Re: [PATCH net-next] net/smc: Unbind smc control from tcp control

Tony Lu
2022-11-24T18:04:49Z

On Thu, Nov 24, 2022 at 03:06:50PM +0100, Alexandra Winter wrote:
>
>
> On 24.11.22 14:00, Alexandra Winter wrote:
> >
> >
> > On 23.11.22 12:25, Tony Lu wrote:
> >> On Wed, Nov 23, 2022 at 12:19:19PM +0100, Jan Karcher wrote:
> >>>
> >>>
> >>> On 23/11/2022 12:13, Tony Lu wrote:
> >>>> On Wed, Nov 23, 2022 at 11:58:30AM +0100, Jan Karcher wrote:
> >>>>> In the past SMC used the values of tcp_{w|r}mem to create the send
> >>>>> buffer and RMB. We now have our own sysctl knobs to tune them without
> >>>>> influencing the TCP default.
> >>>>>
> >>>>> This patch removes the dependency on the TCP control by providing our
> >>>>> own initial values which aim for a low memory footprint.
> >>>>
> >>>> +1, before introducing sysctl knobs of SMC, we were going to get rid of
> >>>> TCP and have SMC own values. Now this does it, So I very much agree with
> >>>> this.
> >>>>
> > 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
> 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 "
> so 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. This does restore the default receive buffer size.
> It should got to net with a Fixes: 0227f058aa29 tag.
> And the descrition should clarify that this restores the default recv buffer size
> (and uncouples from tcp control)
> Maybe the users and the distros would thank you, if you merge them into 1 patch?

AFAIK, updating kernel to the version of upstream will take a long time
for distros and users. Since we have sysctls, maybe we could let
applications and distros use sysctl to set their desired and default
values with documentation. It's a more common way to tweak kernel.

