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

Tony Lu
2022-11-25T07:05:35Z

On Fri, Nov 25, 2022 at 07:15:33AM +0100, Jan Karcher wrote:
>
>
> On 24/11/2022 15:07, Alexandra Winter wrote:
> >
> >
> > On 24.11.22 14:00, Alexandra Winter 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.
>
> The default value is still set to the net.ipv4.tcp_{w|r}mem[1]. This is a
> *very* top level overview about what is happening and *not* a documentation.
> I don't really want to explain the full code flow here.
>
> What we still should do - as Tony aggreed on - is documenting the SMC
> behavior. This is a follow up on my list.

Hello Jan and Alexandra,

It looks like the misalignment of information is causing some trouble,
which is introduced by my patch. Maybe we could have an off-maillist and
online meeting to discussion?

We have some progress updates of scalability, and we are really like the
extension of SMC-D. Also we have some ideas for SMC, in case of
misalignment of information, we'd like to put them on the table and
discuss them earlier. Maybe an online meeting is an efficient way. What
do you think?

If possible, I would prepared the meetings and topics and send them to
everyone first.

Cheers,
Tony Lu

>
> > > > > > > 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?
> >
>
> I am trying to keep the commit title as crisp as possible while providing
> enough information and set the context in the commit message:
>
> "The fixed commit changed the expected behavior of buffersizes set by the
> user using the setsockopt mechanism."
>
> + There is now a whole e-mail thread to consult in case of any further
> questions.
>
> Thank you for your comments
> - Jan
>
> > 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