[PATCH 5/5] io_uring/notif: let userspace know how effective the zero copy usage was

[PATCH 5/5] io_uring/notif: let userspace know how effective the zero copy usage was

[Stefan Metzmacher]

The 2nd cqe for IORING_OP_SEND_ZC has IORING_CQE_F_NOTIF set in cqe->flags
and it will now have the number of successful completed
io_uring_tx_zerocopy_callback() callbacks in the lower 31-bits
of cqe->res, the high bit (0x80000000) is set when
io_uring_tx_zerocopy_callback() was called with success=false.

If cqe->res is still 0, zero copy wasn't used at all.

These values give userspace a change to adjust its strategy
choosing IORING_OP_SEND_ZC or IORING_OP_SEND. And it's a bit
richer than just a simple SO_EE_CODE_ZEROCOPY_COPIED indication.

Fixes: b48c312be05e8 ("io_uring/net: simplify zerocopy send user API")
Fixes: eb315a7d1396b ("tcp: support externally provided ubufs")
Fixes: 1fd3ae8c906c0 ("ipv6/udp: support externally provided ubufs")
Fixes: c445f31b3cfaa ("ipv4/udp: support externally provided ubufs")
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: io-uring@vger.kernel.org
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: netdev@vger.kernel.org
---
 io_uring/notif.c      | 18 ++++++++++++++++++
 net/ipv4/ip_output.c  |  3 ++-
 net/ipv4/tcp.c        |  2 ++
 net/ipv6/ip6_output.c |  3 ++-
 4 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/io_uring/notif.c b/io_uring/notif.c
index e37c6569d82e..b07d2a049931 100644
--- a/io_uring/notif.c
+++ b/io_uring/notif.c
@@ -28,7 +28,24 @@ static void io_uring_tx_zerocopy_callback(struct sk_buff *skb,
 	struct io_notif_data *nd = container_of(uarg, struct io_notif_data, uarg);
 	struct io_kiocb *notif = cmd_to_io_kiocb(nd);
 
+	uarg->zerocopy = uarg->zerocopy & success;
+
+	if (success && notif->cqe.res < S32_MAX)
+		notif->cqe.res++;
+
 	if (refcount_dec_and_test(&uarg->refcnt)) {
+		/*
+		 * If we hit at least one case that
+		 * was not able to use zero copy,
+		 * we set the high bit 0x80000000
+		 * so that notif->cqe.res < 0, means it was
+		 * as least copied once.
+		 *
+		 * The other 31 bits are the success count.
+		 */
+		if (!uarg->zerocopy)
+			notif->cqe.res |= S32_MIN;
+
 		notif->io_task_work.func = __io_notif_complete_tw;
 		io_req_task_work_add(notif);
 	}
@@ -53,6 +70,7 @@ struct io_kiocb *io_alloc_notif(struct io_ring_ctx *ctx)
 
 	nd = io_notif_to_data(notif);
 	nd->account_pages = 0;
+	nd->uarg.zerocopy = 1;
 	nd->uarg.flags = SKBFL_ZEROCOPY_FRAG | SKBFL_DONT_ORPHAN;
 	nd->uarg.callback = io_uring_tx_zerocopy_callback;
 	refcount_set(&nd->uarg.refcnt, 1);
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index d7bd1daf022b..4bdea7a4b2f7 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1032,7 +1032,8 @@ static int __ip_append_data(struct sock *sk,
 				paged = true;
 				zc = true;
 				uarg = msg->msg_ubuf;
-			}
+			} else
+				msg->msg_ubuf->zerocopy = 0;
 		} else if (sock_flag(sk, SOCK_ZEROCOPY)) {
 			uarg = msg_zerocopy_realloc(sk, length, skb_zcopy(skb));
 			if (!uarg)
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 970e9a2cca4a..27a22d470741 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1231,6 +1231,8 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 			uarg = msg->msg_ubuf;
 			net_zcopy_get(uarg);
 			zc = sk->sk_route_caps & NETIF_F_SG;
+			if (!zc)
+				uarg->zerocopy = 0;
 		} else if (sock_flag(sk, SOCK_ZEROCOPY)) {
 			uarg = msg_zerocopy_realloc(sk, size, skb_zcopy(skb));
 			if (!uarg) {
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index f152e51242cb..d85036e91cf7 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1556,7 +1556,8 @@ static int __ip6_append_data(struct sock *sk,
 				paged = true;
 				zc = true;
 				uarg = msg->msg_ubuf;
-			}
+			} else
+				msg->msg_ubuf->zerocopy = 0;
 		} else if (sock_flag(sk, SOCK_ZEROCOPY)) {
 			uarg = msg_zerocopy_realloc(sk, length, skb_zcopy(skb));
 			if (!uarg)
-- 
2.34.1

