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@xxxxxxxxx> > --- Update the comment to be clearer, and account for the improvement to MMU_LAZY_TLB_REFCOUNT comment. Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx> --- 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
