On Fri, Dec 11, 2020 at 9:21 AM SeongJae Park <sjp...@amazon.com> wrote:
>
> From: SeongJae Park <sjp...@amazon.de>
>
> 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
>
> Signed-off-by: SeongJae Park <sjp...@amazon.de>
> ---
> 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);
}
> +}
> +
> +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");
> + if (!fqdir_wq)
> + panic("Could not create fqdir workq");
> + return 0;
> +}
> +
> +pure_initcall(fqdir_wq_init);
> +
> +
> int fqdir_init(struct fqdir **fqdirp, struct inet_frags *f, struct net *net)
> {
> struct fqdir *fqdir = kzalloc(sizeof(*fqdir), GFP_KERNEL);
> --
> 2.17.1
>
