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

[Andrew Morton <akpm@linux-foundation.org>]

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

> From: Ruchi Kandoi <kandoiruchi@google.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@cellrox.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()?