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    SubjectRe: [PATCH tip/core/rcu 01/26] rcu: New rcu_user_enter() and rcu_user_exit() APIs
    On Thu, Aug 30, 2012 at 02:05:18PM -0700, Paul E. McKenney wrote:
    > From: Frederic Weisbecker <fweisbec@gmail.com>
    >
    > RCU currently insists that only idle tasks can enter RCU idle mode, which
    > prohibits an adaptive tickless kernel (AKA nohz cpusets), which in turn
    > would mean that usermode execution would always take scheduling-clock
    > interrupts, even when there is only one task runnable on the CPU in
    > question.
    >
    > This commit therefore adds rcu_user_enter() and rcu_user_exit(), which
    > allow non-idle tasks to enter RCU idle mode. These are quite similar
    > to rcu_idle_enter() and rcu_idle_exit(), respectively, except that they
    > omit the idle-task checks.
    >
    > [ Updated to use "user" flag rather than separate check functions. ]
    >
    > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > Cc: Alessio Igor Bogani <abogani@kernel.org>
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: Avi Kivity <avi@redhat.com>
    > Cc: Chris Metcalf <cmetcalf@tilera.com>
    > Cc: Christoph Lameter <cl@linux.com>
    > Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
    > Cc: Geoff Levand <geoff@infradead.org>
    > Cc: Gilad Ben Yossef <gilad@benyossef.com>
    > Cc: Hakan Akkan <hakanakkan@gmail.com>
    > Cc: Ingo Molnar <mingo@kernel.org>
    > Cc: Kevin Hilman <khilman@ti.com>
    > Cc: Max Krasnyansky <maxk@qualcomm.com>
    > Cc: Peter Zijlstra <peterz@infradead.org>
    > Cc: Stephen Hemminger <shemminger@vyatta.com>
    > Cc: Steven Rostedt <rostedt@goodmis.org>
    > Cc: Sven-Thorsten Dietrich <thebigcorporation@gmail.com>
    > Cc: Thomas Gleixner <tglx@linutronix.de>

    A few suggestions below: an optional microoptimization and some bugfixes.
    With the bugfixes, and with or without the microoptimization:

    Reviewed-by: Josh Triplett <josh@joshtriplett.org>

    > --- a/kernel/rcutree.c
    > +++ b/kernel/rcutree.c
    [...]
    > -static void rcu_idle_enter_common(struct rcu_dynticks *rdtp, long long oldval)
    > +static void rcu_eqs_enter_common(struct rcu_dynticks *rdtp, long long oldval,
    > + bool user)
    > {
    > trace_rcu_dyntick("Start", oldval, 0);
    > - if (!is_idle_task(current)) {
    > + if (!is_idle_task(current) && !user) {

    Microoptimization: putting the !user check first (here and in the exit
    function) would allow the compiler to partially inline rcu_eqs_*_common
    into the two trivial wrappers and constant-fold away the test for !user.

    > +void rcu_idle_enter(void)
    > +{
    > + rcu_eqs_enter(0);
    > +}

    s/0/false/

    > +void rcu_user_enter(void)
    > +{
    > + rcu_eqs_enter(1);
    > +}

    s/1/true/

    > -static void rcu_idle_exit_common(struct rcu_dynticks *rdtp, long long oldval)
    > +static void rcu_eqs_exit_common(struct rcu_dynticks *rdtp, long long oldval,
    > + int user)
    > {
    > smp_mb__before_atomic_inc(); /* Force ordering w/previous sojourn. */
    > atomic_inc(&rdtp->dynticks);
    > @@ -464,7 +490,7 @@ static void rcu_idle_exit_common(struct rcu_dynticks *rdtp, long long oldval)
    > WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
    > rcu_cleanup_after_idle(smp_processor_id());
    > trace_rcu_dyntick("End", oldval, rdtp->dynticks_nesting);
    > - if (!is_idle_task(current)) {
    > + if (!is_idle_task(current) && !user) {

    Same micro-optimization as the enter function.

    > +void rcu_idle_exit(void)
    > +{
    > + rcu_eqs_exit(0);
    > +}

    s/0/false/

    > +void rcu_user_exit(void)
    > +{
    > + rcu_eqs_exit(1);
    > +}

    s/1/true/

    > @@ -539,7 +586,7 @@ void rcu_irq_enter(void)
    > if (oldval)
    > trace_rcu_dyntick("++=", oldval, rdtp->dynticks_nesting);
    > else
    > - rcu_idle_exit_common(rdtp, oldval);
    > + rcu_eqs_exit_common(rdtp, oldval, 1);

    s/1/true/, and likewise in rcu_irq_exit.

    - Josh Triplett


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