[PATCH v4 2/4] lazy tlb: allow lazy tlb mm refcounting to be configurable

[Nicholas Piggin]

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 at gmail.com>

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 at gmail.com>
---
 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