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

[Wenjia Zhang <wenjia@linux.ibm.com>]

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.