Re: [PATCH 5/5] io_uring/notif: let userspace know how effective the zero copy usage was

[Pavel Begunkov]

On 9/16/22 22:36, Stefan Metzmacher wrote:
> The 2nd cqe for IORING_OP_SEND_ZC has IORING_CQE_F_NOTIF set in cqe->flags
> and it will now have the number of successful completed
> io_uring_tx_zerocopy_callback() callbacks in the lower 31-bits
> of cqe->res, the high bit (0x80000000) is set when
> io_uring_tx_zerocopy_callback() was called with success=false.

It has a couple of problems, and because that "simplify uapi"
patch is transitional it doesn't go well with what I'm queuing
for 6.1, let's hold it for a while.


> If cqe->res is still 0, zero copy wasn't used at all.
> 
> These values give userspace a change to adjust its strategy
> choosing IORING_OP_SEND_ZC or IORING_OP_SEND. And it's a bit
> richer than just a simple SO_EE_CODE_ZEROCOPY_COPIED indication.
> 
> Fixes: b48c312be05e8 ("io_uring/net: simplify zerocopy send user API")
> Fixes: eb315a7d1396b ("tcp: support externally provided ubufs")
> Fixes: 1fd3ae8c906c0 ("ipv6/udp: support externally provided ubufs")
> Fixes: c445f31b3cfaa ("ipv4/udp: support externally provided ubufs")
> Signed-off-by: Stefan Metzmacher <metze@samba.org>
> Cc: Pavel Begunkov <asml.silence@gmail.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: io-uring@vger.kernel.org
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: netdev@vger.kernel.org
> ---
>   io_uring/notif.c      | 18 ++++++++++++++++++
>   net/ipv4/ip_output.c  |  3 ++-
>   net/ipv4/tcp.c        |  2 ++
>   net/ipv6/ip6_output.c |  3 ++-
>   4 files changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/io_uring/notif.c b/io_uring/notif.c
> index e37c6569d82e..b07d2a049931 100644
> --- a/io_uring/notif.c
> +++ b/io_uring/notif.c
> @@ -28,7 +28,24 @@ static void io_uring_tx_zerocopy_callback(struct sk_buff *skb,
>   	struct io_notif_data *nd = container_of(uarg, struct io_notif_data, uarg);
>   	struct io_kiocb *notif = cmd_to_io_kiocb(nd);
>   
> +	uarg->zerocopy = uarg->zerocopy & success;
> +
> +	if (success && notif->cqe.res < S32_MAX)
> +		notif->cqe.res++;
> +
>   	if (refcount_dec_and_test(&uarg->refcnt)) {
> +		/*
> +		 * If we hit at least one case that
> +		 * was not able to use zero copy,
> +		 * we set the high bit 0x80000000
> +		 * so that notif->cqe.res < 0, means it was
> +		 * as least copied once.
> +		 *
> +		 * The other 31 bits are the success count.
> +		 */
> +		if (!uarg->zerocopy)
> +			notif->cqe.res |= S32_MIN;
> +
>   		notif->io_task_work.func = __io_notif_complete_tw;
>   		io_req_task_work_add(notif);
>   	}
> @@ -53,6 +70,7 @@ struct io_kiocb *io_alloc_notif(struct io_ring_ctx *ctx)
>   
>   	nd = io_notif_to_data(notif);
>   	nd->account_pages = 0;
> +	nd->uarg.zerocopy = 1;
>   	nd->uarg.flags = SKBFL_ZEROCOPY_FRAG | SKBFL_DONT_ORPHAN;
>   	nd->uarg.callback = io_uring_tx_zerocopy_callback;
>   	refcount_set(&nd->uarg.refcnt, 1);
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index d7bd1daf022b..4bdea7a4b2f7 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -1032,7 +1032,8 @@ static int __ip_append_data(struct sock *sk,
>   				paged = true;
>   				zc = true;
>   				uarg = msg->msg_ubuf;
> -			}
> +			} else
> +				msg->msg_ubuf->zerocopy = 0;
>   		} else if (sock_flag(sk, SOCK_ZEROCOPY)) {
>   			uarg = msg_zerocopy_realloc(sk, length, skb_zcopy(skb));
>   			if (!uarg)
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 970e9a2cca4a..27a22d470741 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1231,6 +1231,8 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>   			uarg = msg->msg_ubuf;
>   			net_zcopy_get(uarg);
>   			zc = sk->sk_route_caps & NETIF_F_SG;
> +			if (!zc)
> +				uarg->zerocopy = 0;
>   		} else if (sock_flag(sk, SOCK_ZEROCOPY)) {
>   			uarg = msg_zerocopy_realloc(sk, size, skb_zcopy(skb));
>   			if (!uarg) {
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index f152e51242cb..d85036e91cf7 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -1556,7 +1556,8 @@ static int __ip6_append_data(struct sock *sk,
>   				paged = true;
>   				zc = true;
>   				uarg = msg->msg_ubuf;
> -			}
> +			} else
> +				msg->msg_ubuf->zerocopy = 0;
>   		} else if (sock_flag(sk, SOCK_ZEROCOPY)) {
>   			uarg = msg_zerocopy_realloc(sk, length, skb_zcopy(skb));
>   			if (!uarg)

