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

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Date: Sat, 13 Jan 2024 15:12:00 +0800
From: Wen Gu <guwen@...ux.alibaba.com>
To: Niklas Schnelle <schnelle@...ux.ibm.com>, wintera@...ux.ibm.com,
 wenjia@...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
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 2024/1/13 00:24, Niklas Schnelle wrote:
> On Thu, 2024-01-11 at 20:00 +0800, 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
> 
> The two support/no support remarks are a bit confusing because support
> here seems to mean "support no-copy mode" while the attribute is more
> like "force copy mode". How about:
> 
> echo 0 > /sys/devices/virtual/smc/loopback-ism/dmb_copy # one DMB mode
> echo 1 > /sys/devices/virtual/smc/loopback-ism/dmb_copy # copy mode
> 

Thank you! Niklas.
That makes it much clearer. It will be improved in next version.

>>
>> 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>
> ---8<---