Re: [Question] atomic_fetch_andnot() in nohz_idle_balance()

[Vincent Guittot]

On Mon, 26 Nov 2018 at 10:30, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Nov 21, 2018 at 11:34:53PM +0100, Andrea Parri wrote:
> > Hi,
> >
> > The comment for the atomic_fetch_andnot() in nohz_idle_balance() says:
> >
> >   "barrier, pairs with nohz_balance_enter_idle(), ensures ..."
> >
> > which, well, does sound a note of warning... ;-)
> >
> > I see that nohz_balance_enter_idle() has an smp_mb__after_atomic() but
> > the comment for the latter suggests that this barrier is pairing with
> > the smp_mb() in _nohz_idle_balance().
> >
> > So, what is the intended pairing barrier for the atomic_fetch_andnot()?
> > what (which memory accesses) do you want "to order" here?
>
> I can't seem to make sense of that comment either; the best I can come
> up with is that it would order the prior NOHZ_KICK_MASK load vs us then
> changing it.
>
> But that would order against kick_ilb(), not enter_idle.
>
> Vincent?

I can't come with a good explanation.
After looking into my email archive, the only explanation that i have
is that the comments remains from a previous iteration of the feature
that was based on a nohz.stats_state mechanism