linux-kernel - Re: [patch 06/11] syslets: core, documentation

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Date:	Wed, 14 Feb 2007 12:14:29 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Benjamin LaHaise <bcrl@...ck.org>
cc:	Russell King <rmk+lkml@....linux.org.uk>,
	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>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 06/11] syslets: core, documentation

On Wed, 14 Feb 2007, Benjamin LaHaise wrote:

> On Wed, Feb 14, 2007 at 11:45:23AM -0800, Davide Libenzi wrote:
> > Sort of, except that the whole thing can complete syncronously w/out 
> > context switches. The real point of the whole fibrils/syslets solution is 
> > that kind of optimization. The solution is as good as it is now, for 
> 
> Except that You Can't Do That (tm).  Try to predict beforehand if the code 
> path being followed will touch the FPU or SSE state, and you can't.  There is 
> no way to avoid the context switch overhead, as you have to preserve things 
> so that whatever state is being returned to the user is as it was.  Unless 
> you plan on resetting the state beforehand, but then you have to call into 
> arch specific code that ends up with a comparable overhead to the context 
> switch.

I think you may have mis-interpreted my words. *When* a schedule would 
block a synco execution try, then you do have a context switch. Noone 
argue that, and the code is clear. The sys_async_exec thread will block, 
and a newly woke up thread will re-emerge to sys_async_exec with a NULL 
returned to userspace. But in a "cachehit" case (no schedule happens 
during the syscall/*let execution), there is no context switch at all. 
That is the whole point of the optimization.



- Davide


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