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SubjectRe: [PATCH 1/3] Adds a read-only "procs" file similar to "tasks" that shows only unique tgids
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@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;
> +}


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