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

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Date:   Mon, 9 Nov 2020 12:12:50 -0500
From:   Chuck Lever <chuck.lever@...cle.com>
To:     Trond Myklebust <trondmy@...merspace.com>
Cc:     Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH RFC] SUNRPC: Use zero-copy to perform socket send
 operations



> On Nov 9, 2020, at 12:08 PM, Trond Myklebust <trondmy@...merspace.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@...g.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@...cle.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?


--
Chuck Lever