subbufs,
 			 struct rchan_callbacks *cb,
 			 void *private_data);
+extern int relay_late_setup_files(struct rchan *chan,
+				  const char *base_filename,
+				  struct dentry *parent);
 extern void relay_close(struct rchan *chan);
 extern void relay_flush(struct rchan *chan);
 extern void relay_subbufs_consumed(struct rchan *chan,

kernel/relay.c +76 -26

index 4c035a8..4bea94a 100644
@@ -378,6 +378,35 @@ void relay_reset(struct rchan *chan)
 }
 EXPORT_SYMBOL_GPL(relay_reset);
 
+static int relay_setup_buf_file(struct rchan *chan,
+				struct rchan_buf *buf,
+				unsigned int cpu)
+{
+	struct dentry *dentry;
+ 	char *tmpname;
+
+	tmpname = kzalloc(NAME_MAX + 1, GFP_KERNEL);
+	if (!tmpname)
+		goto failed;
+	snprintf(tmpname, NAME_MAX, "%s%d", chan->base_filename, cpu);
+
+	/* Create file in fs */
+	dentry = chan->cb->create_buf_file(tmpname, chan->parent,
+					   S_IRUSR, buf, 
+					   &chan->is_global);
+	
+	if (!dentry)
+		goto failed;
+	buf->dentry = dentry;
+
+	kfree(tmpname);
+
+	return 0;
+
+failed:
+	return 1;
+}
+
 /*
  *	relay_open_buf - create a new relay channel buffer
  *

@@ -386,44 +415,30 @@ EXPORT_SYMBOL_GPL(relay_reset);
 static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
 {
  	struct rchan_buf *buf = NULL;
-	struct dentry *dentry;
- 	char *tmpname;
 
  	if (chan->is_global)
 		return chan->buf[0];
 
-	tmpname = kzalloc(NAME_MAX + 1, GFP_KERNEL);
- 	if (!tmpname)
- 		goto end;
- 	snprintf(tmpname, NAME_MAX, "%s%d", chan->base_filename, cpu);
-
 	buf = relay_create_buf(chan);
 	if (!buf)
- 		goto free_name;
+ 		goto end;
+
+ 	if (chan->has_base_filename)
+		if (relay_setup_buf_file(chan, buf, cpu)) goto free_buf;
 
  	buf->cpu = cpu;
  	__relay_reset(buf, 1);
 
-	/* Create file in fs */
- 	dentry = chan->cb->create_buf_file(tmpname, chan->parent, S_IRUSR,
- 					   buf, &chan->is_global);
- 	if (!dentry)
- 		goto free_buf;
-
-	buf->dentry = dentry;
-
  	if(chan->is_global) {
  		chan->buf[0] = buf;
  		buf->cpu = 0;
   	}
 
- 	goto free_name;
+ 	goto end;
 
 free_buf:
  	relay_destroy_buf(buf);
  	buf = NULL;
-free_name:
- 	kfree(tmpname);
 end:
 	return buf;
 }

@@ -508,8 +523,8 @@ static int __cpuinit relay_hotcpu_callback(struct notifier_block *nb,
 
 /**
  *	relay_open - create a new relay channel
- *	@base_filename: base name of files to create
- *	@parent: dentry of parent directory, %NULL for root directory
+ *	@base_filename: base name of files to create, %NULL for buffering only
+ *	@parent: dentry of parent directory, %NULL for root directory or buffer
  *	@subbuf_size: size of sub-buffers
  *	@n_subbufs: number of sub-buffers
  *	@cb: client callback functions

@@ -531,8 +546,6 @@ struct rchan *relay_open(const char *base_filename,
 {
 	unsigned int i;
 	struct rchan *chan;
-	if (!base_filename)
-		return NULL;
 
 	if (!(subbuf_size && n_subbufs))
 		return NULL;

