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

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Aug 31, 2020 at 03:47:17PM -0600, Shuah Khan wrote:
> 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 for the ack. We are still pondering whether the merge these
patches as they have some limitations (the per-CPU data structures may
not fit in the sole data vDSO page).

-- 
Catalin

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