linux-kernel - Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

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Date:	Fri, 2 Mar 2007 12:18:39 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

On Fri, 2 Mar 2007, Ingo Molnar wrote:

> 
> * Davide Libenzi <davidel@...ilserver.org> wrote:
> 
> > [...] We're still missing proper FPU context switch in the 
> > move_user_context(). [...]
> 
> yeah - i'm starting to be of the opinion that the FPU context should 
> stay with the threadlet, exclusively. I.e. when calling a threadlet, the 
> 'outer loop' (the event loop) should not leak FPU context into the 
> threadlet and then expect it to be replicated from whatever random point 
> the threadlet ended up sleeping at. It would be possible, but it just 
> makes no sense. What makes most sense is to just keep the FPU context 
> with the threadlet, and to let the 'new head' use an initial (unused) 
> FPU context. And it's in fact the threadlet that will most likely have 
> an acrive FPU context across a system call, not the outer loop. In other 
> words: no special FPU support needed at all for threadlets (i.e. no 
> flipping needed even) - this behavior just naturally happens in the 
> current implementation. Hm?

I think that the "dirty" FPU context must, at least, follow the new head. 
That's what the userspace sees, and you don't want an async_exec to 
re-emerge with a different FPU context.
I think it should also follow the async thread (old, going-to-sleep, 
thread), since a threadlet might have that dirtied, and as a consequence 
it'll want to find it back when it's re-scheduled.
So, IMO, if the USEDFPU bit is set, we need to sync the dirty  FPU context 
with an early unlazy_fpu(), *and* copy the sync'd FPU context to the new head.
This should really be a fork of the dirty FPU context IMO, and should only 
happen if the USEDFPU bit is set.



- Davide


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