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 18:54:33 -0800
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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.

>> 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.
>
> So I'd think it would be mostly theoretical, but in my testing on a
> VM, setting the timerslack for bash to 10 secs made time sleep 1 take
> ~10.5 seconds. So its apparently not too hard to coalesce fairly far
> out (I need to spend a bit more time to verify that events really
> weren't happening during that time and we're not just doing
> unnecessary delays with the extra slack).

99% sure you're hitting something else;
we look pretty much only 1 ahead in the queue for timers to run to see if
they can be run, once we hit a timer that's not ready yet we stop.
your 10 second ahead is behind a whole bunch of other not-ready ones
so won't even be looked at until its close


> But yea. My main concern is that if we do a consistent 64bit interface
> for all arches in the /proc/<pid>/timerslack_ns interface, it will
> make PR_GET_TIMERSLACK return incorrect results on 32bit systems when
> the slack is >= 2^32.

or we return UINT_MAX for that case. not too hard.