NOMMU systems could easily go without this and save a bit of code
and the refcount atomics, because their mm switch is a no-op. I
haven't flipped them over because haven't audited all arch code to
convert over to using the _lazy_tlb refcounting.
Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
---
arch/Kconfig | 11 +++++++
include/linux/sched/mm.h | 13 ++++++--
kernel/sched/core.c | 68 +++++++++++++++++++++++++++++-----------
kernel/sched/sched.h | 4 ++-
4 files changed, 75 insertions(+), 21 deletions(-)
diff --git a/arch/Kconfig b/arch/Kconfig
index 56b6ccc0e32d..596bf589d74b 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -430,6 +430,17 @@ config ARCH_WANT_IRQS_OFF_ACTIVATE_MM
irqs disabled over activate_mm. Architectures that do IPI based TLB
shootdowns should enable this.
+# Should make this depend on MMU, because there is little use for lazy mm switching
+# with NOMMU. Must audit NOMMU architecture code for lazy mm refcounting first.
+config MMU_LAZY_TLB
+ def_bool y
+ help
+ Enable "lazy TLB" mmu context switching for kernel threads.
+
+config MMU_LAZY_TLB_REFCOUNT
+ def_bool y
+ depends on MMU_LAZY_TLB
+
config ARCH_HAVE_NMI_SAFE_CMPXCHG
bool
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 7157c0f6fef8..bd0f27402d4b 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -51,12 +51,21 @@ 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.
+ */
+ smp_mb();
+ }
}
/**
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e372b613d514..3b79c6cc3a37 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3579,7 +3579,7 @@ 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;
/*
@@ -3598,7 +3598,10 @@ 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".
@@ -3630,6 +3633,8 @@ static struct rq *finish_task_switch(struct task_struct *prev)
* rq->curr, before returning to userspace, for
* {PRIVATE,GLOBAL}_EXPEDITED. This is implicitly provided by
* mmdrop_lazy_tlb().
+ *
+ * This same issue applies to other places that mmdrop_lazy_tlb().
*/
if (mm)
mmdrop_lazy_tlb(mm);
@@ -3719,22 +3724,10 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
calculate_sigpending();
}
-/*
- * context_switch - switch to the new MM and the new thread's register state.
- */
-static __always_inline struct rq *
-context_switch(struct rq *rq, struct task_struct *prev,
- struct task_struct *next, struct rq_flags *rf)
+static __always_inline void
+context_switch_mm(struct rq *rq, struct task_struct *prev,
+ struct task_struct *next)
{
- prepare_task_switch(rq, prev, next);
-
- /*
- * For paravirt, this is coupled with an exit in switch_to to
- * combine the page table reload and the switch backend into
- * one hypercall.
- */
- arch_start_context_switch(prev);
-
/*
* kernel -> kernel lazy + transfer active
* user -> kernel lazy + mmgrab_lazy_tlb() active
@@ -3765,11 +3758,50 @@ context_switch(struct rq *rq, struct task_struct *prev,
if (!prev->mm) { // from kernel
exit_lazy_tlb(prev->active_mm, next);
+#ifdef CONFIG_MMU_LAZY_TLB_REFCOUNT
/* will mmdrop_lazy_tlb() in finish_task_switch(). */
- rq->prev_mm = prev->active_mm;
+ rq->prev_lazy_mm = prev->active_mm;
prev->active_mm = NULL;
+#else
+ /* See membarrier comment in finish_task_switch(). */
+ smp_mb();
+#endif
}
}
+}
+
+static __always_inline void
+context_switch_mm_nolazy(struct rq *rq, struct task_struct *prev,
+ struct task_struct *next)
+{
+ if (!next->mm)
+ next->active_mm = &init_mm;
+ membarrier_switch_mm(rq, prev->active_mm, next->active_mm);
+ switch_mm_irqs_off(prev->active_mm, next->active_mm, next);
+ if (!prev->mm)
+ prev->active_mm = NULL;
+}
+
+/*
+ * context_switch - switch to the new MM and the new thread's register state.
+ */
+static __always_inline struct rq *
+context_switch(struct rq *rq, struct task_struct *prev,
+ struct task_struct *next, struct rq_flags *rf)
+{
+ prepare_task_switch(rq, prev, next);
+
+ /*
+ * For paravirt, this is coupled with an exit in switch_to to
+ * combine the page table reload and the switch backend into
+ * one hypercall.
+ */
+ arch_start_context_switch(prev);
+
+ if (IS_ENABLED(CONFIG_MMU_LAZY_TLB))
+ context_switch_mm(rq, prev, next);
+ else
+ context_switch_mm_nolazy(rq, prev, next);
rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index df80bfcea92e..3b72aec5a2f2 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -950,7 +950,9 @@ 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;
--
2.23.0
