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Date: Fri, 5 Feb 2016 12:32:30 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: John Stultz <john.stultz@linaro.org>
Cc: lkml <linux-kernel@vger.kernel.org>, Ruchi Kandoi <kandoiruchi@google.com>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>, Oren Laadan <orenl@cellrox.com>,
        Rom Lemarchand <romlem@android.com>, Kees Cook <keescook@chromium.org>,
        Android Kernel Team <kernel-team@android.com>
Subject: Re: [PATCH] prctl: Add PR_SET_TIMERSLACK_PID for setting timer
 slack of an arbitrary thread.
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On Fri, 5 Feb 2016 12:23:13 -0800 John Stultz <john.stultz@linaro.org> wrote:

> On Fri, Feb 5, 2016 at 12:13 PM, Andrew Morton
> <akpm@linux-foundation.org> wrote:
> > On Fri,  5 Feb 2016 10:08:43 -0800 John Stultz <john.stultz@linaro.org> wrote:
> >> @@ -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()?
> 
> I'm fine with moving this way.
> 
> Ruchi/Rom: Any objections to that idea?
> 
> Thomas/Arjan: Any other functionality we should consider including
> when adding a syscall to tweak timer slack?

A syscall is quite a bit more fuss - implement it on x86_64, provide a
no-op default in sys_ni.c, add a test suite into
tools/testing/selftests (mainly for arch maintainers), wait for the
various arch maintainers to wire it up.

Fortunately the build system now emits little messages which tell
maintainers that there's a new syscall which needs looking at.

And a manpage will be needed, but a prctl manpage patch would have been
needed anyway.