Subject: Re: [PATCH net] net/smc: Fix expected buffersizes and sync logic

From: Tony Lu <[email-masked]>
Date: Wed, 23 Nov 2022 21:41:06 +0800

On Wed, Nov 23, 2022 at 02:13:04PM +0100, Jan Karcher wrote:
> 
> 
> On 23/11/2022 12:53, Tony Lu wrote:
> > On Wed, Nov 23, 2022 at 11:49:07AM +0100, Jan Karcher wrote:
> > > The fixed commit changed the expected behavior of buffersizes
> > > set by the user using the setsockopt mechanism.
> > > Before the fixed patch the logic for determining the buffersizes used
> > > was the following:
> > > 
> > > default  = net.ipv4.tcp_{w|r}mem[1]
> > > sockopt  = the setsockopt mechanism
> > > val      = the value assigned in default or via setsockopt
> > > sk_buf   = short for sk_{snd|rcv}buf
> > > real_buf = the real size of the buffer (sk_buf_size in __smc_buf_create)
> > > 
> > >    exposed   | net/core/sock.c  |    af_smc.c    |  smc_core.c
> > >              |                  |                |
> > > +---------+ |                  | +------------+ | +-------------------+
> > > | default |----------------------| sk_buf=val |---| real_buf=sk_buf/2 |
> > > +---------+ |                  | +------------+ | +-------------------+
> > >              |                  |                |    ^
> > >              |                  |                |    |
> > > +---------+ | +--------------+ |                |    |
> > > | sockopt |---| sk_buf=val*2 |-----------------------|
> > > +---------+ | +--------------+ |                |
> > >              |                  |                |
> > > 
> > > The fixed patch introduced a dedicated sysctl for smc
> > > and removed the /2 in smc_core.c resulting in the following flow:
> > > 
> > > default  = net.smc.{w|r}mem (which defaults to net.ipv4.tcp_{w|r}mem[1])
> > > sockopt  = the setsockopt mechanism
> > > val      = the value assigned in default or via setsockopt
> > > sk_buf   = short for sk_{snd|rcv}buf
> > > real_buf = the real size of the buffer (sk_buf_size in __smc_buf_create)
> > > 
> > >    exposed   | net/core/sock.c  |    af_smc.c    |  smc_core.c
> > >              |                  |                |
> > > +---------+ |                  | +------------+ | +-----------------+
> > > | default |----------------------| sk_buf=val |---| real_buf=sk_buf |
> > > +---------+ |                  | +------------+ | +-----------------+
> > >              |                  |                |    ^
> > >              |                  |                |    |
> > > +---------+ | +--------------+ |                |    |
> > > | sockopt |---| sk_buf=val*2 |-----------------------|
> > > +---------+ | +--------------+ |                |
> > >              |                  |                |
> > > 
> > > This would result in double of memory used for existing configurations
> > > that are using setsockopt.
> > 
> > Firstly, thanks for your detailed diagrams :-)
> > 
> > And the original decision to use user-provided values rather than
> > value/2 to follow the instructions of the socket manual [1].
> > 
> >    SO_RCVBUF
> >           Sets or gets the maximum socket receive buffer in bytes.
> >           The kernel doubles this value (to allow space for
> >           bookkeeping overhead) when it is set using setsockopt(2),
> >           and this doubled value is returned by getsockopt(2).  The
> >           default value is set by the
> >           /proc/sys/net/core/rmem_default file, and the maximum
> >           allowed value is set by the /proc/sys/net/core/rmem_max
> >           file.  The minimum (doubled) value for this option is 256.
> > 
> > [1] https://man7.org/linux/man-pages/man7/socket.7.html
> > 
> > The user of SMC should know that setsockopt() with SO_{RCV|SND}BUF will
> 
> I totally agree that an educated user of SMC should know about that behavior
> if they decide to use it.
> We do provide our users preload libraries where they can pass preferred
> buffersizes via arguments and we handle the Sockopts for them.
> 
> > double the values in kernel, and getsockopt() will return the doubled
> > values. So that they should use half of the values which are passed to
> > setsockopt(). The original patch tries to make things easier in SMC and
> > let user-space to handle them following the socket manual.
> > 
> > > SMC historically decided to use the explicit value given by the user
> > > to allocate the memory. This is why we used the /2 in smc_core.c.
> > > That logic was not applied to the default value.
> > 
> > Yep, let back to the patch which introduced smc_{w|r}mem knobs, it's a
> > trade-off to follow original logic of SMC, or follow the socket manual.
> > We decides to follow the instruction of manuals in the end.
> 
> I understand the point. I spend a lot of time trying to decide what to do.
> 
> Since it was an intentional decision to not follow the general socket
> option, and we do not have anyone complaining we do not really have a reason
> to change it.
> Changing it means that users with existing configurations would have to
> change their configs on an update or suddenly expect double the memory
> consumption.
> That's why we in the end preffered to stay with the current logic.

I can't agree with you more with the points to follow the historic logic
and not break the user-space applications.

> I'm thinking that maybe - if we stay with the historic logic - we should
> document that desicion somewhere. So that in the future, if a user that
> expects the man page behavior, has a way to understand what SMC is doing.
> What do oyu think?

Yep, we _really_ need to document it if we change the convention.
Actually, I spent a lot of time to find the history about the logic of
buffer (/2 and *2) in SMC. So I'm really in favor of adding
documentation, at least code comments to help others to understand them.

