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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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