[v2, 1/3] powerpc/powernv/npu: Reduce eieio usage when issuing ATSD invalidates

[Michael Ellerman]

On Wed, 2018-10-03 at 18:51:32 UTC, Mark Hairgrove wrote:
> There are two types of ATSDs issued to the NPU: invalidates targeting a
> specific virtual address and invalidates targeting the whole address
> space. In both cases prior to this change, the sequence was:
> 
>     for each NPU
>         - Write the target address to the XTS_ATSD_AVA register
>         - EIEIO
>         - Write the launch value to issue the ATSD
> 
> First, a target address is not required when invalidating the whole
> address space, so that write and the EIEIO have been removed. The AP
> (size) field in the launch is not needed either.
> 
> Second, for per-address invalidates the above sequence is inefficient in
> the common case of multiple NPUs because an EIEIO is issued per NPU. This
> unnecessarily forces the launches of later ATSDs to be ordered with the
> launches of earlier ones. The new sequence only issues a single EIEIO:
> 
>     for each NPU
>         - Write the target address to the XTS_ATSD_AVA register
>     EIEIO
>     for each NPU
>         - Write the launch value to issue the ATSD
> 
> Performance results were gathered using a microbenchmark which creates a
> 1G allocation then uses mprotect with PROT_NONE to trigger invalidates in
> strides across the allocation.
> 
> With only a single NPU active (one GPU) the difference is in the noise for
> both types of invalidates (+/-1%).
> 
> With two NPUs active (on a 6-GPU system) the effect is more noticeable:
> 
>          mprotect rate (GB/s)
> Stride   Before      After      Speedup
> 64K         5.9        6.5          10%
> 1M         31.2       33.4           7%
> 2M         36.3       38.7           7%
> 4M        322.6      356.7          11%
> 
> Signed-off-by: Mark Hairgrove <mhairgrove at nvidia.com>
> Reviewed-by: Alistair Popple <alistair at popple.id.au>

Series applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/7ead15a1442b25e12a6f0791a7c7a5

cheers