netdev - Re: [PATCH net-next 13/15] net/smc: introduce loopback-ism DMB type control

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Date: Fri, 16 Feb 2024 15:25:04 +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 13/15] net/smc: introduce loopback-ism DMB type
 control



On 11.01.24 13:00, Wen Gu wrote:
> This provides a way to {get|set} type of DMB offered by loopback-ism,
> whether it is physically or virtually contiguous memory.
> 
> echo 0 > /sys/devices/virtual/smc/loopback-ism/dmb_type # physically
> echo 1 > /sys/devices/virtual/smc/loopback-ism/dmb_type # virtually
> 
> 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 and DMBs related to loopback-ism will be
> flushed and subsequent DMBs created will be of the desired type.
> 
> The motivation of this control is that physically contiguous DMB has
> best performance but is usually expensive, while the virtually
> contiguous DMB is cheap and perform well in most scenarios, but if
> sndbuf and DMB are merged, virtual DMB will be accessed concurrently
> in Tx and Rx and 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.
> 
I'm courious about why you say that physically contiguous DMB has best 
performance. Because we saw even a bit better perfomance with the 
virtual one than the performance with the physical one.