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    SubjectRe: [PATCH RFC tip/core/rcu 2/5] rcu: Short-circuit normal GPs via expedited GPs
    On Wed, Jul 01, 2015 at 12:03:52PM +0200, Peter Zijlstra wrote:
    > On Tue, Jun 30, 2015 at 02:48:27PM -0700, Paul E. McKenney wrote:
    > > @@ -2121,17 +2137,24 @@ static int __noreturn rcu_gp_kthread(void *arg)
    > > TPS("fqswait"));
    > > rsp->gp_state = RCU_GP_WAIT_FQS;
    > > ret = wait_event_interruptible_timeout(rsp->gp_wq,
    > > - ((gf = READ_ONCE(rsp->gp_flags)) &
    > > - RCU_GP_FLAG_FQS) ||
    > > - (!READ_ONCE(rnp->qsmask) &&
    > > - !rcu_preempt_blocked_readers_cgp(rnp)),
    > > - j);
    > > + ((gf = READ_ONCE(rsp->gp_flags)) &
    > > + RCU_GP_FLAG_FQS) ||
    > > + (!READ_ONCE(rnp->qsmask) &&
    > > + !rcu_preempt_blocked_readers_cgp(rnp)) ||
    > > + rcu_exp_gp_seq_done(rsp->exp_rsp,
    > > + rsp->gp_exp_snap),
    > > + j);
    >
    > How about using a helper function there?
    >
    > static inline bool rcu_gp_done(rsp, rnp)
    > {
    > /* Forced Quiescent State complete */
    > if (READ_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS)
    > return true;
    >
    > /* QS not masked and not blocked by preempted readers */
    > if (!READ_ONCE(rnp->qsmask) &&
    > !rcu_preempt_blocked_readers_cgp(rnp))
    > return true;
    >
    > /* Expedited Grace Period completed */
    > if (rcu_exp_gp_seq_done(rsp))
    > return true;
    >
    > return false;
    > }
    >
    > ret = wait_event_interruptible_timeout(rsp->gp_wq,
    > rcu_gp_done(rsp, rnp), j);

    Fair point, and applies to the original as well, give or take comments
    and naming. Please see below for the corresponding patch. Thoughts?

    Thanx, Paul

    ------------------------------------------------------------------------

    commit a8bb9abbf3690e507d61efe8144cec091ce5fb02
    Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Date: Wed Jul 1 13:50:28 2015 -0700

    rcu: Pull out wait_event*() condition into helper function

    The condition for the wait_event_interruptible_timeout() that waits
    to do the next force-quiescent-state scan is a bit ornate:

    ((gf = READ_ONCE(rsp->gp_flags)) &
    RCU_GP_FLAG_FQS) ||
    (!READ_ONCE(rnp->qsmask) &&
    !rcu_preempt_blocked_readers_cgp(rnp))

    This commit therefore pulls this condition out into a helper function
    and comments its component conditions.

    Reported-by: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    index 4992bfd360b6..2afb8e8c5134 100644
    --- a/kernel/rcu/tree.c
    +++ b/kernel/rcu/tree.c
    @@ -1904,6 +1904,26 @@ static int rcu_gp_init(struct rcu_state *rsp)
    }

    /*
    + * Helper function for wait_event_interruptible_timeout() wakeup
    + * at force-quiescent-state time.
    + */
    +static bool rcu_gp_fqs_check_wake(struct rcu_state *rsp, int *gfp)
    +{
    + struct rcu_node *rnp = rcu_get_root(rsp);
    +
    + /* Someone like call_rcu() requested a force-quiescent-state scan. */
    + *gfp = READ_ONCE(rsp->gp_flags);
    + if (*gfp & RCU_GP_FLAG_FQS)
    + return true;
    +
    + /* The current grace period has completed. */
    + if (!READ_ONCE(rnp->qsmask) && !rcu_preempt_blocked_readers_cgp(rnp))
    + return true;
    +
    + return false;
    +}
    +
    +/*
    * Do one round of quiescent-state forcing.
    */
    static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
    @@ -2067,11 +2087,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
    TPS("fqswait"));
    rsp->gp_state = RCU_GP_WAIT_FQS;
    ret = wait_event_interruptible_timeout(rsp->gp_wq,
    - ((gf = READ_ONCE(rsp->gp_flags)) &
    - RCU_GP_FLAG_FQS) ||
    - (!READ_ONCE(rnp->qsmask) &&
    - !rcu_preempt_blocked_readers_cgp(rnp)),
    - j);
    + rcu_gp_fqs_check_wake(rsp, &gf), j);
    rsp->gp_state = RCU_GP_DONE_FQS;
    /* Locking provides needed memory barriers. */
    /* If grace period done, leave loop. */


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