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

[Trond Myklebust <trondmy@hammerspace.com>]

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?


-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com