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    SubjectRe: [patch 4/6] ia64: Use preempt_schedule_irq
    On Thu, 21 Nov 2013, Peter Zijlstra wrote:

    > On Thu, Nov 21, 2013 at 12:41:44PM +0100, Thomas Gleixner wrote:
    > > On Wed, 20 Nov 2013, Thomas Gleixner wrote:
    > > > On Wed, 20 Nov 2013, Tony Luck wrote:
    > > > > asmlinkage void __sched preempt_schedule_irq(void)
    > > > > {
    > > > > schedule();
    > > > > }
    > > > >
    > > > > Or is life more complicated than that?
    > > >
    > > > Hmm, I think I fubared that and you decided to ignore my patch :)
    > > >
    > > > Let me look at it tomorrow morning with full awake brain cells.
    > >
    > > Ok, wrapped my brain around it. I tripped over the magic asm foo which
    > > has a single need_resched check and schedule point for both sys call
    > > return and interrupt return.
    > >
    > > So you need the schedule_preempt_irq() for kernel preemption from
    > > interrupt return while on a normal syscall preemption a schedule would
    > > be sufficient. But using schedule_preempt_irq() is not harmful here in
    > > any way. It just sets the preempt_active bit also in cases where it
    > > would not be required.
    > >
    > > Even on preempt=n kernels adding the preempt_active bit is completely
    > > harmless. So instead of having an extra function, moving the existing
    > > one out of the ifdef PREEMPT looks like the sanest thing to do.
    > >
    > > Peter, Ingo ?
    >
    > Uhm, preempt_schedule_irq() assumes interrupts are disabled and
    > explicitly returns with interrupts disabled again. Does the ia64
    > callsite conform?

    Yep.

    > If so, schedule() would actually be actively wrong, because that will
    > whinge when called with interrupts disabled, and will return with
    > interrupts enabled.
    >
    > Anyway, I don't object to the patch per se, but it might bloat a few
    > !ia64 kernels for having to carry the extra text.

    Well, we could get rid of quite some other sti/schedule/cli asm magic
    all over the archs.

    Thanks,

    tglx


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