linux-kernel - Re: [v7 0/8] Reduce cross CPU IPI interference

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Message-ID: <CAOtvUMeaL8-9wYYrC-05P95ZcYHnV5as0bjjv9gJSHKN+EFz0w@mail.gmail.com>
Date:	Fri, 10 Feb 2012 22:39:02 +0200
From:	Gilad Ben-Yossef <gilad@...yossef.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Chris Metcalf <cmetcalf@...era.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Christoph Lameter <cl@...ux.com>,
	linux-mm@...ck.org, Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>,
	Sasha Levin <levinsasha928@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Andi Kleen <andi@...stfloor.org>, Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Avi Kivity <avi@...hat.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Kosaki Motohiro <kosaki.motohiro@...il.com>,
	Milton Miller <miltonm@....com>
Subject: Re: [v7 0/8] Reduce cross CPU IPI interference

On Fri, Feb 10, 2012 at 10:29 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Fri, 2012-02-10 at 22:13 +0200, Gilad Ben-Yossef wrote:
>> My current understanding is that if I have a real time task and wish it
>> have a deterministic performance time, you should call mlockall() to lock
>> the program data and text into physical memory so that  a  less often taken
>> branch or access to a new data region will not result in a page fault.
>>
>> You still have to worry about TLB misses on non hardware page table
>> walk architecture, but at least everything is in the  page tables
>>
>> If there is a better way to do this? I'm always happy to learn new
>> ways to do things. :-)
>
> A rt application usually consists of a lot of non-rt parts and a usually
> relatively small rt part. Using mlockall() pins the entire application
> into memory, which while on the safe side is very often entirely too
> much.
>
> The alternative method is to only mlock the text and data used by the rt
> part. You need to be aware of what text runs in your rt part anyway,
> since you need to make sure it is in fact deterministic code.
>
> One of the ways of achieving this is using a special linker section for
> your vetted rt code and mlock()'ing only that text section.
>
> On thread creation, provide a custom allocated (and mlock()'ed) stack
> etc..
>
> Basically, if you can't tell a-priory what code is part of your rt part,
> you don't have an rt part ;-)
>

That I can totally agree with.

I guess mlockall() is still useful as a kind of hack for lazy people,
although if you say that this kind of laziness does not really mix
well with real
time programming I will tend to agree... :-)

Gilad



-- 
Gilad Ben-Yossef
Chief Coffee Drinker
gilad@...yossef.com
Israel Cell: +972-52-8260388
US Cell: +1-973-8260388
http://benyossef.com

"If you take a class in large-scale robotics, can you end up in a
situation where the homework eats your dog?"
 -- Jean-Baptiste Queru
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