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    SubjectRe: [PATCH tip/core/rcu 07/15] rcu: Add event tracing to ->gp_tasks update at GP start
    On Mon, 24 Jul 2017 14:44:36 -0700
    "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

    > There is currently event tracing to track when a task is preempted
    > within a preemptible RCU read-side critical section, and also when that
    > task subsequently reaches its outermost rcu_read_unlock(), but none
    > indicating when a new grace period starts when that grace period must
    > wait on pre-existing readers that have been been preempted at least once
    > since the beginning of their current RCU read-side critical sections.
    >
    > This commit therefore adds an event trace at grace-period start in
    > the case where there are such readers. Note that only the first
    > reader in the list is traced.
    >
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > ---
    > kernel/rcu/tree_plugin.h | 9 ++++++++-
    > 1 file changed, 8 insertions(+), 1 deletion(-)
    >
    > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
    > index 14ba496a13cd..3e3f92e981a1 100644
    > --- a/kernel/rcu/tree_plugin.h
    > +++ b/kernel/rcu/tree_plugin.h
    > @@ -636,10 +636,17 @@ static int rcu_print_task_exp_stall(struct rcu_node *rnp)
    > */
    > static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
    > {
    > + struct task_struct *t;
    > +
    > RCU_LOCKDEP_WARN(preemptible(), "rcu_preempt_check_blocked_tasks() invoked with preemption enabled!!!\n");
    > WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp));
    > - if (rcu_preempt_has_tasks(rnp))
    > + if (rcu_preempt_has_tasks(rnp)) {

    The only function of this if block is to fill the content of the
    trace event, correct?

    What about doing:

    if (trace_rcu_unlock_preempted_task_enabled() &&
    rcu_preempt_has_tasks(rnp)) {

    instead? The trace_rcu_unlock_preempted_task_enabled() is a static
    branch (aka jump_label), which would make the above a constant branch
    when tracing is not enabled, and would keep this from adding any extra
    overhead.

    -- Steve

    > rnp->gp_tasks = rnp->blkd_tasks.next;
    > + t = container_of(rnp->gp_tasks, struct task_struct,
    > + rcu_node_entry);
    > + trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),
    > + rnp->gpnum, t->pid);
    > + }
    > WARN_ON_ONCE(rnp->qsmask);
    > }
    >

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