Re: [PATCH RFC] SUNRPC: Use zero-copy to perform socket send operations

[Eric Dumazet <eric.dumazet@gmail.com>]

On 11/9/20 8:31 PM, Chuck Lever wrote:
> 
> 
>> On Nov 9, 2020, at 1:16 PM, Trond Myklebust <trondmy@hammerspace.com> wrote:
>>
>> On Mon, 2020-11-09 at 12:36 -0500, Chuck Lever wrote:
>>>
>>>
>>>> On Nov 9, 2020, at 12:32 PM, Trond Myklebust <
>>>> trondmy@hammerspace.com> wrote:
>>>>
>>>> On Mon, 2020-11-09 at 12:12 -0500, Chuck Lever wrote:
>>>>>
>>>>>
>>>>>> On Nov 9, 2020, at 12:08 PM, Trond Myklebust
>>>>>> <trondmy@hammerspace.com> wrote:
>>>>>>
>>>>>> On Mon, 2020-11-09 at 11:03 -0500, Chuck Lever wrote:
>>>>>>> Daire Byrne reports a ~50% aggregrate throughput regression
>>>>>>> on
>>>>>>> his
>>>>>>> Linux NFS server after commit da1661b93bf4 ("SUNRPC: Teach
>>>>>>> server
>>>>>>> to
>>>>>>> use xprt_sock_sendmsg for socket sends"), which replaced
>>>>>>> kernel_send_page() calls in NFSD's socket send path with
>>>>>>> calls to
>>>>>>> sock_sendmsg() using iov_iter.
>>>>>>>
>>>>>>> Investigation showed that tcp_sendmsg() was not using zero-
>>>>>>> copy
>>>>>>> to
>>>>>>> send the xdr_buf's bvec pages, but instead was relying on
>>>>>>> memcpy.
>>>>>>>
>>>>>>> Set up the socket and each msghdr that bears bvec pages to
>>>>>>> use
>>>>>>> the
>>>>>>> zero-copy mechanism in tcp_sendmsg.
>>>>>>>
>>>>>>> Reported-by: Daire Byrne <daire@dneg.com>
>>>>>>> BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=209439
>>>>>>> Fixes: da1661b93bf4 ("SUNRPC: Teach server to use
>>>>>>> xprt_sock_sendmsg
>>>>>>> for socket sends")
>>>>>>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>>>>>>> ---
>>>>>>>  net/sunrpc/socklib.c  |    5 ++++-
>>>>>>>  net/sunrpc/svcsock.c  |    1 +
>>>>>>>  net/sunrpc/xprtsock.c |    1 +
>>>>>>>  3 files changed, 6 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> This patch does not fully resolve the issue. Daire reports
>>>>>>> high
>>>>>>> softIRQ activity after the patch is applied, and this
>>>>>>> activity
>>>>>>> seems to prevent full restoration of previous performance.
>>>>>>>
>>>>>>>
>>>>>>> diff --git a/net/sunrpc/socklib.c b/net/sunrpc/socklib.c
>>>>>>> index d52313af82bc..af47596a7bdd 100644
>>>>>>> --- a/net/sunrpc/socklib.c
>>>>>>> +++ b/net/sunrpc/socklib.c
>>>>>>> @@ -226,9 +226,12 @@ static int xprt_send_pagedata(struct
>>>>>>> socket
>>>>>>> *sock, struct msghdr *msg,
>>>>>>>         if (err < 0)
>>>>>>>                 return err;
>>>>>>>  
>>>>>>> +       msg->msg_flags |= MSG_ZEROCOPY;
>>>>>>>         iov_iter_bvec(&msg->msg_iter, WRITE, xdr->bvec,
>>>>>>> xdr_buf_pagecount(xdr),
>>>>>>>                       xdr->page_len + xdr->page_base);
>>>>>>> -       return xprt_sendmsg(sock, msg, base + xdr-
>>>>>>>> page_base);
>>>>>>> +       err = xprt_sendmsg(sock, msg, base + xdr->page_base);
>>>>>>> +       msg->msg_flags &= ~MSG_ZEROCOPY;
>>>>>>> +       return err;
>>>>>>>  }
>>>>>>>  
>>>>>>>  /* Common case:
>>>>>>> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
>>>>>>> index c2752e2b9ce3..c814b4953b15 100644
>>>>>>> --- a/net/sunrpc/svcsock.c
>>>>>>> +++ b/net/sunrpc/svcsock.c
>>>>>>> @@ -1176,6 +1176,7 @@ static void svc_tcp_init(struct
>>>>>>> svc_sock
>>>>>>> *svsk,
>>>>>>> struct svc_serv *serv)
>>>>>>>                 svsk->sk_datalen = 0;
>>>>>>>                 memset(&svsk->sk_pages[0], 0, sizeof(svsk-
>>>>>>>> sk_pages));
>>>>>>>  
>>>>>>> +               sock_set_flag(sk, SOCK_ZEROCOPY);
>>>>>>>                 tcp_sk(sk)->nonagle |= TCP_NAGLE_OFF;
>>>>>>>  
>>>>>>>                 set_bit(XPT_DATA, &svsk->sk_xprt.xpt_flags);
>>>>>>> diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
>>>>>>> index 7090bbee0ec5..343c6396b297 100644
>>>>>>> --- a/net/sunrpc/xprtsock.c
>>>>>>> +++ b/net/sunrpc/xprtsock.c
>>>>>>> @@ -2175,6 +2175,7 @@ static int
>>>>>>> xs_tcp_finish_connecting(struct
>>>>>>> rpc_xprt *xprt, struct socket *sock)
>>>>>>>  
>>>>>>>                 /* socket options */
>>>>>>>                 sock_reset_flag(sk, SOCK_LINGER);
>>>>>>> +               sock_set_flag(sk, SOCK_ZEROCOPY);
>>>>>>>                 tcp_sk(sk)->nonagle |= TCP_NAGLE_OFF;
>>>>>>>  
>>>>>>>                 xprt_clear_connected(xprt);
>>>>>>>
>>>>>>>
>>>>>> I'm thinking we are not really allowed to do that here. The
>>>>>> pages
>>>>>> we
>>>>>> pass in to the RPC layer are not guaranteed to contain stable
>>>>>> data
>>>>>> since they include unlocked page cache pages as well as
>>>>>> O_DIRECT
>>>>>> pages.
>>>>>
>>>>> I assume you mean the client side only. Those issues aren't a
>>>>> factor
>>>>> on the server. Not setting SOCK_ZEROCOPY here should be enough to
>>>>> prevent the use of zero-copy on the client.
>>>>>
>>>>> However, the client loses the benefits of sending a page at a
>>>>> time.
>>>>> Is there a desire to remedy that somehow?
>>>>
>>>> What about splice reads on the server side?
>>>
>>> On the server, this path formerly used kernel_sendpages(), which I
>>> assumed is similar to the sendmsg zero-copy mechanism. How does
>>> kernel_sendpages() mitigate against page instability?
>>
>> It copies the data. 🙂
> 
> tcp_sendmsg_locked() invokes skb_copy_to_page_nocache(), which is
> where Daire's performance-robbing memcpy occurs.
> 
> do_tcp_sendpages() has no such call site. Therefore the legacy
> sendpage-based path has at least one fewer data copy operations.
> 
> What is the appropriate way to make tcp_sendmsg() treat a bvec-bearing
> msghdr like an array of struct page pointers passed to kernel_sendpage() ?
> 


MSG_ZEROCOPY is only accepted if sock_flag(sk, SOCK_ZEROCOPY) is true,
ie if SO_ZEROCOPY socket option has been set earlier.