linux-kernel - Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

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Date:	Wed, 28 Feb 2007 22:23:06 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Ulrich Drepper <drepper@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@....com.au>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Zach Brown <zach.brown@...cle.com>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	"David S. Miller" <davem@...emloft.net>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

* Davide Libenzi <davidel@...ilserver.org> wrote:

> On Wed, 28 Feb 2007, Ingo Molnar wrote:
> 
> > 
> > * Davide Libenzi <davidel@...ilserver.org> wrote:
> > 
> > > My point is, the syslet infrastructure is expensive for the kernel in 
> > > terms of compat, [...]
> > 
> > it is not. Today i've implemented 64-bit syslets on x86_64 and 
> > 32-bit-on-64-bit compat syslets. Both the 64-bit and the 32-bit syslet 
> > (and threadlet) binaries work just fine on a 64-bit kernel, and they 
> > share 99% of the infrastructure. There's only a single #ifdef 
> > CONFIG_COMPAT in kernel/async.c:
> > 
> > #ifdef CONFIG_COMPAT
> > 
> > asmlinkage struct syslet_uatom __user *
> > compat_sys_async_exec(struct syslet_uatom __user *uatom,
> >                       struct async_head_user __user *ahu)
> > {
> >         return __sys_async_exec(uatom, ahu, &compat_sys_call_table,
> >                                 compat_NR_syscalls);
> > }
> > 
> > #endif
> 
> Did you hide all the complexity of the userspace atom decoding inside 
> another function? :)

no, i made the 64-bit and 32-bit structures layout-compatible. This 
makes the 32-bit structure as large as the 64-bit ones, but that's not a 
big issue, compared to the simplifications it brings.

> > But i'm happy to change the syslet API in any sane way, and did so 
> > based on feedback from Jens who is actually using them.
> 
> Wouldn't you agree on a simple/parallel execution engine [...]

the thing is, there's almost zero overhead from having those basic 
things like conditions and the ->next link, and they make it so much 
more capable. As usual my biggest problem is that you are not trying to 
use syslets at all - you are only trying to get rid of them ;-) My 
purpose with syslets is to enable a syslet to do almost anything that 
user-space could do too, as simply as possible. Syslets could even 
allocate user-space memory and then use it (i dont think we actually 
want to do that though). That doesnt mean arbitrary complex code 
/should/ be done via syslets, or that it wont be significantly slower 
than what user-space can do, but i'd not like to artificially dumb the 
engine down. I'm totally willing to simplify/shrink the vectoring of 
arguments and just about anything else, but your proposals so far (such 
as your return-value-embedded-in-atom suggestion) all kill important 
aspects of the engine.

All the existing syslet features were purpose-driven: i actually coded 
up a sample syslet, trying to do something that makes sense, and added 
these features based on that. The engine core takes up maybe 50 lines of 
code.

	Ingo
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