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

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, Jun 16, 2020 at 10:40:04AM +0200, Thomas Gleixner wrote:
> Joel,
> 
> Joel Fernandes <joel@joelfernandes.org> 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 :)

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

 - Joel