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

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Date:	Fri, 5 Feb 2016 18:15:47 -0800
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	John Stultz <john.stultz@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Kees Cook <keescook@...omium.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Ruchi Kandoi <kandoiruchi@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oren Laadan <orenl@...lrox.com>,
	Rom Lemarchand <romlem@...roid.com>,
	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 2/5/2016 4:51 PM, John Stultz wrote:
> On Fri, Feb 5, 2016 at 2:35 PM, John Stultz <john.stultz@...aro.org> wrote:
>> On Fri, Feb 5, 2016 at 12:50 PM, Andrew Morton
>> <akpm@...ux-foundation.org> wrote:
>>> On Fri, 5 Feb 2016 12:44:04 -0800 Kees Cook <keescook@...omium.org> wrote:
>>>> Could this be exposed as a writable /proc entry instead? Like the oom_* stuff?
>>>
>>> /proc/<pid>/timer_slack_ns, guarded by ptrace_may_access(), documented
>>> under Documentation/?  Yup, that would work.  It's there for all
>>> architectures from day one and there is precedent.  It's not as nice,
>>> but /proc nasties will always be with us.
>>
>> Ok. I'll start working on that.
>
> Arjan/Thomas:  One curious thing I noticed here while writing some
> documentation. The timer_slack_ns value in the task struct is a
> unsigned long.
>
> So this means PR_SET_TIMERSLACK limits the maximum slack on 32 bit
> machines to ~4 seconds. Where on 64bit machines it can be quite a bit
> longer (unreasonably long, really :).

originally when we created timerslack, 4 seconds was an eternity and good enough for everyone
by a mile... (assumption was practical upper limit being in the 15 msec range)
and most of the RT guys would only tolerate a little bit of it

is there any real/practial use of going longer than 4 seconds? if there
is then yeah fixing it makes sense.
if it's just theoretical... shrug... 32 bit systems have a bunch of
other limits/differences a well.