linux-kernel - Re: [PATCH -rcu dev 3/3] RFC: rcu/tree: Read dynticks_nmi

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Date:   Mon, 19 Aug 2019 07:40:23 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, rcu@...r.kernel.org
Subject: Re: [PATCH -rcu dev 3/3] RFC: rcu/tree: Read dynticks_nmi_nesting in
 advance

On Mon, Aug 19, 2019 at 02:59:08PM +0200, Frederic Weisbecker wrote:
> On Fri, Aug 16, 2019 at 09:52:42AM -0700, Paul E. McKenney wrote:
> > On Fri, Aug 16, 2019 at 12:24:04PM -0400, Joel Fernandes wrote:
> > > On Thu, Aug 15, 2019 at 10:53:11PM -0400, Joel Fernandes (Google) wrote:
> > > > I really cannot explain this patch, but without it, the "else if" block
> > > > just doesn't execute thus causing the tick's dep mask to not be set and
> > > > causes the tick to be turned off.
> > > > 
> > > > I tried various _ONCE() macros but the only thing that works is this
> > > > patch.
> > > > 
> > > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > > > ---
> > > >  kernel/rcu/tree.c | 3 ++-
> > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index 856d3c9f1955..ac6bcf7614d7 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -802,6 +802,7 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
> > > >  {
> > > >  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> > > >  	long incby = 2;
> > > > +	int dnn = rdp->dynticks_nmi_nesting;
> > > 
> > > I believe the accidental sign extension / conversion from long to int was
> > > giving me an illusion since things started working well. Changing the 'int
> > > dnn' to 'long dnn' gives similar behavior as without this patch! At least I
> > > know now. Please feel free to ignore this particular RFC patch while I debug
> > > this more (over the weekend or early next week). The first 2 patches are
> > > good, just ignore this one.
> > 
> > Ah, good point on the type!  So you were ending up with zero due to the
> > low-order 32 bits of DYNTICK_IRQ_NONIDLE being zero, correct?  If so,
> > the "!rdp->dynticks_nmi_nesting" instead needs to be something like
> > "rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE", which sounds like
> > it is actually worse then the earlier comparison against the constant 2.
> > 
> > Sounds like I should revert the -rcu commit 805a16eaefc3 ("rcu: Force
> > nohz_full tick on upon irq enter instead of exit").
> 
> I can't find that patch so all I can say so far is that its title doesn't
> inspire me much. Do you still need that change for some reason?

It is in -rcu branch dev, but has been rebased.  The current version
is 227482fd4f3e ("rcu: Force nohz_full tick on upon irq enter instead
of exit").

It is not yet clear to me whether this is needed or not.  I -think- that
it is not, but without it, it is possible that some chain of events would
result in the rcu_data structure's ->rcu_urgent_qs field being cleared
before the interrupt-exit code could sample it, which might possibly
result in the tick remaining off.

							Thanx, Paul