Re: [PATCH v3 5/5] x86/entry/64: Bypass enter_from_user_mode on non-context-tracking boots

Frederic Weisbecker
2015-11-16T22:50:56Z

On Mon, Nov 16, 2015 at 11:10:55AM -0800, Andy Lutomirski wrote:
> On Nov 13, 2015 7:26 AM, "Frederic Weisbecker" <fweisbec@gmail.com> wrote:
> >
> > On Thu, Nov 12, 2015 at 12:59:04PM -0800, Andy Lutomirski wrote:
> > > On CONFIG_CONTEXT_TRACKING kernels that have context tracking
> > > disabled at runtime (which includes most distro kernels), we still
> > > have the overhead of a call to enter_from_user_mode in interrupt and
> > > exception entries.
> > >
> > > If jump labels are available, this uses the jump label
> > > infrastructure to skip the call.
> >
> > Looks good. But why are we still calling context tracking code on IRQs at all?
>
> Same reasons as before:
>
> 1. This way the IRQ exit path is almost completely shared with all the
> other exit paths.

I'm all for consolidation in general. Unless it brings bad middle states.

If I knew before that I would have to argue endlessly in order to protest against
these context tracking changes, I would have NACK'ed the x86 consolidation rework in
the state it was while it got merged.

>
> 2. It combines the checks for which context we were in with what CPL
> we entered from.
>
> Part 2 should be complete across the whole x86 kernel soon once the
> 64-bit syscall code gets fixed up.
>
> We should get rid of the duplication in the irq entry hooks. Want to
> help with that?

Which one? The duplication against irq_enter() and irq_exit()?

I think that irq_exit() should be moved to the IRQ very end and perform the
final signal/schedule/preempt_schedule_irq() loop. But it requires a bit of
rework on all archs in order to do that. This could be done iteratively though.

> Presumably we should do the massive remote polling speedup to the nohz code,

Hmm, I don't get what you mean here.

> and we should also teach enter_from_user_mode to transition directly to IRQ state as
> appropriate. Then irq_enter can be much faster.

I don't get what you mean here either. You mean calling irq_enter() from enter_from_user_mode()?