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From: Andy Lutomirski <luto@amacapital.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: David Ahern <dsahern@gmail.com>, Ingo Molnar <mingo@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	tglx@linutronix.de
Subject: Re: deadlock in scheduler enabling HRTICK feature
Date: Thu, 27 Jun 2013 12:18:04 -0700	[thread overview]
Message-ID: <51CC8FEC.8030605@mit.edu> (raw)
In-Reply-To: <20130627105303.GD27378@dyad.programming.kicks-ass.net>

On 06/27/2013 03:53 AM, Peter Zijlstra wrote:
> On Thu, Jun 27, 2013 at 12:43:09PM +0200, Peter Zijlstra wrote:
>> On Wed, Jun 26, 2013 at 10:46:33AM -0600, David Ahern wrote:
>>> On 6/26/13 1:05 AM, Peter Zijlstra wrote:
>>>>> What is the expectation that the feature provides? not a whole lot of
>>>>> documentation on it. I walked down the path wondering if it solved an odd
>>>>> problem we are seeing with the CFS in 2.6.27 kernel.
>>>>
>>>> Its supposed to use hrtimers for slice expiry instead of the regular tick.
>>>
>>> So theoretically CPU bound tasks would get preempted sooner? That was my
>>> guess/hope anyways.
>>
>> Doth the below worketh?
>>
> 
> Related to all this; the reason its not enabled by default is that mucking
> about with hrtimers all the while is god awful expensive.
> 
> I've had ideas about making this a special purpose 'hard-coded' timer in the
> hrtimer guts that's only ever re-programmed when the new value is sooner.
> 
> By making it a 'special' timer we can avoid the whole rb-tree song and dance;
> and by taking 'spurious' short interrupts we can avoid prodding the hardware
> too often.

Supposedly, on really new x86 systems, the TSC deadline timer is so fast
to reprogram that this isn't worth it.

I wonder if the general timing code should have a way to indicate that a
given clockevent is (a) very fast and (b) is actually locked to a
clocksource as opposed to just having a vaguely accurately calibrated
frequency.

--Andy

  parent reply	other threads:[~2013-06-27 19:18 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-06-25 21:05 David Ahern
2013-06-25 21:17 ` Peter Zijlstra
2013-06-25 21:20   ` David Ahern
2013-06-26  7:05     ` Peter Zijlstra
2013-06-26 16:46       ` David Ahern
2013-06-27 10:43         ` Peter Zijlstra
2013-06-27 10:53           ` Peter Zijlstra
2013-06-27 12:28             ` Mike Galbraith
2013-06-27 13:06             ` Ingo Molnar
2013-06-27 19:18             ` Andy Lutomirski [this message]
2013-06-27 20:37               ` Peter Zijlstra
2013-06-27 22:28           ` David Ahern
2013-06-28  9:00             ` Ingo Molnar
2013-06-28  9:18               ` Peter Zijlstra
2013-07-12 13:29                 ` [tip:sched/core] sched: Fix HRTICK tip-bot for Peter Zijlstra
2013-06-28  9:09             ` deadlock in scheduler enabling HRTICK feature Peter Zijlstra
2013-06-28 17:28               ` David Ahern

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