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

Tony Lu
2022-11-28T12:25:53Z

On Mon, Nov 28, 2022 at 09:32:45AM +0100, Wenjia Zhang wrote:
>
>
> On 28.11.22 05:33, Tony Lu wrote:
> > On Fri, Nov 25, 2022 at 11:59:46AM +0100, Alexandra Winter wrote:
> > >
> > >
> > > On 25.11.22 08:05, Tony Lu wrote:
> > > > 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
> > > >
> > >
> > > Thanks a lot for your constructive proposals Tony. Yes, we should have a discussion off-mailinglist
> > > about future topics.
> >
> > I will prepare the discussion off-maillinglist ASAP. The email will be
> > sent out when it's ready. And Jan, What about your opinion?
> >
> > Cheers,
> > Tony Lu
> >
> Hi Tony,
>
> Sorry for the flurry we brought!
>
> It's very nice to know that you got progress on the scalability.
>
> Firstly the off-millinglist is a good idea! Let's know if yor're ready.
>
> About the meetings I would ask for your understanding that I still can not
> give any guarantee. But I would let you know ASAP after I talk to our team.

Sure, looking forward to your reply.

Cheers,
Tony Lu

>
> Best,
> Wenjia
> > >
> > > My remaining concern for this fix is the default values (user does not use setsockopt, nor
> > > changes the new sysfs parameters, nor changes tcp defaults):
> > > > > > > before 0227f058aa29 ("net/smc: Unbind r/w buffer size from clcsock and make them tunable")
> > > send: 16k recv: 64k
> > > > > > > after net/smc: Fix expected buffersizes and sync logic (this patch)
> > > > > > > send: 16k recv: 128k
> > >
> > > @Jan, as this is the only patch you want to send to net, please change the default size of
> > > the receive buffers back to 64k (I don't care how).
> > >
> > >
> > > > >
> > > > > > > > > > > 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