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Date:   Fri, 5 Feb 2021 12:52:32 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
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Subject: Re: [PATCH v10 2/5] sched: CGroup tagging interface for core
 scheduling
Message-ID: <YB0xgL5x1RM7DJ49@hirez.programming.kicks-ass.net>
References: <20210123011704.1901835-1-joel@joelfernandes.org>
 <20210123011704.1901835-3-joel@joelfernandes.org>
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On Fri, Jan 22, 2021 at 08:17:01PM -0500, Joel Fernandes (Google) wrote:

> +/* All active sched_core_cookies */
> +static struct rb_root sched_core_cookies = RB_ROOT;
> +static DEFINE_RAW_SPINLOCK(sched_core_cookies_lock);

> +/*
> + * Returns the following:
> + * a < b  => -1
> + * a == b => 0
> + * a > b  => 1
> + */
> +static int sched_core_cookie_cmp(const struct sched_core_cookie *a,
> +				 const struct sched_core_cookie *b)
> +{
> +#define COOKIE_CMP_RETURN(field) do {		\
> +	if (a->field < b->field)		\
> +		return -1;			\
> +	else if (a->field > b->field)		\
> +		return 1;			\
> +} while (0)					\
> +
> +	COOKIE_CMP_RETURN(task_cookie);
> +	COOKIE_CMP_RETURN(group_cookie);
> +
> +	/* all cookie fields match */
> +	return 0;
> +
> +#undef COOKIE_CMP_RETURN
> +}

AFAICT all this madness exists because cgroup + task interaction, yet
none of that code is actually dependent on cgroups being on.

So this seems to implement semantics that will make two tasks that share
a cookie, but are then placed in different cgroups not actually share.

Is that desired? Can we justify these semantics and the resulting code
complexity.