linux-kernel - Re: [patch 08/13] syslets: x86, add move_user

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Date:	Wed, 21 Feb 2007 20:10:55 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	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 08/13] syslets: x86, add move_user_context() method

On Thu, 22 Feb 2007, Ingo Molnar wrote:

> 
> * Davide Libenzi <davidel@...ilserver.org> wrote:
> 
> > On Wed, 21 Feb 2007, Ingo Molnar wrote:
> > 
> > > From: Ingo Molnar <mingo@...e.hu>
> > > 
> > > add the move_user_context() method to move the user-space
> > > context of one kernel thread to another kernel thread.
> > > User-space might notice the changed TID, but execution,
> > > stack and register contents (general purpose and FPU) are
> > > still the same.
> > 
> > Also signal handling should/must be maintained, on top of TID. You 
> > don't want the user to be presented with a different signal handling 
> > after an sys_async_exec call.
> 
> right now CLONE_SIGNAL and CLONE_SIGHAND is used for new async threads, 
> so they should inherit and share all the signal settings.

Right. Sorry I missed the signal cloning flags (still has to go thru the 
whole code).



> > So NTSK loads a non up2date FPUo, instead of the FPUc that was the 
> > "dirty" context to migrate (since TS_USEDFPU was set). I think you 
> > need an early __unlazy_fpu() in that case, that would turn the above 
> > into:
> 
> yes. My plan is to to avoid all these problems by having a 
> special-purpose sched_yield_to(old_task, new_task) function.
> 
> this, besides being even faster than the default scheduler (because the 
> runqueue balance does not change so no real scheduling decision has to 
> be done - the true scheduling decisions happen later on at async-wakeup 
> time), should also avoid all the FPU races: the FPU just gets flipped 
> between old_task and new_task (and TS_USEDFPU needs to be moved as well, 
> etc.). No intermediate task can come inbetween.
> 
> can you see a hole in this sched_yield_to() method as well?

Not sure till I see the code, but in that case we really need a sync&copy. 
This is really a fork-like for the FPU context IMO. The current "dirty" 
FPU context should follow both OTSK and NTSK.



- Davide


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