linux-kernel - Re: [RFCv2 1/6] sched/core: Add manual jitter classification from cgroup interface

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Message-ID: <20190515162924.GT2589@hirez.programming.kicks-ass.net>
Date:   Wed, 15 May 2019 18:29:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Parth Shah <parth@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        mingo@...hat.com, dietmar.eggemann@....com, dsmythies@...us.net
Subject: Re: [RFCv2 1/6] sched/core: Add manual jitter classification from
 cgroup interface

On Wed, May 15, 2019 at 07:23:17PM +0530, Parth Shah wrote:

> Subject: [RFCv2 1/6] sched/core: Add manual jitter classification from cgroup interface

How can this be v2 ?! I've never seen v1.

> Jitter tasks are usually of less important in terms of performance
> and are short/bursty in characteristics. TurboSched uses this jitter
> classification to pack jitters into the already running busy cores to
> keep the total idle core count high.
> 
> The patch describes the use of UCLAMP mechanism to classify tasks. Patrick
> Bellasi came up with a mechanism to classify tasks from the userspace
> https://lore.kernel.org/lkml/20190402104153.25404-1-patrick.bellasi@arm.com/

The canonical form is:

	https://lkml.kernel.org/r/$MSGID

> This UCLAMP mechanism can be useful in classifying tasks as jitter. Jitters
> can be classified for the cgroup by keeping util.max of the tasks as the
> least(=0). This also provides benefit of giving the least frequency to
> those jitter tasks, which is useful if all jitters are packed onto a
> separate core.
> 
> Use Case with UCLAMP
> ===================
> To create a cgroup with all the tasks classified as jitters;
> 
> ```
> mkdir -p /sys/fs/cgroup/cpu/jitter
> echo 0 > /proc/sys/kernel/sched_uclamp_util_min;
> echo 0 > /sys/fs/cgroup/cpu/jitter/cpu.util.min;
> echo 0 > /sys/fs/cgroup/cpu/jitter/cpu.util.max;
> i=8;
> ./turbo_bench -t 30 -h $i -n $i &
> ./turbo_bench -t 30 -h 0 -n $i &
> echo $! > /sys/fs/cgroup/cpu/jitter/cgroup.procs;
> ```
> 
> Signed-off-by: Parth Shah <parth@...ux.ibm.com>
> ---
>  kernel/sched/core.c  | 9 +++++++++
>  kernel/sched/sched.h | 1 +
>  2 files changed, 10 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d42c0f5eefa9..77aa4aee4478 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7192,6 +7192,15 @@ static int cpu_util_max_write_u64(struct cgroup_subsys_state *css,
>  	tg->uclamp_req[UCLAMP_MAX].value = max_value;
>  	tg->uclamp_req[UCLAMP_MAX].bucket_id = uclamp_bucket_id(max_value);
>  
> +	/*
> +	 * Classify the tasks belonging to the last bucket of MAX UCLAMP as
> +	 * jitters
> +	 */
> +	if (uclamp_bucket_id(max_value) == 0)
> +		tg->turbo_sched_enabled = 1;
> +	else if (tg->turbo_sched_enabled)
> +		tg->turbo_sched_enabled = 0;
> +
>  	/* Update effective clamps to track the most restrictive value */
>  	cpu_util_update_eff(css, UCLAMP_MAX);
>  
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index b4019012d84b..e75ffaf3ff34 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -407,6 +407,7 @@ struct task_group {
>  	struct uclamp_se	uclamp[UCLAMP_CNT];
>  #endif
>  
> +	bool			turbo_sched_enabled;
>  };

Your simple patch has 3 problems:

 - it limits itself; for no apparent reason; to the cgroup interface.

 - it is inconsistent in the terminology; pick either jitter or
   turbo-sched, and I think the latter is a horrid name, it wants to be
   'pack' or something similar. Also, jitter really doesn't make sense
   given the classification.

 - you use '_Bool' in a composite type.