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

[Chuck Lever <chuck.lever@oracle.com>]

> 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?


--
Chuck Lever