[PATCH v2 1/1] net/ipv4/inet_fragment: Batch fqdir destroy works

SeongJae Park
2020-12-10T08:10:33Z

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