Hello,

On Mon, Feb 08, 2021 at 03:29:21PM -0500, Johannes Weiner wrote:

quoted

quoted

@@ -789,6 +793,7 @@ static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu)
 		u64_stats_update_end(&blkg->iostat.sync);
 
 		/* propagate global delta to parent */
+		/* XXX: could skip this if parent is root */
 		if (parent) {
 			u64_stats_update_begin(&parent->iostat.sync);
 			blkg_iostat_set(&delta, &blkg->iostat.cur);

Might as well update this similar to cgroup_base_stat_flush()?

I meant to revisit that, but I'm never 100% confident when it comes to
the interaction and lifetime of css, blkcg and blkg_gq.

Yeah, it does get hairy.

IIUC, the blkg_gq->parent linkage always matches the css parent
linkage; it just exists as an optimization for ancestor walks, which
would otherwise have to do radix lookups when going through the css.

But yes, at least this part is straight-forward.

So with the cgroup_parent() check at the beginning of the function
making sure we're looking at a non-root group, blkg_gq->parent should
also never be NULL and I can do if (paren->parent) directly, right?

I think so.

quoted

quoted

 static void cgroup_base_stat_flush(struct cgroup *cgrp, int cpu)
 {
-	struct cgroup *parent = cgroup_parent(cgrp);
 	struct cgroup_rstat_cpu *rstatc = cgroup_rstat_cpu(cgrp, cpu);
+	struct cgroup *parent = cgroup_parent(cgrp);

Is this chunk intentional?

Yeah, it puts the local variable declarations into reverse christmas
tree ordering to make them a bit easier to read. It's a while-at-it
cleanup, mostly a force of habit. I can drop it if it bothers you.

I don't mind either way. Was just wondering whether it was accidental.

Thanks.

-- 
tejun