Re: [PATCH for-next v2 25/25] io_uring: mutex locked poll hashing

[Hao Xu <hao.xu@linux.dev>]

On 6/14/22 22:37, Pavel Begunkov wrote:
> Currently we do two extra spin lock/unlock pairs to add a poll/apoll
> request to the cancellation hash table and remove it from there.
> 
> On the submission side we often already hold ->uring_lock and tw
> completion is likely to hold it as well. Add a second cancellation hash
> table protected by ->uring_lock. In concerns for latency because of a
> need to have the mutex locked on the completion side, use the new table
> only in following cases:
> 
> 1) IORING_SETUP_SINGLE_ISSUER: only one task grabs uring_lock, so there
>     is no contention and so the main tw hander will always end up
>     grabbing it before calling into callbacks.

This statement seems not true, the io-worker may grab the uring lock,
and that's why the [1] place I marked below is needed, right? Or do I
miss something?

> 
> 2) IORING_SETUP_SQPOLL: same as with single issuer, only one task is
>     using ->uring_lock.

same as above.

> 
> 3) apoll: we normally grab the lock on the completion side anyway to
>     execute the request, so it's free.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>   io_uring/io_uring.c       |   9 +++-
>   io_uring/io_uring_types.h |   4 ++
>   io_uring/poll.c           | 111 ++++++++++++++++++++++++++++++--------
>   3 files changed, 102 insertions(+), 22 deletions(-)
> 
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index 4bead16e57f7..1395176bc2ea 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -731,6 +731,8 @@ static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
>   	hash_bits = clamp(hash_bits, 1, 8);
>   	if (io_alloc_hash_table(&ctx->cancel_table, hash_bits))
>   		goto err;
> +	if (io_alloc_hash_table(&ctx->cancel_table_locked, hash_bits))
> +		goto err;
>   
>   	ctx->dummy_ubuf = kzalloc(sizeof(*ctx->dummy_ubuf), GFP_KERNEL);
>   	if (!ctx->dummy_ubuf)
> @@ -773,6 +775,7 @@ static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
>   err:
>   	kfree(ctx->dummy_ubuf);
>   	kfree(ctx->cancel_table.hbs);
> +	kfree(ctx->cancel_table_locked.hbs);
>   	kfree(ctx->io_bl);
>   	xa_destroy(&ctx->io_bl_xa);
>   	kfree(ctx);
> @@ -3056,6 +3059,7 @@ static __cold void io_ring_ctx_free(struct io_ring_ctx *ctx)
>   	if (ctx->hash_map)
>   		io_wq_put_hash(ctx->hash_map);
>   	kfree(ctx->cancel_table.hbs);
> +	kfree(ctx->cancel_table_locked.hbs);
>   	kfree(ctx->dummy_ubuf);
>   	kfree(ctx->io_bl);
>   	xa_destroy(&ctx->io_bl_xa);
> @@ -3217,12 +3221,13 @@ static __cold void io_ring_ctx_wait_and_kill(struct io_ring_ctx *ctx)
>   		__io_cqring_overflow_flush(ctx, true);
>   	xa_for_each(&ctx->personalities, index, creds)
>   		io_unregister_personality(ctx, index);
> +	if (ctx->rings)
> +		io_poll_remove_all(ctx, NULL, true);
>   	mutex_unlock(&ctx->uring_lock);
>   
>   	/* failed during ring init, it couldn't have issued any requests */
>   	if (ctx->rings) {
>   		io_kill_timeouts(ctx, NULL, true);
> -		io_poll_remove_all(ctx, NULL, true);
>   		/* if we failed setting up the ctx, we might not have any rings */
>   		io_iopoll_try_reap_events(ctx);
>   	}
> @@ -3347,7 +3352,9 @@ static __cold void io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
>   		}
>   
>   		ret |= io_cancel_defer_files(ctx, task, cancel_all);
> +		mutex_lock(&ctx->uring_lock);
>   		ret |= io_poll_remove_all(ctx, task, cancel_all);
> +		mutex_unlock(&ctx->uring_lock);
>   		ret |= io_kill_timeouts(ctx, task, cancel_all);
>   		if (task)
>   			ret |= io_run_task_work();
> diff --git a/io_uring/io_uring_types.h b/io_uring/io_uring_types.h
> index ce2fbe6749bb..557b8e7719c9 100644
> --- a/io_uring/io_uring_types.h
> +++ b/io_uring/io_uring_types.h
> @@ -189,6 +189,7 @@ struct io_ring_ctx {
>   		struct xarray		io_bl_xa;
>   		struct list_head	io_buffers_cache;
>   
> +		struct io_hash_table	cancel_table_locked;
>   		struct list_head	cq_overflow_list;
>   		struct list_head	apoll_cache;
>   		struct xarray		personalities;
> @@ -323,6 +324,7 @@ enum {
>   	/* keep async read/write and isreg together and in order */
>   	REQ_F_SUPPORT_NOWAIT_BIT,
>   	REQ_F_ISREG_BIT,
> +	REQ_F_HASH_LOCKED_BIT,
>   
>   	/* not a real bit, just to check we're not overflowing the space */
>   	__REQ_F_LAST_BIT,
> @@ -388,6 +390,8 @@ enum {
>   	REQ_F_APOLL_MULTISHOT	= BIT(REQ_F_APOLL_MULTISHOT_BIT),
>   	/* recvmsg special flag, clear EPOLLIN */
>   	REQ_F_CLEAR_POLLIN	= BIT(REQ_F_CLEAR_POLLIN_BIT),
> +	/* hashed into ->cancel_hash_locked */
> +	REQ_F_HASH_LOCKED	= BIT(REQ_F_HASH_LOCKED_BIT),
>   };
>   
>   typedef void (*io_req_tw_func_t)(struct io_kiocb *req, bool *locked);
> diff --git a/io_uring/poll.c b/io_uring/poll.c
> index 07157da1c2cb..d20484c1cbb7 100644
> --- a/io_uring/poll.c
> +++ b/io_uring/poll.c
> @@ -93,6 +93,26 @@ static void io_poll_req_delete(struct io_kiocb *req, struct io_ring_ctx *ctx)
>   	spin_unlock(lock);
>   }
>   
> +static void io_poll_req_insert_locked(struct io_kiocb *req)
> +{
> +	struct io_hash_table *table = &req->ctx->cancel_table_locked;
> +	u32 index = hash_long(req->cqe.user_data, table->hash_bits);
> +
> +	hlist_add_head(&req->hash_node, &table->hbs[index].list);
> +}
> +
> +static void io_poll_tw_hash_eject(struct io_kiocb *req, bool *locked)
> +{
> +	struct io_ring_ctx *ctx = req->ctx;
> +
> +	if (req->flags & REQ_F_HASH_LOCKED) {
> +		io_tw_lock(ctx, locked);

                           [1]

> +		hash_del(&req->hash_node);
> +	} else {
> +		io_poll_req_delete(req, ctx);
> +	}
> +}
> +