Re: [RFC][PATCH 6/7] context_tracking: Provide SMP ordering using RCU

[Paul E. McKenney]

On Wed, Sep 22, 2021 at 09:47:59PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 22, 2021 at 09:33:43PM +0200, Peter Zijlstra wrote:
> 
> > Anyway, lemme see if I get your proposal; lets say the counter starts at
> > 0 and is in kernel space.
> > 
> >  0x00(0) - kernel
> >  0x02(2) - user
> >  0x04(0) - kernel
> > 
> > So far so simple, then NMI on top of that goes:
> > 
> >  0x00(0) - kernel
> >  0x03(3) - kernel + nmi
> >  0x04(0) - kernel
> >  0x06(2) - user
> >  0x09(1) - user + nmi
> >  0x0a(2) - user
> > 
> > Which then gives us:
> > 
> >  (0) := kernel
> >  (1) := nmi-from-user
> >  (2) := user
> >  (3) := nmi-from-kernel
> > 
> > Which should work I suppose. But like I said above, I'd be happier if
> > this counter would live in context_tracking rather than RCU.
> 
> Furthermore, if we have this counter, the we can also do things like:
> 
>   seq = context_tracking_seq_cpu(that_cpu);
>   if ((seq & 3) != USER)
>     // nohz_fail, do something
>   set_tsk_thread_flag(curr_task(that_cpu), TIF_DO_SOME_WORK);
>   if (seq == context_tracking_seq_cpu(that_cpu))
>     // success!!
> 
> To remotely set pending state. Allowing yet more NOHZ_FULL fixes, like,
> for example, eliding the text_poke IPIs.

Nice!

There have been several instances where I thought that the extra state
would help RCU, but each time there turned out to be a simpler way to
get things done.  Or that it eventually turned out that RCU didn't need
to care about the difference between idle and nohz_full userspace.

							Thanx, Paul