[PATCH v4 3/4] lazy tlb: shoot lazies, a non-refcounting lazy tlb option

[Nicholas Piggin]

Excerpts from Nicholas Piggin's message of June 5, 2021 11:42 am:
> On big systems, the mm refcount can become highly contented when doing
> a lot of context switching with threaded applications (particularly
> switching between the idle thread and an application thread).
> 
> Abandoning lazy tlb slows switching down quite a bit in the important
> user->idle->user cases, so instead implement a non-refcounted scheme
> that causes __mmdrop() to IPI all CPUs in the mm_cpumask and shoot down
> any remaining lazy ones.
> 
> Shootdown IPIs are some concern, but they have not been observed to be
> a big problem with this scheme (the powerpc implementation generated
> 314 additional interrupts on a 144 CPU system during a kernel compile).
> There are a number of strategies that could be employed to reduce IPIs
> if they turn out to be a problem for some workload.
> 
> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
> ---

Update the comment to be clearer, and account for the improvement
to MMU_LAZY_TLB_REFCOUNT comment.

Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
---
 arch/Kconfig | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 2ad1a505ca55..cf468c9777d8 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -433,15 +433,16 @@ config MMU_LAZY_TLB_REFCOUNT
 	def_bool y
 	depends on !MMU_LAZY_TLB_SHOOTDOWN
 
-# Instead of refcounting the lazy mm struct for kernel thread references
-# (which can cause contention with multi-threaded apps on large multiprocessor
-# systems), this option causes __mmdrop to IPI all CPUs in the mm_cpumask and
-# switch to init_mm if they were using the to-be-freed mm as the lazy tlb. To
-# implement this, architectures must use _lazy_tlb variants of mm refcounting
-# when releasing kernel thread mm references, and mm_cpumask must include at
-# least all possible CPUs in which the mm might be lazy, at the time of the
-# final mmdrop. mmgrab/mmdrop in arch/ code must be switched to _lazy_tlb
-# postfix as necessary.
+# This option allows MMU_LAZY_TLB_REFCOUNT=n. It ensures no CPUs are using an
+# mm as a lazy tlb beyond its last reference count, by shooting down these
+# users before the mm is deallocated. __mmdrop() first IPIs all CPUs that may
+# be using the mm as a lazy tlb, so that they may switch themselves to using
+# init_mm for their active mm. mm_cpumask(mm) is used to determine which CPUs
+# may be using mm as a lazy tlb mm.
+#
+# To implement this, an arch must ensure mm_cpumask(mm) contains at least all
+# possible CPUs in which the mm is lazy, and it must meet the requirements for
+# MMU_LAZY_TLB_REFCOUNT=n (see above).
 config MMU_LAZY_TLB_SHOOTDOWN
 	bool
 
-- 
2.23.0