* [Question] atomic_fetch_andnot() in nohz_idle_balance()
@ 2018-11-21 22:34 Andrea Parri
2018-11-26 9:30 ` Peter Zijlstra
0 siblings, 1 reply; 6+ messages in thread
From: Andrea Parri @ 2018-11-21 22:34 UTC (permalink / raw)
To: Peter Zijlstra, Ingo Molnar, Vincent Guittot; +Cc: linux-kernel
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?
Thanks,
Andrea
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Question] atomic_fetch_andnot() in nohz_idle_balance()
2018-11-21 22:34 [Question] atomic_fetch_andnot() in nohz_idle_balance() Andrea Parri
@ 2018-11-26 9:30 ` Peter Zijlstra
2018-11-26 11:37 ` Vincent Guittot
0 siblings, 1 reply; 6+ messages in thread
From: Peter Zijlstra @ 2018-11-26 9:30 UTC (permalink / raw)
To: Andrea Parri; +Cc: Ingo Molnar, Vincent Guittot, linux-kernel
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?
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Question] atomic_fetch_andnot() in nohz_idle_balance()
2018-11-26 9:30 ` Peter Zijlstra
@ 2018-11-26 11:37 ` Vincent Guittot
2018-11-26 20:44 ` Andrea Parri
0 siblings, 1 reply; 6+ messages in thread
From: Vincent Guittot @ 2018-11-26 11:37 UTC (permalink / raw)
To: Peter Zijlstra; +Cc: andrea.parri, Ingo Molnar, linux-kernel
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
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Question] atomic_fetch_andnot() in nohz_idle_balance()
2018-11-26 11:37 ` Vincent Guittot
@ 2018-11-26 20:44 ` Andrea Parri
2018-11-27 9:01 ` Vincent Guittot
0 siblings, 1 reply; 6+ messages in thread
From: Andrea Parri @ 2018-11-26 20:44 UTC (permalink / raw)
To: Vincent Guittot; +Cc: Peter Zijlstra, Ingo Molnar, linux-kernel
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 ...)
Andrea
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Question] atomic_fetch_andnot() in nohz_idle_balance()
2018-11-26 20:44 ` Andrea Parri
@ 2018-11-27 9:01 ` Vincent Guittot
2018-11-27 10:59 ` Andrea Parri
0 siblings, 1 reply; 6+ messages in thread
From: Vincent Guittot @ 2018-11-27 9:01 UTC (permalink / raw)
To: andrea.parri; +Cc: Peter Zijlstra, Ingo Molnar, linux-kernel
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
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Question] atomic_fetch_andnot() in nohz_idle_balance()
2018-11-27 9:01 ` Vincent Guittot
@ 2018-11-27 10:59 ` Andrea Parri
0 siblings, 0 replies; 6+ messages in thread
From: Andrea Parri @ 2018-11-27 10:59 UTC (permalink / raw)
To: Vincent Guittot; +Cc: Peter Zijlstra, Ingo Molnar, linux-kernel
On Tue, Nov 27, 2018 at 10:01:24AM +0100, Vincent Guittot wrote:
> 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
Ah!, thank you for the clarification. Just sent a clean-up patch for
the comment (but deferring for the _relaxed() change...).
Andrea
>
> >
> > Andrea
^ permalink raw reply [flat|nested] 6+ messages in thread
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2018-11-21 22:34 [Question] atomic_fetch_andnot() in nohz_idle_balance() Andrea Parri
2018-11-26 9:30 ` Peter Zijlstra
2018-11-26 11:37 ` Vincent Guittot
2018-11-26 20:44 ` Andrea Parri
2018-11-27 9:01 ` Vincent Guittot
2018-11-27 10:59 ` Andrea Parri
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