> On Nov 9, 2020, at 12:08 PM, Trond Myklebust <trondmy@xxxxxxxxxxxxxxx> 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@xxxxxxxx>
>> 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@xxxxxxxxxx>
>> ---
>> 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
