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From:   SeongJae Park <sjpark@amazon.com>
To:     <davem@davemloft.net>
CC:     SeongJae Park <sjpark@amazon.de>, <kuba@kernel.org>,
        <kuznet@ms2.inr.ac.ru>, <edumazet@google.com>, <fw@strlen.de>,
        <paulmck@kernel.org>, <netdev@vger.kernel.org>,
        <rcu@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: [PATCH v2 1/1] net/ipv4/inet_fragment: Batch fqdir destroy works
Date:   Thu, 10 Dec 2020 09:08:44 +0100
Message-ID: <20201210080844.23741-2-sjpark@amazon.com>
In-Reply-To: <20201210080844.23741-1-sjpark@amazon.com>
References: <20201210080844.23741-1-sjpark@amazon.com>
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From: SeongJae Park <sjpark@amazon.de>

In 'fqdir_exit()', a work for destroy of the 'fqdir' is enqueued.  The
work function, 'fqdir_work_fn()', calls 'rcu_barrier()'.  In case of
intensive 'fqdir_exit()' (e.g., frequent 'unshare()' systemcalls), this
increased contention could result in unacceptably high latency of
'rcu_barrier()'.  This commit avoids such contention by doing the
destroy work in batched manner, as similar to that of 'cleanup_net()'.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 include/net/inet_frag.h  |  2 +-
 net/ipv4/inet_fragment.c | 28 ++++++++++++++++++++--------
 2 files changed, 21 insertions(+), 9 deletions(-)

diff --git a/include/net/inet_frag.h b/include/net/inet_frag.h
index bac79e817776..558893d8810c 100644
--- a/include/net/inet_frag.h
+++ b/include/net/inet_frag.h
@@ -20,7 +20,7 @@ struct fqdir {
 
 	/* Keep atomic mem on separate cachelines in structs that include it */
 	atomic_long_t		mem ____cacheline_aligned_in_smp;
-	struct work_struct	destroy_work;
+	struct llist_node	destroy_list;
 };
 
 /**
diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
index 10d31733297d..d5c40386a764 100644
--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c
@@ -145,12 +145,19 @@ static void inet_frags_free_cb(void *ptr, void *arg)
 		inet_frag_destroy(fq);
 }
 
+static LLIST_HEAD(destroy_list);
+
 static void fqdir_work_fn(struct work_struct *work)
 {
-	struct fqdir *fqdir = container_of(work, struct fqdir, destroy_work);
-	struct inet_frags *f = fqdir->f;
+	struct llist_node *kill_list;
+	struct fqdir *fqdir, *tmp;
+	struct inet_frags *f;
 
-	rhashtable_free_and_destroy(&fqdir->rhashtable, inet_frags_free_cb, NULL);
+	/* Atomically snapshot the list of fqdirs to destroy */
+	kill_list = llist_del_all(&destroy_list);
+
+	llist_for_each_entry(fqdir, kill_list, destroy_list)
+		rhashtable_free_and_destroy(&fqdir->rhashtable, inet_frags_free_cb, NULL);
 
 	/* We need to make sure all ongoing call_rcu(..., inet_frag_destroy_rcu)
 	 * have completed, since they need to dereference fqdir.
@@ -158,10 +165,13 @@ static void fqdir_work_fn(struct work_struct *work)
 	 */
 	rcu_barrier();
 
-	if (refcount_dec_and_test(&f->refcnt))
-		complete(&f->completion);
+	llist_for_each_entry_safe(fqdir, tmp, kill_list, destroy_list) {
+		f = fqdir->f;
+		if (refcount_dec_and_test(&f->refcnt))
+			complete(&f->completion);
 
-	kfree(fqdir);
+		kfree(fqdir);
+	}
 }
 
 int fqdir_init(struct fqdir **fqdirp, struct inet_frags *f, struct net *net)
@@ -184,10 +194,12 @@ int fqdir_init(struct fqdir **fqdirp, struct inet_frags *f, struct net *net)
 }
 EXPORT_SYMBOL(fqdir_init);
 
+static DECLARE_WORK(fqdir_destroy_work, fqdir_work_fn);
+
 void fqdir_exit(struct fqdir *fqdir)
 {
-	INIT_WORK(&fqdir->destroy_work, fqdir_work_fn);
-	queue_work(system_wq, &fqdir->destroy_work);
+	if (llist_add(&fqdir->destroy_list, &destroy_list))
+		queue_work(system_wq, &fqdir_destroy_work);
 }
 EXPORT_SYMBOL(fqdir_exit);
 
-- 
2.17.1