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

[Alexandra Winter <wintera@linux.ibm.com>]

On 23.11.22 14:41, Tony Lu wrote:
> 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
Iiuc you are changing the default values in this a patch and your other patch:
Default values for real_buf for send and receive:

before 0227f058aa29 ("net/smc: Unbind r/w buffer size from clcsock and make them tunable")
    real_buf=net.ipv4.tcp_{w|r}mem[1]/2   send: 8k  recv: 64k 
    
after 0227f058aa29 ("net/smc: Unbind r/w buffer size from clcsock and make them tunable")
real_buf=net.ipv4.tcp_{w|r}mem[1]   send: 16k (16*1024) recv: 128k (131072) 

after net/smc: Fix expected buffersizes and sync logic
real_buf=net.ipv4.tcp_{w|r}mem[1]   send: 16k (16*1024) recv: 128k (131072) 

after net/smc: Unbind smc control from tcp control
real_buf=SMC_*BUF_INIT_SIZE   send: 16k (16384) recv: 64k (65536)

If my understanding is correct, then I nack this. 
Defaults should be restored to the values before 0227f058aa29.
Otherwise users will notice a change in memory usage that needs to
be avoided or announced more explicitely. (and don't change them twice)
>  
>> - 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