From: Nicholas Piggin Date: Mon, 23 Aug 2021 23:59:18 +0000 (+1000) Subject: lazy tlb: allow lazy tlb mm refcounting to be configurable X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=ccb3587b3ce93b2b959dbace94200d02ae260a5e;p=users%2Fjedix%2Flinux-maple.git lazy tlb: allow lazy tlb mm refcounting to be configurable 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. Link: https://lkml.kernel.org/r/20210605014216.446867-3-npiggin@gmail.com Signed-off-by: Nicholas Piggin Cc: Andy Lutomirski Cc: Anton Blanchard Cc: Benjamin Herrenschmidt Cc: Paul Mackerras Cc: Randy Dunlap Signed-off-by: Andrew Morton Signed-off-by: Stephen Rothwell --- diff --git a/arch/Kconfig b/arch/Kconfig index 129df498a8e1..9dddd38d2bf4 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -425,6 +425,10 @@ config ARCH_WANT_IRQS_OFF_ACTIVATE_MM irqs disabled over activate_mm. Architectures that do IPI based TLB shootdowns should enable this. +# Use normal mm refcounting for MMU_LAZY_TLB kernel thread references. +config MMU_LAZY_TLB_REFCOUNT + def_bool y + config ARCH_HAVE_NMI_SAFE_CMPXCHG bool diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h index f7a0b347fecb..939dee9f5bc8 100644 --- a/include/linux/sched/mm.h +++ b/include/linux/sched/mm.h @@ -52,12 +52,21 @@ static inline void mmdrop(struct mm_struct *mm) /* Helpers for lazy TLB mm refcounting */ static inline void mmgrab_lazy_tlb(struct mm_struct *mm) { - mmgrab(mm); + if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_REFCOUNT)) + mmgrab(mm); } static inline void mmdrop_lazy_tlb(struct mm_struct *mm) { - mmdrop(mm); + if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_REFCOUNT)) { + mmdrop(mm); + } else { + /* + * mmdrop_lazy_tlb must provide a full memory barrier, see the + * membarrier comment finish_task_switch which relies on this. + */ + smp_mb(); + } } /** diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 030348d3e6d2..df4002813f02 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4513,7 +4513,7 @@ static struct rq *finish_task_switch(struct task_struct *prev) __releases(rq->lock) { struct rq *rq = this_rq(); - struct mm_struct *mm = rq->prev_mm; + struct mm_struct *mm = NULL; long prev_state; /* @@ -4532,7 +4532,10 @@ static struct rq *finish_task_switch(struct task_struct *prev) current->comm, current->pid, preempt_count())) preempt_count_set(FORK_PREEMPT_COUNT); - rq->prev_mm = NULL; +#ifdef CONFIG_MMU_LAZY_TLB_REFCOUNT + mm = rq->prev_lazy_mm; + rq->prev_lazy_mm = NULL; +#endif /* * A task struct has one reference for the use as "current". @@ -4668,9 +4671,20 @@ context_switch(struct rq *rq, struct task_struct *prev, switch_mm_irqs_off(prev->active_mm, next->mm, next); if (!prev->mm) { // from kernel - /* will mmdrop_lazy_tlb() in finish_task_switch(). */ - rq->prev_mm = prev->active_mm; +#ifdef CONFIG_MMU_LAZY_TLB_REFCOUNT + /* Will mmdrop_lazy_tlb() in finish_task_switch(). */ + rq->prev_lazy_mm = prev->active_mm; prev->active_mm = NULL; +#else + /* + * Without MMU_LAZY_TLB_REFCOUNT there is no lazy + * tracking (because no rq->prev_lazy_mm) in + * finish_task_switch, so no mmdrop_lazy_tlb(), so no + * memory barrier for membarrier (see the membarrier + * comment in finish_task_switch()). Do it here. + */ + smp_mb(); +#endif } } diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 14a41a243f7b..1a13bb32eba2 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -967,7 +967,9 @@ struct rq { struct task_struct *idle; struct task_struct *stop; unsigned long next_balance; - struct mm_struct *prev_mm; +#ifdef CONFIG_MMU_LAZY_TLB_REFCOUNT + struct mm_struct *prev_lazy_mm; +#endif unsigned int clock_update_flags; u64 clock;