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.
