linux-kernel - Re: [PATCH 0 of 4] Generic AIO by scheduling stacks

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Date:	Wed, 31 Jan 2007 16:51:48 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	unlisted-recipients:; (no To-header on input)
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Zach Brown <zach.brown@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-aio@...ck.org, Suparna Bhattacharya <suparna@...ibm.com>,
	Benjamin LaHaise <bcrl@...ck.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 0 of 4] Generic AIO by scheduling stacks

Nick Piggin wrote:
> Linus Torvalds wrote:
> 
>>
>> On Wed, 31 Jan 2007, Benjamin Herrenschmidt wrote:
>>
>>
>>>> - We would now have some measure of task_struct concurrency.  Read 
>>>> that twice,
>>>> it's scary.  As two fibrils execute and block in turn they'll each be
>>>> referencing current->.  It means that we need to audit task_struct 
>>>> to make sure
>>>> that paths can handle racing as its scheduled away.  The current 
>>>> implementation
>>>> *does not* let preemption trigger a fibril switch.  So one only has 
>>>> to worry
>>>> about racing with voluntary scheduling of the fibril paths.  This 
>>>> can mean
>>>> moving some task_struct members under an accessor that hides them in 
>>>> a struct
>>>> in task_struct so they're switched along with the fibril.  I think 
>>>> this is a
>>>> manageable burden.
>>>
>>>
>>> That's the one scaring me in fact ... Maybe it will end up being an easy
>>> one but I don't feel too comfortable...
>>
>>
>>
>> We actually have almost zero "interesting" data in the task-struct.
>>
>> All the real meat of a task has long since been split up into 
>> structures that can be shared for threading anyway (ie 
>> signal/files/mm/etc).
>>
>> Which is why I'm personally very comfy with just re-using task_struct 
>> as-is.
>>
>> NOTE! This is with the understanding that we *never* do any 
>> preemption. The whole point of the microthreading as far as I'm 
>> concerned is exactly that it is cooperative. It's not preemptive, and 
>> it's emphatically *not* concurrent (ie you'd never have two fibrils 
>> running at the same time on separate CPU's).
> 
> 
> So using stacks to hold state is (IMO) the logical choice to do async
> syscalls, especially once you have a look at some of the other AIO
> stuff going around.
> 
> I always thought that the AIO people didn't do this because they wanted
> to avoid context switch overhead.
> 
> So now if we introduce the context switch overhead back, why do we need
> just another scheduling primitive? What's so bad about using threads? The
> upside is that almost everything is already there and working, and also
> they don't have any of these preemption or concurrency restrictions.

In other words, while I share the appreciation for this clever trick of
using cooperative switching between these little thriblets, I don't
actually feel it is very elegant to then have to change the kernel so
much in order to handle them.

I would be fascinated to see where such a big advantage comes from using
these rather than threads. Maybe we can then improve threads not to suck
so much and everybody wins.

-- 
SUSE Labs, Novell Inc.
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