-- 
Pavel Begunkov

Re: [PATCH 5/5] io_uring/notif: let userspace know how effective the zero copy usage was

[Stefan Metzmacher]

Am 17.09.22 um 11:22 schrieb Pavel Begunkov:
> On 9/16/22 22:36, Stefan Metzmacher wrote:
>> The 2nd cqe for IORING_OP_SEND_ZC has IORING_CQE_F_NOTIF set in cqe->flags
>> and it will now have the number of successful completed
>> io_uring_tx_zerocopy_callback() callbacks in the lower 31-bits
>> of cqe->res, the high bit (0x80000000) is set when
>> io_uring_tx_zerocopy_callback() was called with success=false.
> 
> It has a couple of problems, and because that "simplify uapi"
> patch is transitional it doesn't go well with what I'm queuing
> for 6.1, let's hold it for a while.

Once the current behavior gets released stable, we're no
longer able to change the meaning of cqe.res.

As cqe.res == 0 would mean zero copy wasn't used at all,
which would be the indication for userspace to avoid using SEND_ZC.

But if 6.0 would always return cqe.res = 0, there's no chance for
userspace to have a detection strategy.

And I don't think it will cause a lot of trouble for your 6.1 stuff (assuming
you mean your SENDMSG_ZC code), I was already having that on top
of my test branches, the current one is:
https://git.samba.org/?p=metze/linux/wip.git;a=shortlog;h=refs/heads/io_uring-6.0.0-rc5-metze.08

I plan to test SENDMSG_ZC with Samba next week.

metze

Re: [PATCH 5/5] io_uring/notif: let userspace know how effective the zero copy usage was

[Pavel Begunkov]

On 9/17/22 11:24, Stefan Metzmacher wrote:
> Am 17.09.22 um 11:22 schrieb Pavel Begunkov:
>> On 9/16/22 22:36, Stefan Metzmacher wrote:
>>> The 2nd cqe for IORING_OP_SEND_ZC has IORING_CQE_F_NOTIF set in cqe->flags
>>> and it will now have the number of successful completed
>>> io_uring_tx_zerocopy_callback() callbacks in the lower 31-bits
>>> of cqe->res, the high bit (0x80000000) is set when
>>> io_uring_tx_zerocopy_callback() was called with success=false.
>>
>> It has a couple of problems, and because that "simplify uapi"
>> patch is transitional it doesn't go well with what I'm queuing
>> for 6.1, let's hold it for a while.
> 
> Once the current behavior gets released stable, we're no
> longer able to change the meaning of cqe.res.
> 
> As cqe.res == 0 would mean zero copy wasn't used at all,
> which would be the indication for userspace to avoid using SEND_ZC.
> 
> But if 6.0 would always return cqe.res = 0, there's no chance for
> userspace to have a detection strategy.
> 
> And I don't think it will cause a lot of trouble for your 6.1 stuff (assuming
> you mean your SENDMSG_ZC code), I was already having that on top
> of my test branches, the current one is:
> https://git.samba.org/?p=metze/linux/wip.git;a=shortlog;h=refs/heads/io_uring-6.0.0-rc5-metze.08

Not that one though, 1) I want to shrink ubuf_info as we're a bit out
of space on the io_uring side and it prevents some embedding, so there
won't be additional fields you used. And 2) we want to have a feature
merging completion + notif CQEs into one (useful for UDP and some TCP
cases), but that would mean we'll be using cqe->res for the normal
return value.

