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SubjectRe: [PATCH v6 3/3] blk-cgroup: Optimize blkcg_rstat_flush()
Hello.

On Thu, Jun 02, 2022 at 03:20:20PM -0400, Waiman Long <longman@redhat.com> 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.

> +#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.

> @@ -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)

> 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.


Thanks,
Michal

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