netdev - Re: [PATCH net-next 14/15] net/smc: introduce loopback-ism DMB data copy control

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Date: Fri, 16 Feb 2024 15:25:50 +0100
From: Wenjia Zhang <wenjia@...ux.ibm.com>
To: Wen Gu <guwen@...ux.alibaba.com>, wintera@...ux.ibm.com, hca@...ux.ibm.com,
        gor@...ux.ibm.com, agordeev@...ux.ibm.com, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        jaka@...ux.ibm.com, Gerd Bayer <gbayer@...ux.ibm.com>
Cc: borntraeger@...ux.ibm.com, svens@...ux.ibm.com, alibuda@...ux.alibaba.com,
        tonylu@...ux.alibaba.com, linux-s390@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 14/15] net/smc: introduce loopback-ism DMB data
 copy control



On 11.01.24 13:00, Wen Gu wrote:
> This provides a way to {get|set} whether loopback-ism device supports
> merging sndbuf with peer DMB to eliminate data copies between them.
> 
> echo 0 > /sys/devices/virtual/smc/loopback-ism/dmb_copy # support
> echo 1 > /sys/devices/virtual/smc/loopback-ism/dmb_copy # not support
> 
Besides the same confusing as Niklas already mentioned, the name of the 
option looks not clear enough to what it means. What about:
echo 1 > /sys/devices/virtual/smc/loopback-ism/nocopy_support # merge mode
echo 0 > /sys/devices/virtual/smc/loopback-ism/nocopy_support # copy mode

> The settings take effect after re-activating loopback-ism by:
> 
> echo 0 > /sys/devices/virtual/smc/loopback-ism/active
> echo 1 > /sys/devices/virtual/smc/loopback-ism/active
> 
> After this, the link group related to loopback-ism will be flushed and
> the sndbufs of subsequent connections will be merged or not merged with
> peer DMB.
> 
> The motivation of this control is that the bandwidth will be highly
> improved when sndbuf and DMB are merged, but when virtually contiguous
> DMB is provided and merged with sndbuf, it will be concurrently accessed
> on Tx and Rx, then there will be a bottleneck caused by lock contention
> of find_vmap_area when there are many CPUs and CONFIG_HARDENED_USERCOPY
> is set (see link below). So an option is provided.
> 
> Link: https://lore.kernel.org/all/238e63cd-e0e8-4fbf-852f-bc4d5bc35d5a@linux.alibaba.com/
> Signed-off-by: Wen Gu <guwen@...ux.alibaba.com>
> ---
We tried some simple workloads, and the performance of the no-copy case 
was remarkable. Thus, we're wondering if it is necessary to have the 
tunable setting in this loopback case? Or rather, why do we need the 
copy option? Is that because of the bottleneck caused by using the 
combination of the no-copy and virtually contiguours DMA? Or at least 
let no-copy as the default one.