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Date: Wed, 1 Dec 2021 16:29:07 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
jiangshanlai@...il.com, akpm@...ux-foundation.org,
mathieu.desnoyers@...icios.com, josh@...htriplett.org,
tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
oleg@...hat.com, joel@...lfernandes.org,
Frederic Weisbecker <frederic@...nel.org>,
Neeraj Upadhyay <quic_neeraju@...cinc.com>,
Boqun Feng <boqun.feng@...il.com>,
Uladzislau Rezki <urezki@...il.com>,
Juri Lelli <juri.lelli@...hat.com>,
"Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 13/18] rcu/nocb: Remove rcu_node structure from nocb list when de-offloaded
From: Frederic Weisbecker <frederic@...nel.org>
The nocb_gp_wait() function iterates over all CPUs in its group,
including even those CPUs that have been de-offloaded. This is of
course suboptimal, especially if none of the CPUs within the group are
currently offloaded. This will become even more of a problem once a
nocb kthread is created for all possible CPUs.
Therefore use a standard double linked list to link all the offloaded
rcu_data structures and safely add or delete these structure as we
offload or de-offload them, respectively.
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Uladzislau Rezki <urezki@...il.com>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Joel Fernandes <joel@...lfernandes.org>
Tested-by: Juri Lelli <juri.lelli@...hat.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
kernel/rcu/tree.h | 7 +++++--
kernel/rcu/tree_nocb.h | 45 +++++++++++++++++++++++++++++++++++-------
2 files changed, 43 insertions(+), 9 deletions(-)
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 4f6c67b3ccd5e..5884380f4039c 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -227,8 +227,11 @@ struct rcu_data {
struct swait_queue_head nocb_gp_wq; /* For nocb kthreads to sleep on. */
bool nocb_cb_sleep; /* Is the nocb CB thread asleep? */
struct task_struct *nocb_cb_kthread;
- struct rcu_data *nocb_next_cb_rdp;
- /* Next rcu_data in wakeup chain. */
+ struct list_head nocb_head_rdp; /*
+ * Head of rcu_data list in wakeup chain,
+ * if rdp_gp.
+ */
+ struct list_head nocb_entry_rdp; /* rcu_data node in wakeup chain. */
/* The following fields are used by CB kthread, hence new cacheline. */
struct rcu_data *nocb_gp_rdp ____cacheline_internodealigned_in_smp;
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 2461fe8d0c23a..8e94a5344afed 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -625,7 +625,21 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
* and the global grace-period kthread are awakened if needed.
*/
WARN_ON_ONCE(my_rdp->nocb_gp_rdp != my_rdp);
- for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_cb_rdp) {
+ /*
+ * An rcu_data structure is removed from the list after its
+ * CPU is de-offloaded and added to the list before that CPU is
+ * (re-)offloaded. If the following loop happens to be referencing
+ * that rcu_data structure during the time that the corresponding
+ * CPU is de-offloaded and then immediately re-offloaded, this
+ * loop's rdp pointer will be carried to the end of the list by
+ * the resulting pair of list operations. This can cause the loop
+ * to skip over some of the rcu_data structures that were supposed
+ * to have been scanned. Fortunately a new iteration through the
+ * entire loop is forced after a given CPU's rcu_data structure
+ * is added to the list, so the skipped-over rcu_data structures
+ * won't be ignored for long.
+ */
+ list_for_each_entry_rcu(rdp, &my_rdp->nocb_head_rdp, nocb_entry_rdp, 1) {
bool needwake_state = false;
if (!nocb_gp_enabled_cb(rdp))
@@ -1003,6 +1017,8 @@ static long rcu_nocb_rdp_deoffload(void *arg)
swait_event_exclusive(rdp->nocb_state_wq,
!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB |
SEGCBLIST_KTHREAD_GP));
+ /* Stop nocb_gp_wait() from iterating over this structure. */
+ list_del_rcu(&rdp->nocb_entry_rdp);
/*
* Lock one last time to acquire latest callback updates from kthreads
* so we can later handle callbacks locally without locking.
