linux-kernel - Re: [PATCH 2/3] tracing/stat: simplify rbtree freeing code

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Date:	Tue, 26 May 2009 21:33:25 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] tracing/stat: simplify rbtree freeing code

On Tue, May 26, 2009 at 09:16:08AM +0800, Li Zefan wrote:
> Frederic Weisbecker wrote:
> > On Mon, May 25, 2009 at 04:46:29PM +0800, Li Zefan wrote:
> >> When closing a trace_stat file, we destroy the rbtree constructed during
> >> file open, but there is memory leak that the root node is not freed:
> >>
> >> static struct rb_node *release_next(struct rb_node *node)
> >> {
> >> 	...
> >> 	else {
> >> 		if (!parent)		<-- here we leak @node
> >> 			return NULL;
> >> 	...
> >> }
> >>
> >> This patch keeps removing root node until the tree is empty. We regress
> >> from O(n) to O(nlogn), but since both open() and read() are O(nlogn) and
> >> it's a slow path, this change won't affect scalibility.
> >>
> >> [ Impact: fix memory leak when closing a trace_stat file ]
> >>
> >> Signed-off-by: Li Zefan <lizf@...fujitsu.com>
> >> ---
> >>  kernel/trace/trace_stat.c |   39 +++++----------------------------------
> >>  1 files changed, 5 insertions(+), 34 deletions(-)
> >>
> >> diff --git a/kernel/trace/trace_stat.c b/kernel/trace/trace_stat.c
> >> index 6efbcb4..ed18701 100644
> >> --- a/kernel/trace/trace_stat.c
> >> +++ b/kernel/trace/trace_stat.c
> >> @@ -42,47 +42,18 @@ static DEFINE_MUTEX(all_stat_sessions_mutex);
> >>  /* The root directory for all stat files */
> >>  static struct dentry		*stat_dir;
> >>  
> >> -/*
> >> - * Iterate through the rbtree using a post order traversal path
> >> - * to release the next node.
> >> - * It won't necessary release one at each iteration
> >> - * but it will at least advance closer to the next one
> >> - * to be released.
> >> - */
> >> -static struct rb_node *release_next(struct rb_node *node)
> >> +static void reset_stat_session(struct stat_session *session)
> >>  {
> >>  	struct stat_node *snode;
> >> -	struct rb_node *parent = rb_parent(node);
> >> -
> >> -	if (node->rb_left)
> >> -		return node->rb_left;
> >> -	else if (node->rb_right)
> >> -		return node->rb_right;
> >> -	else {
> >> -		if (!parent)
> >> -			return NULL;
> >> -		if (parent->rb_left == node)
> >> -			parent->rb_left = NULL;
> >> -		else
> >> -			parent->rb_right = NULL;
> >> +	struct rb_root *sroot = &session->stat_root;
> >>  
> >> -		snode = container_of(node, struct stat_node, node);
> >> +	while (!RB_EMPTY_ROOT(sroot)) {
> >> +		snode = rb_entry(sroot->rb_node, struct stat_node, node);
> >> +		rb_erase(&snode->node, sroot);
> > 
> > 
> > 
> > Why not just keep the previous version but sligthly
> > modified:
> > 
> > 
> > 	while (node)
> > 		node = release_next(node);
> > 
> > 	if (!RB_EMPTY_ROOT(root)) {
> > 		node = rb_entry(...)
> > 		kfree(....)
> > 		root = RB_ROOT
> > 	}
> > 
> 
> The easiest fix:
> 
>  		if (!parent)
> -			return NULL;
> -		if (parent->rb_left == node)
> +			;
> +		else if (parent->rb_left == node)
> 
> ;)



Ah, yeah :)


> 
> > Because the problem with rb_erase() is the wasteful color based rebalancing
> > performed whereas here we just need to walk into the tree and free
> > the nodes.
> > 
> > Hm?
> > 
> 
> Less code, less bugs.
> 
> But actually I don't know how costly rb_erase() is, if it's really better to
> avoid rb_erase(), I'll send another fix.



It is more costly because the tree is rebalanced after erasing
each node.
I don't think the change could be really visualized though. Not
for now at least, but it could if a future tracer has a huge mass of
entries.

 
> > Frederic.
> > 

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