Messages in this thread | | | Date | Wed, 23 Jul 2014 07:14:24 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 04/16] rcu: Remove redundant check for an online CPU |
| |
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@linux.vnet.ibm.com> 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@linux.vnet.ibm.com> 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@gmail.com> > >> > > >> > 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
| |