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    Subject[tip:core/rcu] rcu: Accelerate callback processing on CPUs not detecting GP end
    Commit-ID:  b32e9eb6ad29572b4451847d0e8227c9be2b6d69
    Gitweb: http://git.kernel.org/tip/b32e9eb6ad29572b4451847d0e8227c9be2b6d69
    Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    AuthorDate: Thu, 12 Nov 2009 22:35:03 -0800
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Fri, 13 Nov 2009 10:18:36 +0100

    rcu: Accelerate callback processing on CPUs not detecting GP end

    An earlier fix for a race resulted in a situation where the CPUs
    other than the CPU that detected the end of the grace period would
    not process their callbacks until the next grace period started.

    This means that these other CPUs would unnecessarily demand that an
    extra grace period be started.

    This patch eliminates this extra grace period and speeds callback
    processing by propagating rsp->completed to the rcu_node structures
    in the case where the CPU detecting the end of the grace period
    sees no reason to start a new grace period.

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: laijs@cn.fujitsu.com
    Cc: dipankar@in.ibm.com
    Cc: mathieu.desnoyers@polymtl.ca
    Cc: josh@joshtriplett.org
    Cc: dvhltc@us.ibm.com
    Cc: niv@us.ibm.com
    Cc: peterz@infradead.org
    Cc: rostedt@goodmis.org
    Cc: Valdis.Kletnieks@vt.edu
    Cc: dhowells@redhat.com
    LKML-Reference: <1258094104417-git-send-email->
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    kernel/rcutree.c | 18 +++++++++++++++++-
    1 files changed, 17 insertions(+), 1 deletions(-)

    diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    index d802419..b4efb9e 100644
    --- a/kernel/rcutree.c
    +++ b/kernel/rcutree.c
    @@ -676,7 +676,23 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
    struct rcu_node *rnp = rcu_get_root(rsp);

    if (!cpu_needs_another_gp(rsp, rdp)) {
    - spin_unlock_irqrestore(&rnp->lock, flags);
    + if (rnp->completed == rsp->completed) {
    + spin_unlock_irqrestore(&rnp->lock, flags);
    + return;
    + }
    + spin_unlock(&rnp->lock); /* irqs remain disabled. */
    +
    + /*
    + * Propagate new ->completed value to rcu_node structures
    + * so that other CPUs don't have to wait until the start
    + * of the next grace period to process their callbacks.
    + */
    + rcu_for_each_node_breadth_first(rsp, rnp) {
    + spin_lock(&rnp->lock); /* irqs already disabled. */
    + rnp->completed = rsp->completed;
    + spin_unlock(&rnp->lock); /* irqs remain disabled. */
    + }
    + local_irq_restore(flags);
    return;
    }


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