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SubjectRe: [PATCH 8/8] fs/aio: Use rcu_work instead of explicit rcu and work item
Hi Tejun,

sorry for late reply.

On 03/14, Tejun Heo wrote:
>
> Workqueue now has rcu_work. Use it instead of open-coding rcu -> work
> item bouncing.

Yes, but a bit of open-coding may be more efficient...

> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -115,8 +115,7 @@ struct kioctx {
> struct page **ring_pages;
> long nr_pages;
>
> - struct rcu_head free_rcu;
> - struct work_struct free_work; /* see free_ioctx() */
> + struct rcu_work free_rwork; /* see free_ioctx() */

IIUC, you can't easily share rcu_work's, thus every kioctx needs its own
->free_rwork and this looks sub-optimal.

What do you think about the (untested) patch below?

Oleg.


--- a/fs/aio.c
+++ b/fs/aio.c
@@ -115,8 +115,10 @@ struct kioctx {
struct page **ring_pages;
long nr_pages;

- struct rcu_head free_rcu;
- struct work_struct free_work; /* see free_ioctx() */
+ union {
+ struct rcu_head free_rcu;
+ struct llist_node free_llist;
+ };

/*
* signals when all in-flight requests are done
@@ -589,31 +591,38 @@ static int kiocb_cancel(struct aio_kiocb *kiocb)
return cancel(&kiocb->common);
}

+static struct llist_head free_ioctx_llist;
+
/*
* free_ioctx() should be RCU delayed to synchronize against the RCU
* protected lookup_ioctx() and also needs process context to call
* aio_free_ring(), so the double bouncing through kioctx->free_rcu and
* ->free_work.
*/
-static void free_ioctx(struct work_struct *work)
+static void free_ioctx_workfn(struct work_struct *work)
{
- struct kioctx *ctx = container_of(work, struct kioctx, free_work);
+ struct llist_node *llist = llist_del_all(&free_ioctx_llist);
+ struct kioctx *ctx, *tmp;

- pr_debug("freeing %p\n", ctx);
+ llist_for_each_entry_safe(ctx, tmp, llist, free_llist) {
+ pr_debug("freeing %p\n", ctx);

- aio_free_ring(ctx);
- free_percpu(ctx->cpu);
- percpu_ref_exit(&ctx->reqs);
- percpu_ref_exit(&ctx->users);
- kmem_cache_free(kioctx_cachep, ctx);
+ aio_free_ring(ctx);
+ free_percpu(ctx->cpu);
+ percpu_ref_exit(&ctx->reqs);
+ percpu_ref_exit(&ctx->users);
+ kmem_cache_free(kioctx_cachep, ctx);
+ }
}

+static DECLARE_WORK(free_ioctx_work, free_ioctx_workfn);
+
static void free_ioctx_rcufn(struct rcu_head *head)
{
struct kioctx *ctx = container_of(head, struct kioctx, free_rcu);

- INIT_WORK(&ctx->free_work, free_ioctx);
- schedule_work(&ctx->free_work);
+ if (llist_add(&ctx->free_llist, &free_ioctx_llist))
+ schedule_work(&free_ioctx_work);
}

static void free_ioctx_reqs(struct percpu_ref *ref)
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