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From:   Josh Don <joshdon@google.com>
Date:   Fri, 30 Oct 2020 17:42:12 -0700
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Subject: Re: [PATCH v8 -tip 19/26] sched: Add a second-level tag for nested
 CGroup usecase
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
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On Mon, Oct 19, 2020 at 6:45 PM Joel Fernandes (Google)
<joel@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.