[PATCH 1/3] Adds a read-only "procs" file similar to "tasks" that shows only unique tgids

[Li Zefan]

Ben Blum wrote:
> Adds a read-only "procs" file similar to "tasks" that shows only unique tgids
> 
> struct cgroup used to have a bunch of fields for keeping track of the pidlist
> for the tasks file. Those are now separated into a new struct cgroup_pidlist,
> of which two are had, one for procs and one for tasks. The way the seq_file
> operations are set up is changed so that just the pidlist struct gets passed
> around as the private data.
> 
> Interface example: suppose a process with tgid 1000 has additional threads
> with ids 1001 and 1002. The tasks file will show ids 1000, 1001, and 1002, and
> the procs file will only show id 1000.
> 

I think a better demonstration is:

  $ cat tasks
  1000
  1001
  1002

  $ cat procs
  1000

> Possible future functionality is making the procs file writable for purposes
> of adding all threads with the same tgid at once.
> 
> Signed-off-by: Ben Blum <bblum at google.com>
> 
> ---
> 
>  include/linux/cgroup.h |   22 ++--
>  kernel/cgroup.c        |  282 ++++++++++++++++++++++++++++++------------------
>  2 files changed, 190 insertions(+), 114 deletions(-)
...
> +/* is the size difference enough that we should re-allocate the array? */
> +#define PIDLIST_REALLOC_DIFFERENCE(old,new) ((old) - PAGE_SIZE >= (new))
> +static int pidlist_uniq(pid_t **p, int length)
> +{
> +	int src, dest = 1;
> +	pid_t *list = *p;
> +	pid_t *newlist;
> +
> +	/*
> +	 * we presume the 0th element is unique, so i starts at 1. trivial
> +	 * edge cases first; no work needs to be done for either
> +	 */
> +	if (length == 0 || length == 1)
> +		return length;
> +	/* src and dest walk down the list; dest counts unique elements */
> +	for (src = 1; src < length; src++) {
> +		/* find next unique element */
> +		while (list[src] == list[src-1]) {
> +			src++;
> +			if (src == length)
> +				break;

'goto' is better

> +		}
> +		if (src == length)
> +			break;
> +		/* dest always points to where the next unique element goes */
> +		list[dest] = list[src];
> +		dest++;
> +	}
> +	/*
> +	 * if the length difference is large enough, we want to allocate a
> +	 * smaller buffer to save memory. if this fails due to out of memory,
> +	 * we'll just stay with what we've got.
> +	 */
> +	if (PIDLIST_REALLOC_DIFFERENCE(length, dest)) {
> +		newlist = kmalloc(dest * sizeof(pid_t), GFP_KERNEL);

krealloc()

> +		if (newlist) {
> +			memcpy(newlist, list, dest * sizeof(pid_t));
> +			kfree(list);
> +			*p = newlist;
> +		}
> +	}
> +	return dest;
> +}