lkml.org 
[lkml]   [2018]   [Nov]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [Question] atomic_fetch_andnot() in nohz_idle_balance()
On Mon, 26 Nov 2018 at 10:30, Peter Zijlstra <peterz@infradead.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

\
 
 \ /
  Last update: 2018-11-26 12:37    [W:0.091 / U:0.076 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site