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rcu/nocb: Remove rcu_node structure from nocb list when de-offloaded

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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.

Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Frederic Weisbecker 2021-11-23 01:37:03 +01:00 committed by Paul E. McKenney
parent 0598a4d442
commit 2ebc45c44c
2 changed files with 43 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
kernel/rcu/tree.h
View File

@ -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;

45
kernel/rcu/tree_nocb.h
View File

@ -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)],