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Date:   Wed, 8 May 2019 21:21:31 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Patrick Bellasi <patrick.bellasi@arm.com>
Cc:     linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        linux-api@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Tejun Heo <tj@kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Paul Turner <pjt@google.com>,
        Quentin Perret <quentin.perret@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Todd Kjos <tkjos@google.com>,
        Joel Fernandes <joelaf@google.com>,
        Steve Muckle <smuckle@google.com>,
        Suren Baghdasaryan <surenb@google.com>
Subject: Re: [PATCH v8 05/16] sched/core: Allow sched_setattr() to use the
 current policy
Message-ID: <20190508192131.GD32547@worktop.programming.kicks-ass.net>
References: <20190402104153.25404-1-patrick.bellasi@arm.com>
 <20190402104153.25404-6-patrick.bellasi@arm.com>
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On Tue, Apr 02, 2019 at 11:41:41AM +0100, Patrick Bellasi wrote:
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index 22627f80063e..075c610adf45 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -40,6 +40,8 @@
>  /* SCHED_ISO: reserved but not implemented yet */
>  #define SCHED_IDLE		5
>  #define SCHED_DEADLINE		6
> +/* Must be the last entry: used to sanity check attr.policy values */
> +#define SCHED_POLICY_MAX	SCHED_DEADLINE

This is a wee bit sad to put in a uapi header; but yeah, where else :/

Another option would be something like:

enum {
	SCHED_NORMAL = 0,
	SCHED_FIFO = 1,
	SCHED_RR = 2,
	SCHED_BATCH = 3,
	/* SCHED_ISO = 4, reserved */
	SCHED_IDLE = 5,
	SCHED_DEADLINE = 6,
	SCHED_POLICY_NR
};

>  /* Can be ORed in to make sure the process is reverted back to SCHED_NORMAL on fork */
>  #define SCHED_RESET_ON_FORK     0x40000000
> @@ -50,9 +52,11 @@
>  #define SCHED_FLAG_RESET_ON_FORK	0x01
>  #define SCHED_FLAG_RECLAIM		0x02
>  #define SCHED_FLAG_DL_OVERRUN		0x04
> +#define SCHED_FLAG_KEEP_POLICY		0x08
>  
>  #define SCHED_FLAG_ALL	(SCHED_FLAG_RESET_ON_FORK	| \
>  			 SCHED_FLAG_RECLAIM		| \
> -			 SCHED_FLAG_DL_OVERRUN)
> +			 SCHED_FLAG_DL_OVERRUN		| \
> +			 SCHED_FLAG_KEEP_POLICY)
>  
>  #endif /* _UAPI_LINUX_SCHED_H */
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d368ac26b8aa..20efb32e1a7e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4907,8 +4907,17 @@ SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr,
>  	if (retval)
>  		return retval;
>  
> -	if ((int)attr.sched_policy < 0)
> +	/*
> +	 * A valid policy is always required from userspace, unless
> +	 * SCHED_FLAG_KEEP_POLICY is set and the current policy
> +	 * is enforced for this call.
> +	 */
> +	if (attr.sched_policy > SCHED_POLICY_MAX &&
> +	    !(attr.sched_flags & SCHED_FLAG_KEEP_POLICY)) {
>  		return -EINVAL;
> +	}

And given I just looked at those darn SCHED_* things, I now note the
above does 'funny' things when passed: attr.policy=4.

> +	if (attr.sched_flags & SCHED_FLAG_KEEP_POLICY)
> +		attr.sched_policy = SETPARAM_POLICY;
>  
>  	rcu_read_lock();
>  	retval = -ESRCH;