Re: [PATCH v3 0/5] arm64: vdso: getcpu() support

[Shuah Khan <skhan@linuxfoundation.org>]

On 8/19/20 6:13 AM, Mark Brown wrote:
> Some applications, especially tracing ones, benefit from avoiding the
> syscall overhead for getcpu() so it is common for architectures to have
> vDSO implementations. Add one for arm64, using TPIDRRO_EL0 to pass a
> pointer to per-CPU data rather than just store the immediate value in
> order to allow for future extensibility.
> 
> It is questionable if something TPIDRRO_EL0 based is worthwhile at all
> on current kernels, since v4.18 we have had support for restartable
> sequences which can be used to provide a sched_getcpu() implementation
> with generally better performance than the vDSO approach on
> architectures which have that[1]. Work is ongoing to implement this for
> glibc:
> 
>      https://lore.kernel.org/lkml/20200527185130.5604-3-mathieu.desnoyers@efficios.com/
> 
> but is not yet merged and will need similar work for other userspaces.
> The main advantages for the vDSO implementation are the node parameter
> (though this is a static mapping to CPU number so could be looked up
> separately when processing data if it's needed, it shouldn't need to be
> in the hot path) and ease of implementation for users.
> 
> This is currently not compatible with KPTI due to the use of TPIDRRO_EL0
> by the KPTI trampoline, this could be addressed by reinitializing that
> system register in the return path but I have found it hard to justify
> adding that overhead for all users for something that is essentially a
> profiling optimization which is likely to get superceeded by a more
> modern implementation - if there are other uses for the per-CPU data
> then the balance might change here.
> 
> This builds on work done by Kristina Martsenko some time ago but is a
> new implementation.
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d7822b1e24f2df5df98c76f0e94a5416349ff759
> 
> v3:
>   - Rebase on v5.9-rc1.
>   - Drop in progress portions of the series.
> v2:
>   - Rebase on v5.8-rc3.
>   - Add further cleanup patches & a first draft of multi-page support.
> 
> Mark Brown (5):
>    arm64: vdso: Provide a define when building the vDSO
>    arm64: vdso: Add per-CPU data
>    arm64: vdso: Initialise the per-CPU vDSO data
>    arm64: vdso: Add getcpu() implementation
>    selftests: vdso: Support arm64 in getcpu() test
> 
>   arch/arm64/include/asm/processor.h            | 12 +----
>   arch/arm64/include/asm/vdso/datapage.h        | 54 +++++++++++++++++++
>   arch/arm64/kernel/process.c                   | 26 ++++++++-
>   arch/arm64/kernel/vdso.c                      | 33 +++++++++++-
>   arch/arm64/kernel/vdso/Makefile               |  4 +-
>   arch/arm64/kernel/vdso/vdso.lds.S             |  1 +
>   arch/arm64/kernel/vdso/vgetcpu.c              | 48 +++++++++++++++++
>   .../testing/selftests/vDSO/vdso_test_getcpu.c | 10 ++++
>   8 files changed, 172 insertions(+), 16 deletions(-)
>   create mode 100644 arch/arm64/include/asm/vdso/datapage.h
>   create mode 100644 arch/arm64/kernel/vdso/vgetcpu.c
> 

Patches look good to me from selftests perspective. My acked by
for these patches to go through arm64.

Acked-by: Shuah Khan <skhan@linuxfoundation.org>

If you would like me to take these through kselftest tree, give
me your Acks. I can queue these up for 5.10-rc1

thanks,
-- Shuah