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Message-Id: <20201214065312.270062-3-npiggin@gmail.com>
Date: Mon, 14 Dec 2020 16:53:09 +1000
From: Nicholas Piggin <npiggin@...il.com>
To: linux-kernel@...r.kernel.org
Cc: Nicholas Piggin <npiggin@...il.com>, linux-arch@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
Anton Blanchard <anton@...abs.org>,
Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v2 2/5] lazy tlb: allow lazy tlb mm switching to be configurable
Add CONFIG_MMU_LAZY_TLB which can be configured out to disable
the lazy tlb mechanism entirely, and switches to init_mm when
switching to a kernel thread.
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. They
have not been switched over by default because the arch code needs
to be audited and tested for lazy tlb mm refcounting and converted
to _lazy_tlb refcounting if necessary.
CONFIG_MMU_LAZY_TLB_REFCOUNT is also added, but it must always
be enabled if CONFIG_MMU_LAZY_TLB is enabled until the next patch
which provides an alternate scheme.
Signed-off-by: Nicholas Piggin <npiggin@...il.com>
---
arch/Kconfig | 17 +++++++++
include/linux/sched/mm.h | 13 +++++--
kernel/sched/core.c | 75 ++++++++++++++++++++++++++++++----------
kernel/sched/sched.h | 4 ++-
4 files changed, 87 insertions(+), 22 deletions(-)
diff --git a/arch/Kconfig b/arch/Kconfig
index ba4e966484ab..84faaba66364 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -430,6 +430,23 @@ 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.
+ If this is disabled then switching to a kernel thread always
+ switches to init_mm. If mm switches are inexpensive or free
+ (in the case of NOMMU) then this could be disabled.
+
+config MMU_LAZY_TLB_REFCOUNT
+ def_bool y
+ depends on MMU_LAZY_TLB
+ help
+ This must be enabled if MMU_LAZY_TLB is enabled until the next
+ patch.
+
config ARCH_HAVE_NMI_SAFE_CMPXCHG
bool
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 94a117160083..5edf8e942c84 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 c2f8ea43d29b..9c1dc9406e4b 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".
@@ -3722,22 +3725,10 @@ 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
@@ -3766,11 +3757,57 @@ 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_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
}
}
+}
+
+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
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