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

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Date:	Wed, 14 Feb 2007 16:35:44 -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 03:17:59PM -0800, Davide Libenzi wrote:
> > > That's an incorrect assumption.  Every task/thread in the system has FPU 
> > > state associated with it, in part due to the fact that glibc has to change 
> > > some of the rounding mode bits, making them different than the default from 
> > > a freshly initialized state.
> > 
> > IMO I still belive this is not a huge problem. FPU state propagation/copy 
> > can be done in a clever way, once we detect the in-async condition.
> 
> Show me.  clts() and stts() are expensive hardware operations which there 
> is no means of avoiding as control register writes impact the CPU in a not 
> trivial manner.  I've spent far too much time staring at profiles of what 
> goes on in the context switch code in the process of looking for optimizations 
> on this very issue to be ignored on this point.

The trivial case is the cachehit case. Everything flows like usual since 
we don't swap threads.
If we're going to sleep, __async_schedule has to save/copy (depending if 
TS_USEDFPU is set) the current FPU state to the newly selected service 
thread (return-to-userspace thread).
When a fault eventually happen in the new userspace thread, context is 
restored.



- Davide


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