Messages in this thread | | | Date | Mon, 16 Nov 2015 23:50:49 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH v3 5/5] x86/entry/64: Bypass enter_from_user_mode on non-context-tracking boots |
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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()?
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