Cheers,
Tony Lu
 
> - Jan
> 
> > 
> > Cheers,
> > Tony Lu
> > 
> > > Since we now have our own sysctl, which is also exposed to the user,
> > > we should sync the logic in a way that both values are the real value
> > > used by our code and shown by smc_stats. To achieve this this patch
> > > changes the behavior to:
> > > 
> > > default  = net.smc.{w|r}mem (which defaults to net.ipv4.tcp_{w|r}mem[1])
> > > sockopt  = the setsockopt mechanism
> > > val      = the value assigned in default or via setsockopt
> > > sk_buf   = short for sk_{snd|rcv}buf
> > > real_buf = the real size of the buffer (sk_buf_size in __smc_buf_create)
> > > 
> > >    exposed   | net/core/sock.c  |    af_smc.c     |  smc_core.c
> > >              |                  |                 |
> > > +---------+ |                  | +-------------+ | +-----------------+
> > > | default |----------------------| sk_buf=val*2|---|real_buf=sk_buf/2|
> > > +---------+ |                  | +-------------+ | +-----------------+
> > >              |                  |                 |    ^
> > >              |                  |                 |    |
> > > +---------+ | +--------------+ |                 |    |
> > > | sockopt |---| sk_buf=val*2 |------------------------|
> > > +---------+ | +--------------+ |                 |
> > >              |                  |                 |
> > > 
> > > This way both paths follow the same pattern and the expected behavior
> > > is re-established.
> > > 
> > > Fixes: 0227f058aa29 ("net/smc: Unbind r/w buffer size from clcsock and make them tunable")
> > > Signed-off-by: Jan Karcher <[email-masked]>
> > > Reviewed-by: Wenjia Zhang <[email-masked]>
> > > ---
> > >   net/smc/af_smc.c   | 9 +++++++--
> > >   net/smc/smc_core.c | 8 ++++----
> > >   2 files changed, 11 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
> > > index 036532cf39aa..a8c84e7bac99 100644
> > > --- a/net/smc/af_smc.c
> > > +++ b/net/smc/af_smc.c
> > > @@ -366,6 +366,7 @@ static void smc_destruct(struct sock *sk)
> > >   static struct sock *smc_sock_alloc(struct net *net, struct socket *sock,
> > >   				   int protocol)
> > >   {
> > > +	int buffersize_without_overhead;
> > >   	struct smc_sock *smc;
> > >   	struct proto *prot;
> > >   	struct sock *sk;
> > > @@ -379,8 +380,12 @@ static struct sock *smc_sock_alloc(struct net *net, struct socket *sock,
> > >   	sk->sk_state = SMC_INIT;
> > >   	sk->sk_destruct = smc_destruct;
> > >   	sk->sk_protocol = protocol;
> > > -	WRITE_ONCE(sk->sk_sndbuf, READ_ONCE(net->smc.sysctl_wmem));
> > > -	WRITE_ONCE(sk->sk_rcvbuf, READ_ONCE(net->smc.sysctl_rmem));
> > > +	buffersize_without_overhead =
> > > +		min_t(int, READ_ONCE(net->smc.sysctl_wmem), INT_MAX / 2);
> > > +	WRITE_ONCE(sk->sk_sndbuf, buffersize_without_overhead * 2);
> > > +	buffersize_without_overhead =
> > > +		min_t(int, READ_ONCE(net->smc.sysctl_rmem), INT_MAX / 2);
> > > +	WRITE_ONCE(sk->sk_rcvbuf, buffersize_without_overhead * 2);
> > >   	smc = smc_sk(sk);
> > >   	INIT_WORK(&smc->tcp_listen_work, smc_tcp_listen_work);
> > >   	INIT_WORK(&smc->connect_work, smc_connect_work);
> > > diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
> > > index 00fb352c2765..36850a2ae167 100644
> > > --- a/net/smc/smc_core.c
> > > +++ b/net/smc/smc_core.c
> > > @@ -2314,10 +2314,10 @@ static int __smc_buf_create(struct smc_sock *smc, bool is_smcd, bool is_rmb)
> > >   	if (is_rmb)
> > >   		/* use socket recv buffer size (w/o overhead) as start value */
> > > -		sk_buf_size = smc->sk.sk_rcvbuf;
> > > +		sk_buf_size = smc->sk.sk_rcvbuf / 2;
> > >   	else
> > >   		/* use socket send buffer size (w/o overhead) as start value */
> > > -		sk_buf_size = smc->sk.sk_sndbuf;
> > > +		sk_buf_size = smc->sk.sk_sndbuf / 2;
> > >   	for (bufsize_short = smc_compress_bufsize(sk_buf_size, is_smcd, is_rmb);
> > >   	     bufsize_short >= 0; bufsize_short--) {
> > > @@ -2376,7 +2376,7 @@ static int __smc_buf_create(struct smc_sock *smc, bool is_smcd, bool is_rmb)
> > >   	if (is_rmb) {
> > >   		conn->rmb_desc = buf_desc;
> > >   		conn->rmbe_size_short = bufsize_short;
> > > -		smc->sk.sk_rcvbuf = bufsize;
> > > +		smc->sk.sk_rcvbuf = bufsize * 2;
> > >   		atomic_set(&conn->bytes_to_rcv, 0);
> > >   		conn->rmbe_update_limit =
> > >   			smc_rmb_wnd_update_limit(buf_desc->len);
> > > @@ -2384,7 +2384,7 @@ static int __smc_buf_create(struct smc_sock *smc, bool is_smcd, bool is_rmb)
> > >   			smc_ism_set_conn(conn); /* map RMB/smcd_dev to conn */
> > >   	} else {
> > >   		conn->sndbuf_desc = buf_desc;
> > > -		smc->sk.sk_sndbuf = bufsize;
> > > +		smc->sk.sk_sndbuf = bufsize * 2;
> > >   		atomic_set(&conn->sndbuf_space, bufsize);
> > >   	}
> > >   	return 0;
> > > -- 
> > > 2.34.1