The rcu_gp_kthread_wake() function checks for three conditions before waking up
grace period kthreads:
* Is the thread we are trying to wake up the current thread?
* Are the gp_flags zero? (all threads wait on non-zero gp_flags condition)
* Is there no thread created for this flavour, hence nothing to wake up?
If any one of these condition is true, we do not call wake_up().
In rcu_report_qs_rsp(), I added a pr_info() call testing if any of the above
conditions is true, in which case we can avoid calling wake_up(). It turns out
that quite a few actually are. Most of the cases where we can avoid is
condition 2
above and condition 1 also occurs quite often. Condition 3 never happens.
I could not test the wake_up() in force_quiescent_state() as that is not
triggered trivially, but I am assuming we can replace wake_up() there too.
Hence this commit tries to avoid calling wake_up() whenever we can by using
rcu_gp_kthread_wake() function.
One concern is the comment which states that we need a memory barrier at that
location which is being implied by the wake_up(). Should we put an smp_mb() and
just not rely on the barrier provided by wake_up()? Thoughts?
Signed-off-by: Pranith Kumar <bobby.pr...@gmail.com>
---
kernel/rcu/tree.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 72e0b1f..d0e0d6e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1938,7 +1938,8 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp,
unsigned long flags)
{
WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
- wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */
+ /* Memory barrier implied by wake_up() path. */
+ rcu_gp_kthread_wake(rsp);
}
/*
@@ -2516,7 +2517,8 @@ static void force_quiescent_state(struct rcu_state *rsp)
ACCESS_ONCE(rsp->gp_flags) =
ACCESS_ONCE(rsp->gp_flags) | RCU_GP_FLAG_FQS;
raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
- wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */
+ /* Memory barrier implied by wake_up() path. */
+ rcu_gp_kthread_wake(rsp);
}
/*
--
2.0.1
--
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