linux-kernel - Re: [PATCH] prctl: Add PR_SET_TIMERSLACK_PID for setting timer slack of an arbitrary thread.

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Message-Id: <20160205121353.63d65c6b33a153bd9f863d8b@linux-foundation.org>
Date:	Fri, 5 Feb 2016 12:13:53 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Ruchi Kandoi <kandoiruchi@...gle.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oren Laadan <orenl@...lrox.com>,
	Rom Lemarchand <romlem@...roid.com>,
	Kees Cook <keescook@...omium.org>,
	Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH] prctl: Add PR_SET_TIMERSLACK_PID for setting timer
 slack of an arbitrary thread.

On Fri,  5 Feb 2016 10:08:43 -0800 John Stultz <john.stultz@...aro.org> wrote:

> From: Ruchi Kandoi <kandoiruchi@...gle.com>
> 
> This allows power/performance management software to set timer
> slack for other threads according to its policy for the thread
> (such as when the thread is designated foreground vs. background
> activity)
> 
> Second argument is similar to PR_SET_TIMERSLACK, if non-zero
> then the slack is set to that value otherwise sets it to the
> default for the thread.
> 
> Takes PID of the thread as the third argument.
> 
> This interface checks that the calling task has permissions to
> to use PTRACE_MODE_ATTACH on the target task, so that we can
> ensure arbitrary apps do not change the timer slack for other
> apps.
> 
> Additional fixes from Ruchi and Micha Kalfon <micha@...lrox.com>
> have been folded into this patch to make it easier to reivew.
> 
> ...
>
> @@ -2218,6 +2222,27 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>  	case PR_GET_TID_ADDRESS:
>  		error = prctl_get_tid_address(me, (int __user **)arg2);
>  		break;
> +	case PR_SET_TIMERSLACK_PID:
> +		rcu_read_lock();
> +		tsk = find_task_by_vpid((pid_t)arg3);

hm, as far as I can tell this is the first instance in which prctl() is
used to play with a task other than "current".  Maybe this isn't a good
precedent.

If you look at all the other diddle-other-task functions in
kernel/sys.c, you'll see that they are standalone syscalls.  What
you've done here is just a bit lazy: added what is effectively a new
standalone syscall, only it has been hidden within the prctl() switch
statement.

I don't see a practical problem with this - we could have implemented
all those other syscalls as prctl submodes as well.  But we didn't...

IOW, it would be more consistent to add sys_set_timer_slack()?