[PATCH v2 4/9] cgroup: rstat: add WARN_ON_ONCE() if flushing outside task context

From: Yosry Ahmed <yosryahmed@google.com>
To: "Tejun Heo" <tj@kernel.org>, "Josef Bacik" <josef@toxicpanda.com>,
	"Jens Axboe" <axboe@kernel.dk>,
	"Zefan Li" <lizefan.x@bytedance.com>,
	"Johannes Weiner" <hannes@cmpxchg.org>,
	"Michal Hocko" <mhocko@kernel.org>,
	"Roman Gushchin" <roman.gushchin@linux.dev>,
	"Shakeel Butt" <shakeelb@google.com>,
	"Muchun Song" <muchun.song@linux.dev>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Michal Koutný" <mkoutny@suse.com>
Cc: Vasily Averin <vasily.averin@linux.dev>,
	cgroups@vger.kernel.org,  linux-block@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	 bpf@vger.kernel.org, Yosry Ahmed <yosryahmed@google.com>
Subject: [PATCH v2 4/9] cgroup: rstat: add WARN_ON_ONCE() if flushing outside task context
Date: Tue, 28 Mar 2023 22:16:39 +0000	[thread overview]
Message-ID: <20230328221644.803272-5-yosryahmed@google.com> (raw)
In-Reply-To: <20230328221644.803272-1-yosryahmed@google.com>

rstat flushing is too expensive to perform in irq context.
The previous patch removed the only context that may invoke an rstat
flush from irq context, add a WARN_ON_ONCE() to detect future
violations, or those that we are not aware of.

Ideally, we wouldn't flush with irqs disabled either, but we have one
context today that does so in mem_cgroup_usage(). Forbid callers from
irq context for now, and hopefully we can also forbid callers with irqs
disabled in the future when we can get rid of this callsite.

Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
---
 kernel/cgroup/rstat.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index d3252b0416b6..c2571939139f 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -176,6 +176,8 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
 {
 	int cpu;
 
+	/* rstat flushing is too expensive for irq context */
+	WARN_ON_ONCE(!in_task());
 	lockdep_assert_held(&cgroup_rstat_lock);
 
 	for_each_possible_cpu(cpu) {
-- 
2.40.0.348.gf938b09366-goog