@@ -1066,6 +1082,17 @@ static long rcu_nocb_rdp_offload(void *arg)
return -EINVAL;
pr_info("Offloading %d\n", rdp->cpu);
+
+ /*
+ * Cause future nocb_gp_wait() invocations to iterate over
+ * structure, resetting ->nocb_gp_sleep and waking up the related
+ * "rcuog". Since nocb_gp_wait() in turn locks ->nocb_gp_lock
+ * before setting ->nocb_gp_sleep again, we are guaranteed to
+ * iterate this newly added structure before "rcuog" goes to
+ * sleep again.
+ */
+ list_add_tail_rcu(&rdp->nocb_entry_rdp, &rdp->nocb_gp_rdp->nocb_head_rdp);
+
/*
* Can't use rcu_nocb_lock_irqsave() before SEGCBLIST_LOCKING
* is set.
@@ -1268,7 +1295,6 @@ static void __init rcu_organize_nocb_kthreads(void)
int nl = 0; /* Next GP kthread. */
struct rcu_data *rdp;
struct rcu_data *rdp_gp = NULL; /* Suppress misguided gcc warn. */
- struct rcu_data *rdp_prev = NULL;
if (!cpumask_available(rcu_nocb_mask))
return;
@@ -1288,8 +1314,8 @@ static void __init rcu_organize_nocb_kthreads(void)
/* New GP kthread, set up for CBs & next GP. */
gotnocbs = true;
nl = DIV_ROUND_UP(rdp->cpu + 1, ls) * ls;
- rdp->nocb_gp_rdp = rdp;
rdp_gp = rdp;
+ INIT_LIST_HEAD(&rdp->nocb_head_rdp);
if (dump_tree) {
if (!firsttime)
pr_cont("%s\n", gotnocbscbs
@@ -1302,12 +1328,11 @@ static void __init rcu_organize_nocb_kthreads(void)
} else {
/* Another CB kthread, link to previous GP kthread. */
gotnocbscbs = true;
- rdp->nocb_gp_rdp = rdp_gp;
- rdp_prev->nocb_next_cb_rdp = rdp;
if (dump_tree)
pr_cont(" %d", cpu);
}
- rdp_prev = rdp;
+ rdp->nocb_gp_rdp = rdp_gp;
+ list_add_tail(&rdp->nocb_entry_rdp, &rdp_gp->nocb_head_rdp);
}
if (gotnocbs && dump_tree)
pr_cont("%s\n", gotnocbscbs ? "" : " (self only)");
@@ -1369,6 +1394,7 @@ static void show_rcu_nocb_state(struct rcu_data *rdp)
{
char bufw[20];
char bufr[20];
+ struct rcu_data *nocb_next_rdp;
struct rcu_segcblist *rsclp = &rdp->cblist;
bool waslocked;
bool wassleep;
@@ -1376,11 +1402,16 @@ static void show_rcu_nocb_state(struct rcu_data *rdp)
if (rdp->nocb_gp_rdp == rdp)
show_rcu_nocb_gp_state(rdp);
+ nocb_next_rdp = list_next_or_null_rcu(&rdp->nocb_gp_rdp->nocb_head_rdp,
+ &rdp->nocb_entry_rdp,
+ typeof(*rdp),
+ nocb_entry_rdp);
+
sprintf(bufw, "%ld", rsclp->gp_seq[RCU_WAIT_TAIL]);
sprintf(bufr, "%ld", rsclp->gp_seq[RCU_NEXT_READY_TAIL]);
pr_info(" CB %d^%d->%d %c%c%c%c%c%c F%ld L%ld C%d %c%c%s%c%s%c%c q%ld %c CPU %d%s\n",
rdp->cpu, rdp->nocb_gp_rdp->cpu,
- rdp->nocb_next_cb_rdp ? rdp->nocb_next_cb_rdp->cpu : -1,
+ nocb_next_rdp ? nocb_next_rdp->cpu : -1,
"kK"[!!rdp->nocb_cb_kthread],
"bB"[raw_spin_is_locked(&rdp->nocb_bypass_lock)],
"cC"[!!atomic_read(&rdp->nocb_lock_contended)],
--
2.31.1.189.g2e36527f23
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