On 6/8/22 12:57, Michal Koutný wrote:
Hello.
On Thu, Jun 02, 2022 at 03:20:20PM -0400, Waiman Long <longman@xxxxxxxxxx> wrote:
As it is likely that not all the percpu blkg_iostat_set's has been
updated since the last flush, those stale blkg_iostat_set's don't need
to be flushed in this case.
Yes, there's no point to flush stats for idle devices if there can be
many of them. Good idea.
+static struct llist_node *fetch_delete_blkcg_llist(struct llist_head *lhead)
+{
+ return xchg(&lhead->first, &llist_last);
+}
+
+static struct llist_node *fetch_delete_lnode_next(struct llist_node *lnode)
+{
+ struct llist_node *next = READ_ONCE(lnode->next);
+ struct blkcg_gq *blkg = llist_entry(lnode, struct blkg_iostat_set,
+ lnode)->blkg;
+
+ WRITE_ONCE(lnode->next, NULL);
+ percpu_ref_put(&blkg->refcnt);
+ return next;
+}
Idea/just asking: would it make sense to generalize this into llist.c
(this is basically llist_del_first() + llist_del_all() with a sentinel)?
For the sake of reusability.
I have thought about that. It can be done as a follow-up patch to add a
sentinel version into llist and use that instead. Of course, I can also
update this patchset to include that.
+#define blkcg_llist_for_each_entry_safe(pos, node, nxt) \
+ for (; (node != &llist_last) && \
+ (pos = llist_entry(node, struct blkg_iostat_set, lnode), \
+ nxt = fetch_delete_lnode_next(node), true); \
+ node = nxt)
+
It's good hygiene to parenthesize the args.
I am aware of that. I will certainly add that if it is a generic macro
that can have many users.
@@ -2011,9 +2092,16 @@ void blk_cgroup_bio_start(struct bio *bio)
}
bis->cur.ios[rwd]++;
+ if (!READ_ONCE(bis->lnode.next)) {
+ struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
+
+ llist_add(&bis->lnode, lhead);
+ percpu_ref_get(&bis->blkg->refcnt);
+ }
+
When a blkg's cgroup is rmdir'd, what happens with the lhead list?
We have cgroup_rstat_exit() in css_free_rwork_fn() that ultimately flushes rstats.
init_and_link_css however adds reference form blkcg->css to cgroup->css.
The blkcg->css would be (transitively) pinned by the lhead list and
hence would prevent the final flush (when refs drop to zero). Seems like
a cyclic dependency.
Luckily, there's also per-subsys flushing in css_release which could be
moved after rmdir (offlining) but before last ref is gone:
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index adb820e98f24..d830e6a8fb3b 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5165,11 +5165,6 @@ static void css_release_work_fn(struct work_struct *work)
if (ss) {
/* css release path */
- if (!list_empty(&css->rstat_css_node)) {
- cgroup_rstat_flush(cgrp);
- list_del_rcu(&css->rstat_css_node);
- }
-
cgroup_idr_replace(&ss->css_idr, NULL, css->id);
if (ss->css_released)
ss->css_released(css);
@@ -5279,6 +5274,11 @@ static void offline_css(struct cgroup_subsys_state *css)
css->flags &= ~CSS_ONLINE;
RCU_INIT_POINTER(css->cgroup->subsys[ss->id], NULL);
+ if (!list_empty(&css->rstat_css_node)) {
+ cgroup_rstat_flush(css->cgrp);
+ list_del_rcu(&css->rstat_css_node);
+ }
+
wake_up_all(&css->cgroup->offline_waitq);
}
(not tested)
Good point.
Your change may not be enough since there could be update after the
flush which will pin the blkg and hence blkcg. I guess one possible
solution may be to abandon the llist and revert back to list iteration
when offline. I need to think a bit more about that.
u64_stats_update_end_irqrestore(&bis->sync, flags);
if (cgroup_subsys_on_dfl(io_cgrp_subsys))
- cgroup_rstat_updated(bio->bi_blkg->blkcg->css.cgroup, cpu);
+ cgroup_rstat_updated(blkcg->css.cgroup, cpu);
Maybe bundle the lhead list maintenace with cgroup_rstat_updated() under
cgroup_subsys_on_dfl()? The stats can be read on v1 anyway.
I don't quite understand here. The change is not specific to v1 or v2.
What do you mean by the stat is readable on v1?
Cheers,
Longman
