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    Subject[PATCH tip/core/rcu 10/11] rcu: make force_quiescent_state() start grace period if needed
    Date
    From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    Grace periods cannot be started while force_quiescent_state() is
    active. This is OK in that the affected CPUs will try again later,
    but it does induce needless grace-period delays. This patch causes
    rcu_start_gp() to record a failed attempt to start a grace period.
    When force_quiescent_state() prepares to return, it then starts the
    grace period if there was such a failed attempt.

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    ---
    kernel/rcutree.c | 8 ++++++++
    kernel/rcutree.h | 5 +++++
    2 files changed, 13 insertions(+), 0 deletions(-)

    diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    index d920285..55e8f6e 100644
    --- a/kernel/rcutree.c
    +++ b/kernel/rcutree.c
    @@ -660,6 +660,8 @@ 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) || rsp->fqs_active) {
    + if (cpu_needs_another_gp(rsp, rdp))
    + rsp->fqs_need_gp = 1;
    if (rnp->completed == rsp->completed) {
    spin_unlock_irqrestore(&rnp->lock, flags);
    return;
    @@ -1239,6 +1241,12 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
    break;
    }
    rsp->fqs_active = 0;
    + if (rsp->fqs_need_gp) {
    + spin_unlock(&rsp->fqslock); /* irqs remain disabled */
    + rsp->fqs_need_gp = 0;
    + rcu_start_gp(rsp, flags); /* releases rnp->lock */
    + return;
    + }
    spin_unlock(&rnp->lock); /* irqs remain disabled */
    unlock_fqs_ret:
    spin_unlock_irqrestore(&rsp->fqslock, flags);
    diff --git a/kernel/rcutree.h b/kernel/rcutree.h
    index edb6fae..bd5d78a 100644
    --- a/kernel/rcutree.h
    +++ b/kernel/rcutree.h
    @@ -278,6 +278,11 @@ struct rcu_state {
    /* Force QS state. */
    u8 fqs_active; /* force_quiescent_state() */
    /* is running. */
    + u8 fqs_need_gp; /* A CPU was prevented from */
    + /* starting a new grace */
    + /* period because */
    + /* force_quiescent_state() */
    + /* was running. */
    long gpnum; /* Current gp number. */
    long completed; /* # of last completed gp. */

    --
    1.5.2.5


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