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

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Message-ID: <47C84DCD.5040803@qualcomm.com>
Date:	Fri, 29 Feb 2008 10:24:13 -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:
> 
>>> -allow stop_mahcine_run() to call a function on all cpus. Calling 
>>> stop_machine_run() with a 'ALL_CPUS' invokes this new behavior.
>>>  stop_machine_run() proceeds as normal until the calling cpu has 
>>>  invoked 'fn'. Then, we tell all the other cpus to call 'fn'.
>> Jason, we're actually trying to reduce the usage of the stop_machine 
>> in general. [...]
> 
> please talk in your own name. Stop-machine is a very elegant tool that 
> simplifies a lot of hard things in the kernel and is reasonably fast as 
> well. We've just recently added two new usages of it and more are 
> planned.
> 
> _you_ might be the one who wants to 'reduce the usage of stop_machine' - 
> but that means it is _you_ who first has to convert a number of very 
> difficult pieces of code to "something else".
Sure I started the discussion but I suppose you missed Andi's and other 
replies. All I said that people should think twice before relying on it.
btw I'm ok if I _am_ the _one_ who has to convert those pieces of code, that's 
part of the fun :). But if people keep adding stuff which uses stom_machine 
that may be pretty difficult :).

btw Being an RT guy you do not think that stop machine is evil ? I mean from 
the overhead and especially latency perspective. By overhead I mean if you 
have 100+ cpu box that Paul and other guys have mentioned here. Every single 
CPU has to be frozen. You said it's reasonably fast. I guess it depends what's 
reasonable. And from the latency perspective all bets are off. We have no 
guaranties whatsoever as to hold long it will take for cpu X to get frozen 
(there numerous factors here) and all the other cpus have to wait for it.
As I said for some things there is just no other way but to use the 
stop_machine but we should try to minimize that as much as possible.

Max

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