linux-kernel - Re: [patch 1/2] add ALL_CPUS option to stop_machine

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Date:	Fri, 29 Feb 2008 11:58:03 -0800
From:	Max Krasnyanskiy <maxk@...lcomm.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Jason Baron <jbaron@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [patch 1/2] add ALL_CPUS option to stop_machine_run()

Ingo Molnar wrote:
> * Max Krasnyanskiy <maxk@...lcomm.com> wrote:
> 
>> btw Being an RT guy you do not think that stop machine is evil ? [...]
> 
> i'm not "an RT guy", -rt is just one of the many projects i've been 
> involved with.
> 
> and no, i dont think stop machine is "evil" - it's currently the best 
> way to do certain things. If you can solve it better then sure, i'm 
> awaiting your patches - but the only patch i saw from you so far was the 
> one that turned off stop-machine for isolated cpus - which was 
> incredibly broken and ignored the problem altogether.
Ingo, I got it. My patch was a hack. Moving on. Seriously there is no need to 
say it ten thousand times ;-).

You clipped the part where I elaborated what exactly is evil about the stop 
machine. I clearly said that yes for some things there is just no other way 
but in general we should _try_ to avoid it. Note that I did not say "we must" 
I'm saying we should try.

> Right now the answer is: "if you want to do hard RT then avoid doing 
> things like loading modules". (which you should avoid while doing 
> hard-RT anyway)
That's just not practical. Sure you can have some kind of stripped down 
machine but then you loose a lot of flexibility. Again "should" is the keyword 
here. For a lot of workloads hard-RT has to coexist with a bunch of other things.

Max

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