Re: [PATCH v3 1/1] net/ipv4/inet_fragment: Batch fqdir destroy works

On Fri, 11 Dec 2020 09:43:41 +0100 Eric Dumazet [off-list ref] wrote:

On Fri, Dec 11, 2020 at 9:21 AM SeongJae Park [off-list ref] wrote:

quoted

From: SeongJae Park <redacted>

For each 'fqdir_exit()' call, a work for destroy of the 'fqdir' is
enqueued.  The work function, 'fqdir_work_fn()', internally 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 'rcu_barrier()' and subsequent lightweight works
in a batched manner using a dedicated singlethread worker, as similar to
that of 'cleanup_net()'.

Not sure why you submit a patch series with a single patch.

Your cover letter contains interesting info that would better be
captured in this changelog IMO

I thought someone might think this is not a kernel issue but the reproducer is
insane or 'rcu_barrier()' needs modification.  I wanted to do such discussion
on the coverletter.  Seems I misjudged.  I will make this single patch and move
the detailed information here from the next version.

quoted

Signed-off-by: SeongJae Park <redacted>
---
 include/net/inet_frag.h  |  1 +
 net/ipv4/inet_fragment.c | 45 +++++++++++++++++++++++++++++++++-------
 2 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/include/net/inet_frag.h b/include/net/inet_frag.h
index bac79e817776..48cc5795ceda 100644
--- a/include/net/inet_frag.h
+++ b/include/net/inet_frag.h

@@ -21,6 +21,7 @@ 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       free_list;
 };

 /**

diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
index 10d31733297d..a6fc4afcc323 100644
--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c

@@ -145,12 +145,17 @@ static void inet_frags_free_cb(void *ptr, void *arg)
                inet_frag_destroy(fq);
 }

-static void fqdir_work_fn(struct work_struct *work)
+static struct workqueue_struct *fqdir_wq;
+static LLIST_HEAD(free_list);
+
+static void fqdir_free_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 free */
+       kill_list = llist_del_all(&free_list);

        /* We need to make sure all ongoing call_rcu(..., inet_frag_destroy_rcu)
         * have completed, since they need to dereference fqdir.

@@ -158,12 +163,38 @@ 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, free_list) {
+               f = fqdir->f;
+               if (refcount_dec_and_test(&f->refcnt))
+                       complete(&f->completion);

-       kfree(fqdir);
+               kfree(fqdir);
+       }
 }

+static DECLARE_WORK(fqdir_free_work, fqdir_free_fn);
+
+static void fqdir_work_fn(struct work_struct *work)
+{
+       struct fqdir *fqdir = container_of(work, struct fqdir, destroy_work);
+
+       rhashtable_free_and_destroy(&fqdir->rhashtable, inet_frags_free_cb, NULL);
+
+       if (llist_add(&fqdir->free_list, &free_list))
+               queue_work(fqdir_wq, &fqdir_free_work);

I think you misunderstood me.

Since this fqdir_free_work will have at most one instance, you can use
system_wq here, there is no risk of abuse.

My suggestion was to not use system_wq for fqdir_exit(), to better
control the number
 of threads in rhashtable cleanups.

void fqdir_exit(struct fqdir *fqdir)
{
        INIT_WORK(&fqdir->destroy_work, fqdir_work_fn);
-       queue_work(system_wq, &fqdir->destroy_work);
+      queue_work(fqdir_wq, &fqdir->destroy_work);
}

Oh, got it.  I definitely misunderstood.  My fault, sorry.

quoted

+}
+
+static int __init fqdir_wq_init(void)
+{
+       fqdir_wq = create_singlethread_workqueue("fqdir");

And here, I suggest to use a non ordered work queue, allowing one
thread per cpu, to allow concurrent rhashtable cleanups

Also "fqdir" name is rather vague, this is an implementation detail ?

fqdir_wq =create_workqueue("inet_frag_wq");

So, what you are suggesting is to use a dedicated non-ordered work queue
(fqdir_wq) for rhashtable cleanup and do the remaining works with system_wq in
the batched manner, right?  IOW, doing below change on top of this patch.

--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c

@@ -145,7 +145,7 @@ static void inet_frags_free_cb(void *ptr, void *arg)
                inet_frag_destroy(fq);
 }

-static struct workqueue_struct *fqdir_wq;
+static struct workqueue_struct *inet_frag_wq;
 static LLIST_HEAD(free_list);

 static void fqdir_free_fn(struct work_struct *work)

@@ -181,14 +181,14 @@ static void fqdir_work_fn(struct work_struct *work)
        rhashtable_free_and_destroy(&fqdir->rhashtable, inet_frags_free_cb, NULL);

        if (llist_add(&fqdir->free_list, &free_list))
-               queue_work(fqdir_wq, &fqdir_free_work);
+               queue_work(system_wq, &fqdir_free_work);
 }

 static int __init fqdir_wq_init(void)
 {
-       fqdir_wq = create_singlethread_workqueue("fqdir");
-       if (!fqdir_wq)
-               panic("Could not create fqdir workq");
+       inet_frag_wq = create_workqueue("inet_frag_wq");
+       if (!inet_frag_wq)
+               panic("Could not create inet frag workq");
        return 0;
 }

@@ -218,7 +218,7 @@ EXPORT_SYMBOL(fqdir_init);
 void fqdir_exit(struct fqdir *fqdir)
 {
        INIT_WORK(&fqdir->destroy_work, fqdir_work_fn);
-       queue_work(system_wq, &fqdir->destroy_work);
+       queue_work(inet_frag_wq, &fqdir->destroy_work);
 }
 EXPORT_SYMBOL(fqdir_exit);

If I'm still misunderstanding, please let me know.


Thanks,
SeongJae Park