Re: [tip:x86/asm] x86/asm/entry/64: Migrate error and IRQ exit work to C and remove old assembly code

[Paul E. McKenney]

On Tue, Aug 11, 2015 at 03:59:37PM -0700, Andy Lutomirski wrote:
> On Tue, Aug 11, 2015 at 3:49 PM, Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> > On Tue, Aug 11, 2015 at 03:25:04PM -0700, Andy Lutomirski wrote:
> >> Can you explain to me what context tracking does that rcu_irq_enter
> >> and vtime_account_irq_enter don't do that's expensive?  Frankly, I'd
> >> rather drop everything except the context tracking callback.
> >
> > Irqs have their own hooks in the generic code. irq_enter() and irq_exit().
> > And those take care of RCU and time accounting already. So arch code really
> > doesn't need to care about that.
> 
> I'd love to have irq_enter_from_user and irq_enter_from_kernel instead.

RCU would need to know about irq_enter_from_user(), but could blithely
ignore irq_enter_from_kernel().  Unless irq_enter_from_kernel() is called
from the idle loop, in which case RCU would need to know.  All that aside,
the overhead of rcu_irq_enter() when called from non-idle kernel mode
should be relatively small.  So just telling RCU about all the interrupts
is actually not a bad strategy.

> > context tracking exists for the sole purpose of tracking states that don't
> > have generic hooks. Those are syscalls and exceptions.
> >
> > Besides, rcu_user_exit() is more costly than rcu_irq_enter() which have been
> > designed for the very purpose of providing a fast RCU tracking for non sleepable
> > code (which needs rcu_user_exit()).
> 
> So rcu_user_exit is slower because it's okay to sleep after calling it?
> 
> Would it be possible to defer the overhead until we actually try to
> sleep rather than doing it on entry?  (I have no idea what's going on
> under the hood.)

Nor do I, at least not until someone tells me what .config they are
using.  NO_HZ_FULL, NO_HZ_FULL_SYSIDLE, and RCU_FAST_NO_HZ make a
difference in this case.

> Anyway, irq_enter_from_user would solve this problem completely.
> 
> >
> > I've been thinking about pushing down syscalls and exceptions to generic
> > handlers. It might work for syscalls btw. But many exceptions have only
> > arch handlers, or significant amount of work is done on the arch level
> > which might make use of RCU (eg: breakpoint handlers on x86).
> 
> I'm trying to port the meat of the x86 syscall code to C.  Maybe the
> result will generalize.  The exit code is already in C (in -tip).

That does sound like a good thing!

							Thanx, Paul

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