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

Alexandra Winter
2022-11-24T14:07:20Z

On 24.11.22 14:00, Alexandra Winter wrote:
>
>
> 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]
Jan, you explained to me: "the minima is 16Kib. This is enforced in smc_compress_bufsize
which would move any value <= 16Kib into bucket 0 - which is 16KiB "
net.ipv4.tcp_wmem[1] defaults to 8Kib. So in the default case (unchanged net.ipv4.tcp_wmem[1])
the default for the send path is not net.ipv4.tcp_wmem[1]. Should be clarified here.
>>>>> 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
see above: send: 16k 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)
See above, I stand corrected. However this patch fixes/restores the buffersize
for setsockopt, but not for the default recieve path.
Could you please clarify that in the title and description?

Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
>>
>>> - 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