Messages in this thread | | | From | Pranith Kumar <> | Date | Wed, 23 Jul 2014 11:07:55 -0400 | Subject | Re: [PATCH 04/16] rcu: Remove redundant check for an online CPU |
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On Wed, Jul 23, 2014 at 10:14 AM, Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > 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(). >
OK, since invoke_rcu_core() is being called from multiple places, we cannot remove the check there.
But we can remove the check from __call_rcu_core(). Since we are going to check again in invoke_rcu_core(), no? We will call invoke_rcu_core() but we will return if the check if false.
The trade-off here is between a function call and an extra smp_processor_id(). The only reason I am trying to do this is because smp_processor_id() disables and enables interrupts which is costly and we can avoid one such check.
-- Pranith
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