linux-kernel - Re: [patch 04/13] syslets: core code

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Date:	Sat, 24 Feb 2007 14:25:18 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Kyle Moffett <mrmacman_g4@....com>
cc:	Ingo Molnar <mingo@...e.hu>,
	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>,
	Ulrich Drepper <drepper@...hat.com>,
	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 04/13] syslets: core code

On Sat, 24 Feb 2007, Kyle Moffett wrote:

> On Feb 24, 2007, at 16:10:33, Davide Libenzi wrote:
> > On Sat, 24 Feb 2007, Ingo Molnar wrote:
> > > the on/off calls are shaped in a way that makes them ultimately
> > > vsyscall-able - the kernel only needs to know about the fact that we are
> > > in a threadlet (so that the scheduler can do its special
> > > push-head-to-another-context thing) - and this can be signalled via a
> > > small user-space-side info structure as well, put into the TLS.
> > 
> > IMO it's not a matter of speed. We'll have those two new syscalls, that I
> > don't see other practical use for. IMO the best thing would be to hide all
> > inside the sys_threadlet_exec (or whatever name).
> 
> No, it absolutely is a matter of speed.  The reason to have those two
> implemented that way is so that they can be implemented as vsyscalls
> completely in userspace.  This means that on most modern platforms you can
> implement the "make a threadlet when I block" semantic without even touching
> kernel-mode.  The way it's set up all you'd have to do is save some
> parameters, set up a new callstack, and poke a "1" into a memory address in
> the TLS.  To stop, you effectively just poke a "0" into the spot in the TLS
> and either return or terminate the thread.

Right. I don't why but I got the implression Ingo's threadlet_exec example 
was just sketch code to be moved in a syscall. That's why I was talking 
about a sys_threadlet_exec. But yeah, it makes a lot of sense to turn 
threadlet_exec in a glibc thing, and play everything in userspace at that 
point.



- Davide


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