| Message ID | 20200819121318.52158-1-broonie@kernel.org (mailing list archive) |
|---|---|
| Headers | show |
| Series | arm64: vdso: getcpu() support | expand |
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
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).
On Tue, Sep 01, 2020 at 10:25:52AM +0100, Catalin Marinas wrote: > 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). They definitely don't fit, I did have some half-written proof of concept patches that I posted that extend this but I was waiting to see if there was any interest in a vDSO getcpu() at all before taking it further. Vincenzo's work on doing the multipage user data that he announced at Plumbers would cover it as well, I hadn't been aware of that.
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