Messages in this thread | | | From | Vincent Guittot <> | Date | Tue, 27 Nov 2018 10:01:24 +0100 | Subject | Re: [Question] atomic_fetch_andnot() in nohz_idle_balance() |
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On Mon, 26 Nov 2018 at 21:44, Andrea Parri <andrea.parri@amarulasolutions.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@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 > > 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
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