Re: [PATCH] tracing: Fix wasted memory in saved_cmdlines logic

[Tim Chen]

On Thu, 2024-02-08 at 10:53 -0500, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
> 
> While looking at improving the saved_cmdlines cache I found a huge amount
> of wasted memory that should be used for the cmdlines.
> 
> The tracing data saves pids during the trace. At sched switch, if a trace
> occurred, it will save the comm of the task that did the trace. This is
> saved in a "cache" that maps pids to comms and exposed to user space via
> the /sys/kernel/tracing/saved_cmdlines file. Currently it only caches by
> default 128 comms.
> 
> The structure that uses this creates an array to store the pids using
> PID_MAX_DEFAULT (which is usually set to 32768). This causes the structure
> to be of the size of 131104 bytes on 64 bit machines.
> 
> In hex: 131104 = 0x20020, and since the kernel allocates generic memory in
> powers of two, the kernel would allocate 0x40000 or 262144 bytes to store
> this structure. That leaves 131040 bytes of wasted space.
> 
> Worse, the structure points to an allocated array to store the comm names,
> which is 16 bytes times the amount of names to save (currently 128), which
> is 2048 bytes. Instead of allocating a separate array, make the structure
> end with a variable length string and use the extra space for that.
> 
> This is similar to a recommendation that Linus had made about eventfs_inode names:
> 
>   https://lore.kernel.org/all/20240130190355.11486-5-torvalds@xxxxxxxxxxxxxxxxxxxx/
> 
> Instead of allocating a separate string array to hold the saved comms,
> have the structure end with: char saved_cmdlines[]; and round up to the
> next power of two over sizeof(struct saved_cmdline_buffers) + num_cmdlines * TASK_COMM_LEN
> It will use this extra space for the saved_cmdline portion.
> 
> Now, instead of saving only 128 comms by default, by using this wasted
> space at the end of the structure it can save over 8000 comms and even
> saves space by removing the need for allocating the other array.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 939c7a4f04fcd ("tracing: Introduce saved_cmdlines_size file")
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>

> ---
>  kernel/trace/trace.c | 73 +++++++++++++++++++++-----------------------
>  1 file changed, 34 insertions(+), 39 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 2a7c6fd934e9..0b3e60b827f7 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2320,7 +2320,7 @@ struct saved_cmdlines_buffer {
>  	unsigned *map_cmdline_to_pid;
>  	unsigned cmdline_num;
>  	int cmdline_idx;
> -	char *saved_cmdlines;
> +	char saved_cmdlines[];
>  };
>  static struct saved_cmdlines_buffer *savedcmd;
>  
> @@ -2334,47 +2334,54 @@ static inline void set_cmdline(int idx, const char *cmdline)
>  	strncpy(get_saved_cmdlines(idx), cmdline, TASK_COMM_LEN);
>  }
>  
> -static int allocate_cmdlines_buffer(unsigned int val,
> -				    struct saved_cmdlines_buffer *s)
> +static void free_saved_cmdlines_buffer(struct saved_cmdlines_buffer *s)
> +{
> +	int order = get_order(sizeof(*s) + s->cmdline_num * TASK_COMM_LEN);
> +
> +	kfree(s->map_cmdline_to_pid);
> +	free_pages((unsigned long)s, order);
> +}
> +
> +static struct saved_cmdlines_buffer *allocate_cmdlines_buffer(unsigned int val)
>  {
> +	struct saved_cmdlines_buffer *s;
> +	struct page *page;
> +	int orig_size, size;
> +	int order;
> +
> +	/* Figure out how much is needed to hold the given number of cmdlines */
> +	orig_size = sizeof(*s) + val * TASK_COMM_LEN;
> +	order = get_order(orig_size);
> +	size = 1 << (order + PAGE_SHIFT);
> +	page = alloc_pages(GFP_KERNEL, order);
> +	if (!page)
> +		return NULL;
> +
> +	s = page_address(page);
> +	memset(s, 0, sizeof(*s));
> +
> +	/* Round up to actual allocation */
> +	val = (size - sizeof(*s)) / TASK_COMM_LEN;
> +	s->cmdline_num = val;
> +
>  	s->map_cmdline_to_pid = kmalloc_array(val,
>  					      sizeof(*s->map_cmdline_to_pid),
>  					      GFP_KERNEL);
> -	if (!s->map_cmdline_to_pid)
> -		return -ENOMEM;
> -
> -	s->saved_cmdlines = kmalloc_array(TASK_COMM_LEN, val, GFP_KERNEL);
> -	if (!s->saved_cmdlines) {
> -		kfree(s->map_cmdline_to_pid);
> -		return -ENOMEM;
> -	}
>  
>  	s->cmdline_idx = 0;
> -	s->cmdline_num = val;
>  	memset(&s->map_pid_to_cmdline, NO_CMDLINE_MAP,
>  	       sizeof(s->map_pid_to_cmdline));
>  	memset(s->map_cmdline_to_pid, NO_CMDLINE_MAP,
>  	       val * sizeof(*s->map_cmdline_to_pid));
>  
> -	return 0;
> +	return s;
>  }
>  
>  static int trace_create_savedcmd(void)
>  {
> -	int ret;
> -
> -	savedcmd = kmalloc(sizeof(*savedcmd), GFP_KERNEL);
> -	if (!savedcmd)
> -		return -ENOMEM;
> -
> -	ret = allocate_cmdlines_buffer(SAVED_CMDLINES_DEFAULT, savedcmd);
> -	if (ret < 0) {
> -		kfree(savedcmd);
> -		savedcmd = NULL;
> -		return -ENOMEM;
> -	}
> +	savedcmd = allocate_cmdlines_buffer(SAVED_CMDLINES_DEFAULT);
>  
> -	return 0;
> +	return savedcmd ? 0 : -ENOMEM;
>  }
>  
>  int is_tracing_stopped(void)
> @@ -6056,26 +6063,14 @@ tracing_saved_cmdlines_size_read(struct file *filp, char __user *ubuf,
>  	return simple_read_from_buffer(ubuf, cnt, ppos, buf, r);
>  }
>  
> -static void free_saved_cmdlines_buffer(struct saved_cmdlines_buffer *s)
> -{
> -	kfree(s->saved_cmdlines);
> -	kfree(s->map_cmdline_to_pid);
> -	kfree(s);
> -}
> -
>  static int tracing_resize_saved_cmdlines(unsigned int val)
>  {
>  	struct saved_cmdlines_buffer *s, *savedcmd_temp;
>  
> -	s = kmalloc(sizeof(*s), GFP_KERNEL);
> +	s = allocate_cmdlines_buffer(val);
>  	if (!s)
>  		return -ENOMEM;
>  
> -	if (allocate_cmdlines_buffer(val, s) < 0) {
> -		kfree(s);
> -		return -ENOMEM;
> -	}
> -
>  	preempt_disable();
>  	arch_spin_lock(&trace_cmdline_lock);
>  	savedcmd_temp = savedcmd;