Re: [PATCH 04/16] rcu: Remove redundant check for an online CPU

[Paul E. McKenney]

On Wed, Jul 23, 2014 at 10:01:14AM -0400, Pranith Kumar wrote:
> On Wed, Jul 23, 2014 at 9:41 AM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > On Wed, Jul 23, 2014 at 09:23:47AM -0400, Pranith Kumar wrote:
> >> On Wed, Jul 23, 2014 at 8:09 AM, Paul E. McKenney
> >> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >> > On Wed, Jul 23, 2014 at 01:09:41AM -0400, Pranith Kumar wrote:
> >> >> rcu_prcess_callbacks() is the softirq handler for RCU which is raised from
> >> >> invoke_rcu_core() which is called from __call_rcu_core().
> >> >>
> >> >> Each of these three functions checks if the cpu is online. We can remove the
> >> >> redundant ones. This commit removes one of these redundant check.
> >> >>
> >> >> Signed-off-by: Pranith Kumar <bobby.prani@xxxxxxxxx>
> >> >
> >> > Sorry, but no.  There can be a long delay between raise_softirq() and
> >> > this function starting, particularly if ksoftirqd gets involved.  The
> >> > CPU could easily go offline in the meantime.
> >> >
> >>
> >> That makes sense. I guess one of the other two checks in
> >> __call_rcu_core() or invoke_rcu_core() can go then?
> >
> > What would have to be the case for this suggested change to be safe?
> > Does that condition in fact hold?
> >
> 
> The only scenario which is unsafe is when this thread is preempted and
> scheduled on a dying CPU. In that case we should turn off interrupts.
> 
> Actually, checking the code, I see that interrupts are turned off
> already before we call _call_rcu_core(). So I am not sure such a case
> will happen. On the other hand invoke_rcu_core() will be a one line
> function which might as well be in-lined to avoid the double cpu
> online check.
> 
> What am I missing?

I am not sure.  Perhaps the fact that __call_rcu_core() doesn't call
invoke_rcu_core() unless the condition holds (which means that you
cannnot remove the check from __call_rcu_core()) or maybe the fact that
invoke_rcu_core() is called from many other places, which means that
you might not be able to remove the check from invoke_rcu_core().

							Thanx, Paul

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