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



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