Re: [PATCH x86/entry: Force rcu_irq_enter() when in idle task

[Andy Lutomirski]

On Tue, Jun 16, 2020 at 7:30 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Jun 16, 2020 at 10:40:04AM +0200, Thomas Gleixner wrote:
> > Joel,
> >
> > Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> writes:
> > > On Fri, Jun 12, 2020 at 03:55:00PM +0200, Thomas Gleixner wrote:
> > >> This is also correct vs. user mode entries in NOHZ full scenarios because
> > >> user mode entries bring RCU out of EQS and force the RCU irq nesting state
> > >
> > > I had to re-read this sentence a couple of times. The 'user mode entries'
> > > sounds like 'entry into user mode'. It would be good to reword it to 'IRQ
> > > entry in user mode'.
> >
> > :)
> >
> > > My knowledge predates the rcu-watching reworks so apologies on the below
> > > question but I still didn't fully follow why when the idle task behaves
> > > differently from being in user mode. Even with user mode we should have:
> > >
> > > <user mode>  (in EQS)
> > >   <irq entry>   <---------- exits EQS so now rcu is watching
> > >      <softirq entry in the exit path>
> > >         <timer tick irq entry>  <-- the buggy !watching logic prevents rcu_irq_enter
> > >         -> report QS since tick thinks it is first level.
> > >
> > > Is there a subtlety here I'm missing? I checked the RCU code and I did not
> > > see anywhere that rcu_user_enter() makes it behave differently. Both
> > > rcu_user_enter() and rcu_idle_enter() call rcu_eqs_enter().
> >
> > The interrupt hit user mode entry does:
> >
> >     idtentry_enter_cond_rcu()
> >       if (user_mode(regs))
> >          enter_from_user_mode()
> >            user_exit_irqoff()
> >              __context_tracking_exit(CONTEXT_USER)
> >                rcu_user_exit()
> >                  rcu_eqs_exit(1)
> >                    ...
> >                    WRITE_ONCE(rdp->dynticks_nmi_nesting,
> >                               DYNTICK_IRQ_NONIDLE);
> >
> > i.e. for interrupts which enter from user mode we are not invoking
> > rcu_irq_enter() at all.
> >
> > The return from interrupt path has nothing to do with that because
> > that's an entry in kernel mode.
>
> Hi Thomas,
> Ah, IRQ entry in user mode triggers the context-tracking path. Makes sense now, thanks.
>
> This will help me when I have to propose to get rid of dynticks_nmi_nesting again :)
>

Propose away, but just keep in mind that horrible architectures like
x86 really can nest non-maskable interrupts that hit kernel code more
than one deep.

--Andy