linux-kernel - Re: [PATCH v10 2/5] sched: CGroup tagging interface for core scheduling

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Date:   Fri, 5 Feb 2021 12:52:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        torvalds@...ux-foundation.org, fweisbec@...il.com,
        keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        pjt@...gle.com, rostedt@...dmis.org, derkling@...gle.com,
        benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
        chris.hyser@...cle.com, Ben Segall <bsegall@...gle.com>,
        Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v10 2/5] sched: CGroup tagging interface for core
 scheduling

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.