Messages in this thread | | | From | Pranith Kumar <> | Date | Wed, 23 Jul 2014 11:21:24 -0400 | Subject | Re: [PATCH 04/16] rcu: Remove redundant check for an online CPU |
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On Wed, Jul 23, 2014 at 11:07 AM, Pranith Kumar <bobby.prani@gmail.com> wrote: > 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. >
Ah, I just checked and the above is not true. We don't disable interrupts, we do a this_cpu_read().
How many newbie cards do I have left :)
-- Pranith
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