Cheers,
Tony Lu

>
>
> >
> >>>>>
> >>>>> Signed-off-by: Jan Karcher <jaka@linux.ibm.com>
> >>>>> Reviewed-by: Wenjia Zhang <wenjia@linux.ibm.com>
> >>>>> ---
> >>>>> Documentation/networking/smc-sysctl.rst | 4 ++--
> >>>>> net/smc/smc_core.h | 6 ++++--
> >>>>> net/smc/smc_sysctl.c | 10 ++++++----
> >>>>> 3 files changed, 12 insertions(+), 8 deletions(-)
> >>>>>
> >>>>> diff --git a/Documentation/networking/smc-sysctl.rst b/Documentation/networking/smc-sysctl.rst
> >>>>> index 6d8acdbe9be1..a1c634d3690a 100644
> >>>>> --- a/Documentation/networking/smc-sysctl.rst
> >>>>> +++ b/Documentation/networking/smc-sysctl.rst
> >>>>> @@ -44,7 +44,7 @@ smcr_testlink_time - INTEGER
> >>>>> wmem - INTEGER
> >>>>> Initial size of send buffer used by SMC sockets.
> >>>>> - The default value inherits from net.ipv4.tcp_wmem[1].
> >>>>> + The default value aims for a small memory footprint and is set to 16KiB.
> >>>>> The minimum value is 16KiB and there is no hard limit for max value, but
> >>>>> only allowed 512KiB for SMC-R and 1MiB for SMC-D.
> >>>>> @@ -53,7 +53,7 @@ wmem - INTEGER
> >>>>> rmem - INTEGER
> >>>>> Initial size of receive buffer (RMB) used by SMC sockets.
> >>>>> - The default value inherits from net.ipv4.tcp_rmem[1].
> >>>>> + The default value aims for a small memory footprint and is set to 64KiB.
> >>>>> The minimum value is 16KiB and there is no hard limit for max value, but
> >>>>> only allowed 512KiB for SMC-R and 1MiB for SMC-D.
> >>>>> diff --git a/net/smc/smc_core.h b/net/smc/smc_core.h
> >>>>> index 285f9bd8e232..67c3937f341d 100644
> >>>>> --- a/net/smc/smc_core.h
> >>>>> +++ b/net/smc/smc_core.h
> >>>>> @@ -206,8 +206,10 @@ struct smc_rtoken { /* address/key of remote RMB */
> >>>>> u32 rkey;
> >>>>> };
> >>>>> -#define SMC_BUF_MIN_SIZE 16384 /* minimum size of an RMB */
> >>>>> -#define SMC_RMBE_SIZES 16 /* number of distinct RMBE sizes */
> >>>>> +#define SMC_SNDBUF_INIT_SIZE 16384 /* initial size of send buffer */
> >>>>> +#define SMC_RCVBUF_INIT_SIZE 65536 /* initial size of receive buffer */
> >>>>> +#define SMC_BUF_MIN_SIZE 16384 /* minimum size of an RMB */
> >>>>> +#define SMC_RMBE_SIZES 16 /* number of distinct RMBE sizes */
> There is one blank added to these lines? Why? They still don't align.
> >>>>> /* theoretically, the RFC states that largest size would be 512K,
> >>>>> * i.e. compressed 5 and thus 6 sizes (0..5), despite
> >>>>> * struct smc_clc_msg_accept_confirm.rmbe_size being a 4 bit value (0..15)
> >>>>> diff --git a/net/smc/smc_sysctl.c b/net/smc/smc_sysctl.c
> >>>>> index b6f79fabb9d3..a63aa79d4856 100644
> >>>>> --- a/net/smc/smc_sysctl.c
> >>>>> +++ b/net/smc/smc_sysctl.c
> >>>>> @@ -19,8 +19,10 @@
> >>>>> #include "smc_llc.h"
> >>>>> #include "smc_sysctl.h"
> >>>>> -static int min_sndbuf = SMC_BUF_MIN_SIZE;
> >>>>> -static int min_rcvbuf = SMC_BUF_MIN_SIZE;
> >>>>> +static int initial_sndbuf = SMC_SNDBUF_INIT_SIZE;
> >>>>> +static int initial_rcvbuf = SMC_RCVBUF_INIT_SIZE;
> >>>>> +static int min_sndbuf = SMC_BUF_MIN_SIZE;
> >>>>> +static int min_rcvbuf = SMC_BUF_MIN_SIZE;
> > Broken formatting
> >>>>> static struct ctl_table smc_table[] = {
> >>>>> {
> >>>>> @@ -88,8 +90,8 @@ int __net_init smc_sysctl_net_init(struct net *net)
> >>>>> net->smc.sysctl_autocorking_size = SMC_AUTOCORKING_DEFAULT_SIZE;
> >>>>> net->smc.sysctl_smcr_buf_type = SMCR_PHYS_CONT_BUFS;
> >>>>> net->smc.sysctl_smcr_testlink_time = SMC_LLC_TESTLINK_DEFAULT_TIME;
> >>>>> - WRITE_ONCE(net->smc.sysctl_wmem, READ_ONCE(net->ipv4.sysctl_tcp_wmem[1]));
> >>>>> - WRITE_ONCE(net->smc.sysctl_rmem, READ_ONCE(net->ipv4.sysctl_tcp_rmem[1]));
> >>>>> + WRITE_ONCE(net->smc.sysctl_wmem, initial_sndbuf);
> >>>>> + WRITE_ONCE(net->smc.sysctl_rmem, initial_rcvbuf);
> >>>>
> >>>> Maybe we can use SMC_{SND|RCV}BUF_INIT_SIZE macro directly, instead of
> >>>> new variables.
> >>>
> >>> The reason i created the new variables is that min_{snd|rcv}buf also have
> >>> their own variables. I know it is not needed but thought it was cleaner.
> >>> If you have a strong opinion on using the value directly i can change it.
> >>> Please let me know if you want it changed.
> >>
> >> Yep, it's okay for me to use variables or macros. Just let it be.
> > I think it's better coding style to use the macros instead of unneccessary variables.
> > At least the variables could be defined as const.
> >>
> >> Reviewed-by: Tony Lu <tonylu@linux.alibaba.com>
> >>
> >> Cheers,
> >> Tony Lu
> >>
> >>>
> >>> - Jan
> >>>>
> >>>> Cheers,
> >>>> Tony Lu
> >>>>
> >>>>> return 0;
> >>>>> --
> >>>>> 2.34.1