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    Subject[PATCH tip/core/rcu 05/11] rcu: eliminate local variable lastcomp from force_quiescent_state()
    Date
    From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    Because rsp->fqs_active is set to 1 across force_quiescent_state()'s
    switch statement, rcu_start_gp() will refrain from starting a new
    grace period during this time. Therefore, rsp->gpnum is constant,
    and can be propagated to all uses of lastcomp, eliminating this local
    variable.

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    ---
    kernel/rcutree.c | 10 +++-------
    kernel/rcutree.h | 2 --
    2 files changed, 3 insertions(+), 9 deletions(-)

    diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    index 1d8cfb1..62b6433 100644
    --- a/kernel/rcutree.c
    +++ b/kernel/rcutree.c
    @@ -1191,7 +1191,6 @@ static int rcu_process_dyntick(struct rcu_state *rsp, long lastcomp,
    static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
    {
    unsigned long flags;
    - long lastcomp;
    struct rcu_node *rnp = rcu_get_root(rsp);
    u8 forcenow;
    u8 gpdone;
    @@ -1207,7 +1206,6 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
    goto unlock_fqs_ret; /* no emergency and done recently. */
    rsp->n_force_qs++;
    spin_lock(&rnp->lock); /* irqs already disabled */
    - lastcomp = rsp->gpnum - 1;
    rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
    if(!rcu_gp_in_progress(rsp)) {
    rsp->n_force_qs_ngp++;
    @@ -1228,7 +1226,7 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
    break; /* So gcc recognizes the dead code. */

    /* Record dyntick-idle state. */
    - gpdone = rcu_process_dyntick(rsp, lastcomp,
    + gpdone = rcu_process_dyntick(rsp, rsp->gpnum - 1,
    dyntick_save_progress_counter);
    spin_lock(&rnp->lock); /* irqs already disabled */
    if (gpdone)
    @@ -1239,11 +1237,9 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)

    /* Update state, record completion counter. */
    forcenow = 0;
    - if (lastcomp + 1 == rsp->gpnum &&
    - lastcomp == rsp->completed) {
    + if (rsp->gpnum - 1 == rsp->completed) {
    forcenow = rsp->signaled == RCU_SAVE_COMPLETED;
    rsp->signaled = RCU_FORCE_QS;
    - rsp->completed_fqs = lastcomp;
    }
    if (!forcenow)
    break;
    @@ -1253,7 +1249,7 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)

    /* Check dyntick-idle state, send IPI to laggarts. */
    spin_unlock(&rnp->lock); /* irqs remain disabled */
    - gpdone = rcu_process_dyntick(rsp, rsp->completed_fqs,
    + gpdone = rcu_process_dyntick(rsp, rsp->gpnum - 1,
    rcu_implicit_dynticks_qs);

    /* Leave state in case more forcing is required. */
    diff --git a/kernel/rcutree.h b/kernel/rcutree.h
    index dc386a7..5348561 100644
    --- a/kernel/rcutree.h
    +++ b/kernel/rcutree.h
    @@ -296,8 +296,6 @@ struct rcu_state {
    long orphan_qlen; /* Number of orphaned cbs. */
    spinlock_t fqslock; /* Only one task forcing */
    /* quiescent states. */
    - long completed_fqs; /* Value of completed @ snap. */
    - /* Protected by fqslock. */
    unsigned long jiffies_force_qs; /* Time at which to invoke */
    /* force_quiescent_state(). */
    unsigned long n_force_qs; /* Number of calls to */
    --
    1.5.2.5


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