Re: [PATCH v4 3/4] lazy tlb: shoot lazies, a non-refcounting lazy tlb option
From: Nicholas Piggin <npiggin@gmail.com>
Date: 2021-06-08 03:15:39
Also in:
linux-arch, linux-mm, lkml
Subsystem:
the rest · Maintainer:
Linus Torvalds
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@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@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
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2.23.0