@@ -547,12 +560,17 @@ struct rchan *relay_open(const char *base_filename,
 	chan->alloc_size = FIX_SIZE(subbuf_size * n_subbufs);
 	chan->parent = parent;
 	chan->private_data = private_data;
-	strlcpy(chan->base_filename, base_filename, NAME_MAX);
+	if (base_filename) {
+		chan->has_base_filename = 1;
+		strlcpy(chan->base_filename, base_filename, NAME_MAX);
+	} else {
+		chan->has_base_filename = 0;
+	}
 	setup_callbacks(chan, cb);
 	kref_init(&chan->kref);
 
 	mutex_lock(&relay_channels_mutex);
-	for_each_online_cpu(i) {
+	for_each_present_cpu(i) {
 		chan->buf[i] = relay_open_buf(chan, i);
 		if (!chan->buf[i])
 			goto free_bufs;

@@ -563,7 +581,7 @@ struct rchan *relay_open(const char *base_filename,
 	return chan;
 
 free_bufs:
-	for_each_online_cpu(i) {
+	for_each_present_cpu(i) {
 		if (!chan->buf[i])
 			break;
 		relay_close_buf(chan->buf[i]);

@@ -576,6 +594,38 @@ free_bufs:
 EXPORT_SYMBOL_GPL(relay_open);
 
 /**
+ *	relay_late_setup_files - triggers file creation
+ *	@chan: channel to operate on
+ *	@base_filename: base name of files to create
+ *	@parent: dentry of parent directory, %NULL for root directory
+ *
+ *	Returns 0 is successful, non-zero otherwise.
+ *
+ *	Use to setup files for a previously buffer-only channel.
+ *	Useful to do early tracing in kernel, before VFS is up, for example.
+ */
+int relay_late_setup_files(struct rchan *chan,
+			   const char *base_filename,
+			   struct dentry *parent)
+{
+	unsigned int i;
+	
+	if (!chan || !base_filename)
+		return 1;
+	
+	strlcpy(chan->base_filename, base_filename, NAME_MAX);
+	chan->has_base_filename = 1;
+	chan->parent = parent;
+
+	mutex_lock(&relay_channels_mutex);
+	for_each_present_cpu(i)
+		relay_setup_buf_file(chan, chan->buf[i], i);
+	mutex_unlock(&relay_channels_mutex);
+
+	return 0;
+}
+		
+/**
  *	relay_switch_subbuf - switch to a new sub-buffer
  *	@buf: channel buffer
  *	@length: size of current event

Signature · 1 line

1.5.2.5

I like this :) But could we trace event sooner if we did not depend on vmalloc to allocate the buffers ?

I am thinking of receiving a kernel parameter that would reserve a buffer in the linear kernel mapping (kmalloc ?), or to compile-in a static buffer, which could be used by relay instead of the vmalloc'd buffer. That would permit to trace the memory allocator and some other very early stuff. Basically, relay would have it's "memory pool" to manage and would do its allocations from this fixed buffer. No free would be required.

Mathieu

Quoted 11 lines · On Fri, 4 Apr 2008 12:06:23 -0400 Mathieu Desnoyers…

I like this :) But could we trace event sooner if we did not depend on vmalloc to allocate the buffers ?

I am thinking of receiving a kernel parameter that would reserve a buffer in the linear kernel mapping (kmalloc ?), or to compile-in a static buffer, which could be used by relay instead of the vmalloc'd buffer. That would permit to trace the memory allocator and some other very early stuff. Basically, relay would have it's "memory pool" to manage and would do its allocations from this fixed buffer. No free would be required.

Quoted 1 line

Mathieu

Sure. I'm doing this as part of my GSoC application (well, I haven't been accepted yet). I had also proposed this to Pekka Enberg, my mentor, but he was reluctant to hack the relay code more invasively. Now that you agree with this, I'll work on it.

kmalloc() doesn't work before kmem_cache_init() and relay code uses kmalloc(), not vmalloc(), IIRC.

I would go another way. Have the relay client call __get_free_pages() or similar function, allocate the buffer and pass it on to a variant of relay_open(), let's name it relay_early_open(). __get_free_pages() can be used before kmem_cache_init() as far as I know. No free is required, since early tracing code implies it's built into the kernel and not as a module. What do you say?

Will you merge this or wait until I finish the very early tracing stuff?

Cheers,
Eduard

Then I guess we would depend on page_alloc_init(). It's not that bad, but I guess the question is : do we prefer to let users specify the buffer size to reserve for tracing on the kernel command line (that would require __get_free_pages()) or do we compile-in a static buffer size, which can be specified in the kernel configuration. I think providing both could be an option : flexibility _and_ *very* early tracing.

