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

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Message-Id: <21665A89-1BEE-459D-8ED1-ACCB352CF8A4@mac.com>
Date:	Sat, 24 Feb 2007 17:08:47 -0500
From:	Kyle Moffett <mrmacman_g4@....com>
To:	Davide Libenzi <davidel@...ilserver.org>
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 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.

Cheers,
Kyle Moffett
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