Re: [PATCH 09/16] rcu: Remove redundant check for online cpu

Pranith Kumar
2014-07-23T15:12:22Z

On Wed, Jul 23, 2014 at 10:23 AM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Wed, Jul 23, 2014 at 10:12:54AM -0400, Pranith Kumar wrote:
>> On Wed, Jul 23, 2014 at 9:50 AM, Paul E. McKenney
>> <paulmck@linux.vnet.ibm.com> wrote:
>> > On Wed, Jul 23, 2014 at 08:59:06AM -0400, Pranith Kumar wrote:
>> >> On Wed, Jul 23, 2014 at 8:21 AM, Paul E. McKenney
>> >> <paulmck@linux.vnet.ibm.com> wrote:
>> >> > On Wed, Jul 23, 2014 at 01:09:46AM -0400, Pranith Kumar wrote:
>> >> >> There are two checks for an online CPU if two if() conditions. This commit
>> >> >> simplies this by replacing it with only one check for the online CPU.
>> >> >>
>> >> >> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
>> >> >
>> >> > I admit that it is very early in the morning my time, but I don't see
>> >> > this change as preserving the semantics in all cases. Please recheck
>> >> > your changes to the second check.
>> >> >
>> >> > Thanx, Paul
>> >>
>> >> I guess you must be thrown off by the complementary checks, the first
>> >> check is for cpu_online() and second is for cpu_is_offline(). :)
>> >>
>> >> Previously, if a cpu is offline, the first condition is false and the
>> >> second condition is true, so we return from the second if() condition.
>> >> The same semantics are being preserved.
>> >
>> > Fair enough!
>> >
>> > Nevertheless, I am not seeing this as a simplification.
>>
>> I am not sure what you mean here, do you mean that both the checks are
>> actually required?
>
> I mean that the current compound tests each mean something. Pulling out
> the offline test adds lines of code and obscures that meaning. This means
> that it is easier (for me, anyway) to see why the current code is correct
> than it is to see why your suggested change is correct.
>

That is a valid point. I did not mean to reduce readability of the
code. Just trying to avoid the overhead of smp_processor_id().

Not sure if you would prefer this, but how about the following?

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f1ba773..3a26008 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2516,15 +2516,16 @@ static void __call_rcu_core(struct rcu_state
*rsp, struct rcu_data *rdp,
{
bool needwake;

+ bool awake = cpu_online(smp_processor_id);
/*
* If called from an extended quiescent state, invoke the RCU
* core in order to force a re-evaluation of RCU's idleness.
*/
- if (!rcu_is_watching() && cpu_online(smp_processor_id()))
+ if (!rcu_is_watching() && awake)
invoke_rcu_core();

/* If interrupts were disabled or CPU offline, don't invoke RCU core. */
- if (irqs_disabled_flags(flags) || cpu_is_offline(smp_processor_id()))
+ if (irqs_disabled_flags(flags) || !awake)
return;

/*

>
>> >> --
>> >> Pranith.
>> >>
>> >>
>> >>
>> >> >
>> >> >> ---
>> >> >> kernel/rcu/tree.c | 9 ++++++---
>> >> >> 1 file changed, 6 insertions(+), 3 deletions(-)
>> >> >>
>> >> >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> >> >> index 5dcbf36..8d598a2 100644
>> >> >> --- a/kernel/rcu/tree.c
>> >> >> +++ b/kernel/rcu/tree.c
>> >> >> @@ -2602,15 +2602,18 @@ static void __call_rcu_core(struct rcu_state *rsp, struct rcu_data *rdp,
>> >> >> {
>> >> >> bool needwake;
>> >> >>
>> >> >> + if (!cpu_online(smp_processor_id()))
>> >> >> + return;
>> >> >> +
>> >> >> /*
>> >> >> * If called from an extended quiescent state, invoke the RCU
>> >> >> * core in order to force a re-evaluation of RCU's idleness.
>> >> >> */
>> >> >> - if (!rcu_is_watching() && cpu_online(smp_processor_id()))
>> >> >> + if (!rcu_is_watching())
>> >> >> invoke_rcu_core();
>> >> >>
>> >> >> - /* If interrupts were disabled or CPU offline, don't invoke RCU core. */
>> >> >> - if (irqs_disabled_flags(flags) || cpu_is_offline(smp_processor_id()))
>> >> >> + /* If interrupts were disabled, don't invoke RCU core. */
>> >> >> + if (irqs_disabled_flags(flags))
>> >> >> return;
>> >> >>
>> >> >> /*
>> >> >> --
>> >> >> 2.0.0.rc2
>> >> >>
>> >> >
>> >>
>> >>
>> >>
>> >> --
>> >> Pranith
>> >>
>> >
>>
>>
>>
>> --
>> Pranith
>>
>

--
Pranith