linux-kernel - Re: [PATCH v3 5/5] x86/entry/64: Bypass enter_from_user

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Date:	Mon, 16 Nov 2015 23:50:49 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Borislav Petkov <bp@...en8.de>, Brian Gerst <brgerst@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3 5/5] x86/entry/64: Bypass enter_from_user_mode on
 non-context-tracking boots

On Mon, Nov 16, 2015 at 11:10:55AM -0800, Andy Lutomirski wrote:
> On Nov 13, 2015 7:26 AM, "Frederic Weisbecker" <fweisbec@...il.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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