> On Nov 9, 2020, at 12:32 PM, Trond Myklebust <trondmy@xxxxxxxxxxxxxxx> wrote:
>
> On Mon, 2020-11-09 at 12:12 -0500, Chuck Lever wrote:
>>
>>
>>> 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?
>
> 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
