linux-kernel - Re: Reconciling rcu_irq_enter()/rcu_nmi

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Date:	Thu, 16 Jul 2015 21:49:21 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: Reconciling rcu_irq_enter()/rcu_nmi_enter() with context tracking

On Thu, Jul 16, 2015 at 09:29:07PM -0700, Paul E. McKenney wrote:
> On Thu, Jul 16, 2015 at 06:53:15PM -0700, Andy Lutomirski wrote:
> > For reasons that mystify me a bit, we currently track context tracking
> > state separately from rcu's watching state.  This results in strange
> > artifacts: nothing generic cause IRQs to enter CONTEXT_KERNEL, and we
> > can nest exceptions inside the IRQ handler (an example would be
> > wrmsr_safe failing), and, in -next, we splat a warning:
> > 
> > https://gist.github.com/sashalevin/a006a44989312f6835e7
> > 
> > I'm trying to make context tracking more exact, which will fix this
> > issue (the particular splat that Sasha hit shouldn't be possible when
> > I'm done), but I think it would be nice to unify all of this stuff.
> > Would it be plausible for us to guarantee that RCU state is always in
> > sync with context tracking state?  If so, we could maybe simplify
> > things and have fewer state variables.
> 
> A noble goal.  Might even be possible, and maybe even advantageous.
> 
> But it is usually easier to say than to do.  RCU really does need to make
> some adjustments when the state changes, as do the other subsystems.
> It might or might not be possible to do the transitions atomically.
> And if the transitions are not atomic, there will still be weird code
> paths where (say) the processor is considered non-idle, but RCU doesn't
> realize it yet.  Such a code path could not safely use rcu_read_lock(),
> so you still need RCU to be able to scream if someone tries it.
> Contrariwise, if there is a code path where the processor is considered
> idle, but RCU thinks it is non-idle, that code path can stall
> grace periods.  (Yes, not a problem if the code path is short enough.
> At least if the underlying VCPU is making progres...)
> 
> Still, I cannot prove that it is impossible, and if it is possible,
> then as you say, there might well be benefits.
> 
> > Doing this for NMIs might be weird.  Would it make sense to have a
> > CONTEXT_NMI that's somehow valid even if the NMI happened while
> > changing context tracking state.
> 
> Face it, NMIs are weird.  ;-)
> 
> > Thoughts?  As it stands, I think we might already be broken for real:
> > 
> > Syscall -> user_exit.  Perf NMI hits *during* user_exit.  Perf does
> > copy_from_user_nmi, which can fault, causing do_page_fault to get
> > called, which calls exception_enter(), which can't be a good thing.
> > 
> > RCU is okay (sort of) because of rcu_nmi_enter, but this seems very fragile.
> 
> Actually, I see more cases where people forget irq_enter() than
> rcu_nmi_enter().  "We will just nip in quickly and do something without
> actually letting the irq system know.  Oh, and we want some event tracing
> in that code path."  Boom!
> 
> > Thoughts?  As it stands, I need to do something because -tip and thus
> > -next spews occasional warnings.
> 
> Tell me more?

And for completeness, RCU also has the following requirements on the
state-transition mechanism:

1.	It must be possible to reliably sample some other CPU's state.
	This is an energy-efficiency requirement, as RCU is not normally
	permitted to wake up idle CPUs.  Nor nohz CPUs, for that matter.

2.	RCU must be able to track passage through idle and nohz states.
	In other words, if RCU samples at t=0 and finds that the CPU
	is executing (say) in kernel mode, and RCU samples again at
	t=10 and again finds that the CPU is executing in kernel mode,
	RCU needs to be able to determine whether or not that CPU passed
	through idle or nohz betweentimes.

3.	In some configurations, RCU needs to be able to block entry into
	nohz state, both for idle and userspace.

Probably others as well...

							Thanx, Paul

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