On Mon, Oct 19, 2020 at 6:45 PM Joel Fernandes (Google)
<j...@joelfernandes.org> wrote:
>
> +static unsigned long cpu_core_get_group_cookie(struct task_group *tg)
> +{
> + unsigned long color = 0;
> +
> + if (!tg)
> + return 0;
> +
> + for (; tg; tg = tg->parent) {
> + if (tg->core_tag_color) {
> + WARN_ON_ONCE(color);
> + color = tg->core_tag_color;
> + }
> +
> + if (tg->core_tagged) {
> + unsigned long cookie = ((unsigned long)tg << 8) |
> color;
> + cookie &= (1UL << (sizeof(unsigned long) * 4)) - 1;
> + return cookie;
> + }
> + }
> +
> + return 0;
> +}
I'm a bit wary of how core_task_cookie and core_group_cookie are
truncated to the lower half of their bits and combined into the
overall core_cookie. Now that core_group_cookie is further losing 8
bits to color, that leaves (in the case of 32 bit unsigned long) only
8 bits to uniquely identify the group contribution to the cookie.
Also, I agree that 256 colors is likely adequate, but it would be nice
to avoid this restriction.
I'd like to propose the following alternative, which involves creating
a new struct to represent the core cookie:
struct core_cookie {
unsigned long task_cookie;
unsigned long group_cookie;
unsigned long color;
/* can be further extended with arbitrary fields */
struct rb_node node;
refcount_t;
};
struct rb_root core_cookies; /* (sorted), all active core_cookies */
seqlock_t core_cookies_lock; /* protects against removal/addition to
core_cookies */
struct task_struct {
...
unsigned long core_cookie; /* (struct core_cookie *) */
}
A given task stores the address of a core_cookie struct in its
core_cookie field. When we reconfigure a task's
color/task_cookie/group_cookie, we can first look for an existing
core_cookie that matches those settings, or create a new one.
