Re: [RFC 1/2] rcu: Do prepare and cleanup idle depending on in_nmi()

[Paul E. McKenney]

On Wed, Jun 20, 2018 at 11:43:35AM -0400, Steven Rostedt wrote:
> On Wed, 20 Jun 2018 07:50:58 -0700
> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Wed, Jun 20, 2018 at 05:47:19PM +0900, Byungchul Park wrote:
> > > Get rid of dependency on ->dynticks_nmi_nesting.
> > > 
> > > Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
> > > ---
> > >  kernel/rcu/tree.c | 22 ++++++++++------------
> > >  1 file changed, 10 insertions(+), 12 deletions(-)
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index deb2508..59ae94e 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -797,6 +797,11 @@ void rcu_nmi_exit(void)
> > >  		return;
> > >  	}
> > > 
> > > +	if (!in_nmi()) {  
> > 
> > Is in_nmi() sufficiently reliable for use here?  In the past, there have
> > been tracepoints that invoked these functions between the time that the
> > handlers were entered and the time that software updated the state so that
> > the various handler-check functions (such as in_nmi()) would return true.
> > 
> > Steve, has there been any change in this situation?
> 
> There shouldn't be any "trace events", but what we had to deal with was
> function tracing. And in the near future, we will be getting "function
> based events" that will allow you to create an event in any function.
> 
> That said, even the function tracer shouldn't be called from the time
> the NMI triggers to "in_nmi()" is set. Because there's some function
> tracer callbacks that should not be executed from an NMI, and I use
> in_nmi() to determine if they get called or not.

OK, so in theory this change is safe from a tracing perspective.  But
it does add conditionals to a fastpath.

Byungchul, is there any reason to make this change other than preparation
for your second patch?

							Thanx, Paul