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SubjectRe: [patch 08/13] syslets: x86, add move_user_context() method
On Thu, 22 Feb 2007, Ingo Molnar wrote:

>
> * Davide Libenzi <davidel@xmailserver.org> wrote:
>
> > On Wed, 21 Feb 2007, Ingo Molnar wrote:
> >
> > > From: Ingo Molnar <mingo@elte.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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