We can disable the success counting in this case, but it's not nice,
and we can't actually count in io_uring_tx_zerocopy_callback() as in
the patch (racy). Also, the callback will be called multiple times for
a number of different reasons like io_uring flush or net splitting skbs.
The number won't be much useful and unnecessary exposes internal details,
so I think F_COPIED in cqe->flags is a better option.

It's a good question though whether we need more versatile reporting and
how to do it right. Probably should be optional and not a part of IO path,
e.g. send(MSG_PROBE) / ioctl / proc stats / etc.

> I plan to test SENDMSG_ZC with Samba next week.

Awesome

-- 
Pavel Begunkov

Re: [PATCH 5/5] io_uring/notif: let userspace know how effective the zero copy usage was

[Stefan Metzmacher]

Am 21.09.22 um 14:04 schrieb Pavel Begunkov:
> On 9/17/22 11:24, Stefan Metzmacher wrote:
>> Am 17.09.22 um 11:22 schrieb Pavel Begunkov:
>>> On 9/16/22 22:36, Stefan Metzmacher wrote:
>>>> The 2nd cqe for IORING_OP_SEND_ZC has IORING_CQE_F_NOTIF set in cqe->flags
>>>> and it will now have the number of successful completed
>>>> io_uring_tx_zerocopy_callback() callbacks in the lower 31-bits
>>>> of cqe->res, the high bit (0x80000000) is set when
>>>> io_uring_tx_zerocopy_callback() was called with success=false.
>>>
>>> It has a couple of problems, and because that "simplify uapi"
>>> patch is transitional it doesn't go well with what I'm queuing
>>> for 6.1, let's hold it for a while.
>>
>> Once the current behavior gets released stable, we're no
>> longer able to change the meaning of cqe.res.
>>
>> As cqe.res == 0 would mean zero copy wasn't used at all,
>> which would be the indication for userspace to avoid using SEND_ZC.
>>
>> But if 6.0 would always return cqe.res = 0, there's no chance for
>> userspace to have a detection strategy.
>>
>> And I don't think it will cause a lot of trouble for your 6.1 stuff (assuming
>> you mean your SENDMSG_ZC code), I was already having that on top
>> of my test branches, the current one is:
>> https://git.samba.org/?p=metze/linux/wip.git;a=shortlog;h=refs/heads/io_uring-6.0.0-rc5-metze.08
> 
> Not that one though, 1) I want to shrink ubuf_info as we're a bit out
> of space on the io_uring side and it prevents some embedding, so there
> won't be additional fields you used. And 2) we want to have a feature
> merging completion + notif CQEs into one (useful for UDP and some TCP
> cases), but that would mean we'll be using cqe->res for the normal
> return value.

But wouldn't that just don't have the MORE flag set, and be just
like IO_SEND?

> We can disable the success counting in this case, but it's not nice,
> and we can't actually count in io_uring_tx_zerocopy_callback() as in
> the patch (racy). Also, the callback will be called multiple times for
> a number of different reasons like io_uring flush or net splitting skbs.
> The number won't be much useful and unnecessary exposes internal details,
> so I think F_COPIED in cqe->flags is a better option.

Ok, that's better than nothing.

> It's a good question though whether we need more versatile reporting and
> how to do it right. Probably should be optional and not a part of IO path,
> e.g. send(MSG_PROBE) / ioctl / proc stats / etc.
> 
>> I plan to test SENDMSG_ZC with Samba next week.
> 
> Awesome

Currently I'm fighting with converting samba's libtevent to
use POLL_ADD and io_uring_submit_and_wait_timeout().
Which turns out to be much more complex that it should be.
And IORING_POLL_ADD_LEVEL doesn't seem to work at all...
But I'll write a separate mail about my findings in that area...

metze