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

Jan Karcher
2022-11-23T13:31:47Z

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'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?

- 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 <jaka@linux.ibm.com>
>> Reviewed-by: Wenjia Zhang <wenjia@linux.ibm.com>
>> ---
>> 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