Re: [PATCH v4 2/5] x86, traps: Track entry into and exit from IST context

[Andy Lutomirski <luto@amacapital.net>]

On Fri, Nov 21, 2014 at 3:38 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Fri, Nov 21, 2014 at 03:06:48PM -0800, Andy Lutomirski wrote:
>> On Fri, Nov 21, 2014 at 2:55 PM, Paul E. McKenney
>> <paulmck@linux.vnet.ibm.com> wrote:
>> > On Fri, Nov 21, 2014 at 02:19:17PM -0800, Andy Lutomirski wrote:
>> >> On Fri, Nov 21, 2014 at 2:07 PM, Paul E. McKenney
>> >> <paulmck@linux.vnet.ibm.com> wrote:
>> >> > On Fri, Nov 21, 2014 at 01:32:50PM -0800, Andy Lutomirski wrote:
>> >> >> On Fri, Nov 21, 2014 at 1:26 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> >> >> > We currently pretend that IST context is like standard exception
>> >> >> > context, but this is incorrect.  IST entries from userspace are like
>> >> >> > standard exceptions except that they use per-cpu stacks, so they are
>> >> >> > atomic.  IST entries from kernel space are like NMIs from RCU's
>> >> >> > perspective -- they are not quiescent states even if they
>> >> >> > interrupted the kernel during a quiescent state.
>> >> >> >
>> >> >> > Add and use ist_enter and ist_exit to track IST context.  Even
>> >> >> > though x86_32 has no IST stacks, we track these interrupts the same
>> >> >> > way.
>> >> >>
>> >> >> I should add:
>> >> >>
>> >> >> I have no idea why RCU read-side critical sections are safe inside
>> >> >> __do_page_fault today.  It's guarded by exception_enter(), but that
>> >> >> doesn't do anything if context tracking is off, and context tracking
>> >> >> is usually off. What am I missing here?
>> >> >
>> >> > Ah!  There are three cases:
>> >> >
>> >> > 1.      Context tracking is off on a non-idle CPU.  In this case, RCU is
>> >> >         still paying attention to CPUs running in both userspace and in
>> >> >         the kernel.  So if a page fault happens, RCU will be set up to
>> >> >         notice any RCU read-side critical sections.
>> >> >
>> >> > 2.      Context tracking is on on a non-idle CPU.  In this case, RCU
>> >> >         might well be ignoring userspace execution: NO_HZ_FULL and
>> >> >         all that.  However, as you pointed out, in this case the
>> >> >         context-tracking code lets RCU know that we have entered the
>> >> >         kernel, which means that RCU will again be paying attention to
>> >> >         RCU read-side critical sections.
>> >> >
>> >> > 3.      The CPU is idle.  In this case, RCU is ignoring the CPU, so
>> >> >         if we take a page fault when context tracking is off, life
>> >> >         will be hard.  But the kernel is not supposed to take page
>> >> >         faults in the idle loop, so this is not a problem.
>> >>
>> >> I guess so, as long as there are really no page faults in the idle loop.
>> >
>> > As far as I know, there are not.  If there are, someone needs to let
>> > me know!  ;-)
>> >
>> >> There are, however, machine checks in the idle loop, and maybe kprobes
>> >> (haven't checked), so I think this patch might fix real bugs.
>> >
>> > If you can get ISTs from the idle loop, then the patch is needed.
>> >
>> >> > Just out of curiosity...  Can an NMI occur in IST context?  If it can,
>> >> > I need to make rcu_nmi_enter() and rcu_nmi_exit() deal properly with
>> >> > nested calls.
>> >>
>> >> Yes, and vice versa.  That code looked like it handled nesting
>> >> correctly, but I wasn't entirely sure.
>> >
>> > It currently does not, please see below patch.  Are you able to test
>> > nesting?  It would be really cool if you could do so -- I have no
>> > way to test this patch.
>>
>> I can try.  It's sort of easy -- I'll put an int3 into do_nmi and add
>> a fixup to avoid crashing.
>>
>> What should I look for?  Should I try to force full nohz on and assert
>> something?  I don't really know how to make full nohz work.
>
> You should look for the WARN_ON_ONCE() calls in rcu_nmi_enter() and
> rcu_nmi_exit() to fire.

No warning with or without your patch, maybe because all of those
returns skip the labels.

Also, an NMI can happen *during* rcu_nmi_enter or rcu_nmi_exit.  Is
that okay?  Should those dynticks_nmi_nesting++ things be local_inc
and local_dec_and_test?

That dynticks_nmi_nesting thing seems scary to me.  Shouldn't the code
unconditionally increment dynticks_nmi_nesting in rcu_nmi_enter and
unconditionally decrement it in rcu_nmi_exit?

--Andy

>
>                                                         Thanx, Paul
>
>> >> Also, just to make sure: are we okay if rcu_nmi_enter() is called
>> >> before exception_enter if context tracking is on and we came directly
>> >> from userspace?
>> >
>> > If I understand correctly, this will result in context tracking invoking
>> > rcu_user_enter(), which will result in the rcu_dynticks counter having an
>> > odd value.  In that case, rcu_nmi_enter() will notice that RCU is already
>> > paying attention to this CPU via its check of atomic_read(&rdtp->dynticks)
>> > & 0x1), and will thus just return.  The matching rcu_nmi_exit() will
>> > notice that the nesting count is zero, and will also just return.
>> >
>> > Thus, everything works in that case.
>> >
>> > In contrast, if rcu_nmi_enter() was invoked from the idle loop, it
>> > would see that RCU is not paying attention to this CPU and that the
>> > NMI nesting depth (which rcu_nmi_enter() increments) used to be zero.
>> > It would then atomically increment rtdp->dynticks, forcing RCU to start
>> > paying attention to this CPU.  The matching rcu_nmi_exit() will see
>> > that the nesting count was non-zero, but became zero when decremented.
>> > This will cause rcu_nmi_exit() to atomically increment rtdp->dynticks,
>> > which will tell RCU to stop paying attention to this CPU.
>> >
>> >                                                         Thanx, Paul
>> >
>> > ------------------------------------------------------------------------
>> >
>> > rcu: Make rcu_nmi_enter() handle nesting
>> >
>> > Andy Lutomirski is introducing ISTs into x86, which from RCU's
>> > viewpoint are NMIs.  Because ISTs and NMIs can nest, rcu_nmi_enter()
>> > and rcu_nmi_exit() must now correctly handle nesting.  As luck would
>> > have it, rcu_nmi_exit() handles nesting but rcu_nmi_enter() does not.
>> > This patch therefore makes rcu_nmi_enter() handle nesting.
>>
>> Thanks.  Should I add this to v5 of my series?
>>
>> --Andy
>>
>> >
>> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>> >
>> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> > index 8749f43f3f05..875421aff6e3 100644
>> > --- a/kernel/rcu/tree.c
>> > +++ b/kernel/rcu/tree.c
>> > @@ -770,7 +770,8 @@ void rcu_nmi_enter(void)
>> >         if (rdtp->dynticks_nmi_nesting == 0 &&
>> >             (atomic_read(&rdtp->dynticks) & 0x1))
>> >                 return;
>> > -       rdtp->dynticks_nmi_nesting++;
>> > +       if (rdtp->dynticks_nmi_nesting++ != 0)
>> > +               return; /* Nested NMI/IST/whatever. */
>> >         smp_mb__before_atomic();  /* Force delay from prior write. */
>> >         atomic_inc(&rdtp->dynticks);
>> >         /* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
>> >
>>
>>
>>
>> --
>> Andy Lutomirski
>> AMA Capital Management, LLC
>>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC