[RFC PATCH 6/7] lazy tlb: allow lazy tlb mm switching to be configurable

Nicholas Piggin
2020-07-10T01:57:38Z

NOMMU systems could easily go without this and save a bit of code
and the mm refcounting, 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@gmail.com>
---
arch/Kconfig | 7 +++++
include/linux/sched/mm.h | 12 ++++++---
kernel/sched/core.c | 55 +++++++++++++++++++++++++++-------------
kernel/sched/sched.h | 4 ++-
4 files changed, 55 insertions(+), 23 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 8cc35dc556c7..2daf8fe6146a 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -411,6 +411,13 @@ config MMU_GATHER_NO_GATHER
bool
depends on MMU_GATHER_TABLE_FREE

+# Would like to make this depend on MMU, because there is little use for lazy mm switching
+# with NOMMU, but have to audit NOMMU architecture code first.
+config MMU_LAZY_TLB
+ def_bool y
+ help
+ Enable "lazy TLB" mmu context switching for kernel threads.
+
config ARCH_HAVE_NMI_SAFE_CMPXCHG
bool

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 110d4ad21de6..2c2b20e2ccc7 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -53,18 +53,22 @@ 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))
+ mmgrab(mm);
}

static inline void mmdrop_lazy_tlb(struct mm_struct *mm)
{
- mmdrop(mm);
+ if (IS_ENABLED(CONFIG_MMU_LAZY_TLB))
+ mmdrop(mm);
}

static inline void mmdrop_lazy_tlb_smp_mb(struct mm_struct *mm)
{
- /* This depends on mmdrop providing a full smp_mb() */
- mmdrop(mm);
+ if (IS_ENABLED(CONFIG_MMU_LAZY_TLB))
+ mmdrop(mm); /* This depends on mmdrop providing a full smp_mb() */
+ else
+ smp_mb();
}

/*
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d19f2f517f6c..14b4fae6f6e3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3253,7 +3253,7 @@ 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;

/*
@@ -3272,7 +3272,10 @@ 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
+ mm = rq->prev_lazy_mm;
+ rq->prev_lazy_mm = NULL;
+#endif

/*
* A task struct has one reference for the use as "current".
@@ -3393,22 +3396,11 @@ 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);
-
+#ifdef CONFIG_MMU_LAZY_TLB
/*
* kernel -> kernel lazy + transfer active
* user -> kernel lazy + mmgrab_lazy_tlb() active
@@ -3440,10 +3432,37 @@ context_switch(struct rq *rq, struct task_struct *prev,
exit_lazy_tlb(prev->active_mm, next);

/* 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
+ 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;
+#endif
+}
+
+/*
+ * 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);
+
+ context_switch_mm(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 877fb08eb1b0..b196dd885d33 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -929,7 +929,9 @@ struct rq {
struct task_struct *idle;
struct task_struct *stop;
unsigned long next_balance;
- struct mm_struct *prev_mm;
+#ifdef CONFIG_MMU_LAZY_TLB
+ struct mm_struct *prev_lazy_mm;
+#endif

unsigned int clock_update_flags;
u64 clock;
--
2.23.0