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

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Date:   Mon, 9 Nov 2020 18:16:34 +0000
From:   Trond Myklebust <trondmy@...merspace.com>
To:     "chuck.lever@...cle.com" <chuck.lever@...cle.com>
CC:     "linux-nfs@...r.kernel.org" <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 Mon, 2020-11-09 at 12:36 -0500, Chuck Lever wrote:
> 
> 
> > On Nov 9, 2020, at 12:32 PM, Trond Myklebust <
> > trondmy@...merspace.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@...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?
> > 
> > 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?
> 

It copies the data. 🙂

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