linux-kernel - Re: [Question] atomic_fetch_andnot() in nohz_idle

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Date:   Tue, 27 Nov 2018 10:01:24 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     andrea.parri@...rulasolutions.com
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [Question] atomic_fetch_andnot() in nohz_idle_balance()

On Mon, 26 Nov 2018 at 21:44, Andrea Parri
<andrea.parri@...rulasolutions.com> wrote:
>
> On Mon, Nov 26, 2018 at 12:37:00PM +0100, Vincent Guittot wrote:
> > On Mon, 26 Nov 2018 at 10:30, Peter Zijlstra <peterz@...radead.org> 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
>
> I'm afraid I still can't help your comment...  put in other terms, would
> you feel "unconfortable" with _relax()ing the andnot()?  (and if so ...)

so I think that the comment is useless and can be removed because it
was referring to a line code above the comment that was present in a
previous iteration of the patchset. This line disappeared in final
version but the comment has stayed.

If your question is: can we use atomic_fetch_andnot_relaxed() instead
of atomic_fetch_andnot() in nohz_idle_balance() ?
I think that it's possible

>
>   Andrea