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Subject[tip: core/rcu] rcu/nocb: Prepare for fine-grained deferred wakeup
The following commit has been merged into the core/rcu branch of tip:

Commit-ID: 870905169da8bfae0570df013efe860d33251b0f
Gitweb: https://git.kernel.org/tip/870905169da8bfae0570df013efe860d33251b0f
Author: Frederic Weisbecker <frederic@kernel.org>
AuthorDate: Tue, 23 Feb 2021 01:10:10 +01:00
Committer: Paul E. McKenney <paulmck@kernel.org>
CommitterDate: Wed, 12 May 2021 12:10:23 -07:00

rcu/nocb: Prepare for fine-grained deferred wakeup

Tuning the deferred wakeup level must be done from a safe wakeup
point. Currently those sites are:

* ->nocb_timer
* user/idle/guest entry
* CPU down
* softirq/rcuc

All of these sites perform the wake up for both RCU_NOCB_WAKE and
RCU_NOCB_WAKE_FORCE.

In order to merge ->nocb_timer and ->nocb_bypass_timer together, we plan
to add a new RCU_NOCB_WAKE_BYPASS that really should be deferred until
a timer fires so that we don't wake up the NOCB-gp kthread too early.

To prepare for that, this commit specifies the per-callsite wakeup
level/limit.

Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
[ paulmck: Fix non-NOCB rcu_nocb_need_deferred_wakeup() definition. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tree.c | 2 +-
kernel/rcu/tree.h | 2 +-
kernel/rcu/tree_plugin.h | 17 +++++++++--------
3 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8e78b24..5f1545a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3911,7 +3911,7 @@ static int rcu_pending(int user)
check_cpu_stall(rdp);

/* Does this CPU need a deferred NOCB wakeup? */
- if (rcu_nocb_need_deferred_wakeup(rdp))
+ if (rcu_nocb_need_deferred_wakeup(rdp, RCU_NOCB_WAKE))
return 1;

/* Is this a nohz_full CPU in userspace or idle? (Ignore RCU if so.) */
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index b280a84..2510e86 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -433,7 +433,7 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
bool *was_alldone, unsigned long flags);
static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_empty,
unsigned long flags);
-static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp);
+static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level);
static bool do_nocb_deferred_wakeup(struct rcu_data *rdp);
static void rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp);
static void rcu_spawn_cpu_nocb_kthread(int cpu);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index db28e31..e2e5e49 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2352,13 +2352,14 @@ static int rcu_nocb_cb_kthread(void *arg)
}

/* Is a deferred wakeup of rcu_nocb_kthread() required? */
-static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp)
+static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level)
{
- return READ_ONCE(rdp->nocb_defer_wakeup) > RCU_NOCB_WAKE_NOT;
+ return READ_ONCE(rdp->nocb_defer_wakeup) >= level;
}

/* Do a deferred wakeup of rcu_nocb_kthread(). */
-static bool do_nocb_deferred_wakeup_common(struct rcu_data *rdp)
+static bool do_nocb_deferred_wakeup_common(struct rcu_data *rdp,
+ int level)
{
unsigned long flags;
int ndw;
@@ -2367,7 +2368,7 @@ static bool do_nocb_deferred_wakeup_common(struct rcu_data *rdp)

raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);

- if (!rcu_nocb_need_deferred_wakeup(rdp_gp)) {
+ if (!rcu_nocb_need_deferred_wakeup(rdp_gp, level)) {
raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
return false;
}
@@ -2384,7 +2385,7 @@ static void do_nocb_deferred_wakeup_timer(struct timer_list *t)
{
struct rcu_data *rdp = from_timer(rdp, t, nocb_timer);

- do_nocb_deferred_wakeup_common(rdp);
+ do_nocb_deferred_wakeup_common(rdp, RCU_NOCB_WAKE);
}

/*
@@ -2397,8 +2398,8 @@ static bool do_nocb_deferred_wakeup(struct rcu_data *rdp)
if (!rdp->nocb_gp_rdp)
return false;

- if (rcu_nocb_need_deferred_wakeup(rdp->nocb_gp_rdp))
- return do_nocb_deferred_wakeup_common(rdp);
+ if (rcu_nocb_need_deferred_wakeup(rdp->nocb_gp_rdp, RCU_NOCB_WAKE))
+ return do_nocb_deferred_wakeup_common(rdp, RCU_NOCB_WAKE);
return false;
}

@@ -2939,7 +2940,7 @@ static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
{
}

-static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp)
+static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level)
{
return false;
}
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