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    Subject[PATCH RFC tip/core/rcu 01/16] rcu: substitute set_need_resched for sending resched IPIs
    Date
    This patch adds a check to __rcu_pending() that does a local
    set_need_resched() if the current CPU is holding up the current grace
    period and if force_quiescent_state() will be called soon. The goal is
    to reduce the probability that force_quiescent_state() will need to do
    smp_send_reschedule(), which sends an IPI and is therefore more expensive
    on most architectures.

    Signed-off-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    ---
    kernel/rcutree.c | 10 ++++++++++
    1 files changed, 10 insertions(+), 0 deletions(-)

    diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    index 3ec8160..e54c123 100644
    --- a/kernel/rcutree.c
    +++ b/kernel/rcutree.c
    @@ -1499,6 +1499,16 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)

    /* Is the RCU core waiting for a quiescent state from this CPU? */
    if (rdp->qs_pending) {
    +
    + /*
    + * If force_quiescent_state() coming soon and this CPU
    + * needs a quiescent state, and this is either RCU-sched
    + * or RCU-bh, force a local reschedule.
    + */
    + if (!rdp->preemptable &&
    + ULONG_CMP_LT(ACCESS_ONCE(rsp->jiffies_force_qs) - 1,
    + jiffies))
    + set_need_resched();
    rdp->n_rp_qs_pending++;
    return 1;
    }
    --
    1.7.0


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