So making this relay interface flexible enough to receive a buffer either reserved by __get_free_pages() or a static buffer seems like a good compromise. It would also have to receive the buffer size from the caller.

As far as I am concerned, I can only merge this in the LTTng project to try it. I've added Tom Zanussi, the author of relay, who might have some ideas to share with us. I would wait until we have a more precise idea of the API before I modify LTTng to start using it.

Regards,

Mathieu

relay_open() can now handle NULL base_filename, to register a buffer-only channel. Using a new function, relay_late_setup_files(), one can assign files after creating the channel, thus allowing for doing early tracing in the kernel, before VFS is up.

Signed-off-by Eduard - Gabriel Munteanu <[email protected]>

---
 Documentation/filesystems/relay.txt |   11 ++++
 include/linux/relay.h               |    4 ++
 kernel/relay.c                      |  102 ++++++++++++++++++++++++++---------
 3 files changed, 91 insertions(+), 26 deletions(-)

Documentation/filesystems/relay.txt +1 -0

index 094f2d2..b59c228 100644
@@ -161,6 +161,7 @@ TBD(curr. line MT:/API/)
     relay_close(chan)
     relay_flush(chan)
     relay_reset(chan)
+    relay_late_setup_files(chan, base_filename, parent)
 
   channel management typically called on instigation of userspace:
 

Documentation/filesystems/relay.txt +3 -0

+relay_late_setup_files() when the kernel is ready to handle files, to expose
+the buffered data to the userspace.
+
 Channel 'modes'
 ---------------
 

kernel/relay.c +9 -0

index 4c035a8..4bea94a 100644
@@ -378,6 +378,35 @@ void relay_reset(struct rchan *chan)
 }
 EXPORT_SYMBOL_GPL(relay_reset);
 
+static int relay_setup_buf_file(struct rchan *chan,
+				struct rchan_buf *buf,
+				unsigned int cpu)
+{
+	struct dentry *dentry;
+ 	char *tmpname;
+
+	tmpname = kzalloc(NAME_MAX + 1, GFP_KERNEL);
+	if (!tmpname)

kernel/relay.c +10 -4

+	buf->dentry = dentry;
+
+	kfree(tmpname);
+
+	return 0;
+
+failed:
+	return 1;
+}
+
 /*
  *	relay_open_buf - create a new relay channel buffer
  *

@@ -386,44 +415,30 @@ EXPORT_SYMBOL_GPL(relay_reset);
 static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
 {
  	struct rchan_buf *buf = NULL;
-	struct dentry *dentry;
- 	char *tmpname;
 
  	if (chan->is_global)
 		return chan->buf[0];
 
-	tmpname = kzalloc(NAME_MAX + 1, GFP_KERNEL);
- 	if (!tmpname)

kernel/relay.c +1 -11

  	buf->cpu = cpu;
  	__relay_reset(buf, 1);
 
-	/* Create file in fs */
- 	dentry = chan->cb->create_buf_file(tmpname, chan->parent, S_IRUSR,
- 					   buf, &chan->is_global);
- 	if (!dentry)
- 		goto free_buf;
-
-	buf->dentry = dentry;
-
  	if(chan->is_global) {
  		chan->buf[0] = buf;
  		buf->cpu = 0;
   	}
 
- 	goto free_name;
+ 	goto end;
 
 free_buf:
  	relay_destroy_buf(buf);
  	buf = NULL;
-free_name:
- 	kfree(tmpname);
 end:
 	return buf;
 }

kernel/relay.c +19 -0

+ *
+ *	Use to setup files for a previously buffer-only channel.
+ *	Useful to do early tracing in kernel, before VFS is up, for example.
+ */
+int relay_late_setup_files(struct rchan *chan,
+			   const char *base_filename,
+			   struct dentry *parent)
+{
+	unsigned int i;
+	
+	if (!chan || !base_filename)
+		return 1;
+	
+	strlcpy(chan->base_filename, base_filename, NAME_MAX);
+	chan->has_base_filename = 1;
+	chan->parent = parent;
+
+	mutex_lock(&relay_channels_mutex);
+	for_each_present_cpu(i)