linux-kernel - Re: [PATCH RFC 3/8] sched/cpufreq_schedutil: make worker kthread be SCHED

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Date:   Tue, 23 May 2017 20:52:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <juri.lelli@....com>
Cc:     mingo@...hat.com, rjw@...ysocki.net, viresh.kumar@...aro.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        tglx@...utronix.de, vincent.guittot@...aro.org,
        rostedt@...dmis.org, luca.abeni@...tannapisa.it,
        claudio@...dence.eu.com, tommaso.cucinotta@...tannapisa.it,
        bristot@...hat.com, mathieu.poirier@...aro.org, tkjos@...roid.com,
        joelaf@...gle.com, andresoportus@...gle.com,
        morten.rasmussen@....com, dietmar.eggemann@....com,
        patrick.bellasi@....com, Ingo Molnar <mingo@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH RFC 3/8] sched/cpufreq_schedutil: make worker kthread be
 SCHED_DEADLINE

On Tue, May 23, 2017 at 09:53:46AM +0100, Juri Lelli wrote:
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index e2a6c7b3510b..72723859ef74 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -48,5 +48,6 @@
>   */
>  #define SCHED_FLAG_RESET_ON_FORK	0x01
>  #define SCHED_FLAG_RECLAIM		0x02
> +#define SCHED_FLAG_SPECIAL		0x04
>  
>  #endif /* _UAPI_LINUX_SCHED_H */
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7fc2011c3ce7..ba57e2ef9aef 100644

> @@ -4205,7 +4212,9 @@ static int __sched_setscheduler(struct task_struct *p,
>  	}
>  
>  	if (attr->sched_flags &
> -		~(SCHED_FLAG_RESET_ON_FORK | SCHED_FLAG_RECLAIM))
> +		~(SCHED_FLAG_RESET_ON_FORK |
> +		  SCHED_FLAG_RECLAIM |
> +		  SCHED_FLAG_SPECIAL))
>  		return -EINVAL;
>  
>  	/*

Could we pretty please not expose this gruesome hack to userspace?

So if you stick it in attr->sched_flags, use a high bit and don't put it
in a uapi header. Also make the flags check explicitly fail on it when
@user. Such that only _nocheck() (and thus kernel) callers have access
to it.

Also, there's not nearly enough warnings and other derisory comments
near it.