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

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Date: Fri, 12 Jan 2024 17:24:19 +0100
From: Niklas Schnelle <schnelle@...ux.ibm.com>
To: Wen Gu <guwen@...ux.alibaba.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 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

> 
> 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>
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