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

Paul E. McKenney
2014-07-23T15:30:37Z

On Wed, Jul 23, 2014 at 11:11:45AM -0400, Pranith Kumar wrote:
> 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?

If you change the "awake" to something like "am_online", I could get
behind this one.

Thanx, Paul

> 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
>