Excerpts from Nicholas Piggin's message of June 5, 2021 11:42 am: > Add CONFIG_MMU_TLB_REFCOUNT which enables refcounting of the lazy tlb mm > when it is context switched. This can be disabled by architectures that > don't require this refcounting if they clean up lazy tlb mms when the > last refcount is dropped. Currently this is always enabled, which is > what existing code does, so the patch is effectively a no-op. > > Rename rq->prev_mm to rq->prev_lazy_mm, because that's what it is. > > Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx> Can I give you a couple of incremental patches for 2/4 and 3/4 to improve the implementation requirement comments a bit for benefit of other archs. Thanks, Nick -- Explain the requirements for lazy tlb mm refcounting in the comment, to help with archs that may want to disable this by some means other than MMU_LAZY_TLB_SHOOTDOWN. Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx> --- arch/Kconfig | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/arch/Kconfig b/arch/Kconfig index 1cff045cdde6..39d8c7dcf220 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -419,6 +419,16 @@ config ARCH_WANT_IRQS_OFF_ACTIVATE_MM shootdowns should enable this. # Use normal mm refcounting for MMU_LAZY_TLB kernel thread references. +# MMU_LAZY_TLB_REFCOUNT=n can improve the scalability of context switching +# to/from kernel threads when the same mm is running on a lot of CPUs (a large +# multi-threaded application), by reducing contention on the mm refcount. +# +# This can be disabled if the architecture ensures no CPUs are using an mm as a +# "lazy tlb" beyond its final refcount (i.e., by the time __mmdrop frees the mm +# or its kernel page tables). This could be arranged by arch_exit_mmap(), or +# final exit(2) TLB flush, for example. arch code must also ensure the +# _lazy_tlb variants of mmgrab/mmdrop are used when dropping the lazy reference +# to a kthread ->active_mm (non-arch code has been converted already). config MMU_LAZY_TLB_REFCOUNT def_bool y -- 2.23.0
