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SubjectRe: [PATCH 4/7] x86,tlb: make lazy TLB mode lazier
On Thu, Jul 19, 2018 at 09:45:47AM -0700, Andy Lutomirski wrote:
> After some grepping, there are very few users. The
> only nontrivial ones are the ones in kernel/ and mm/mmu_context.c that
> are involved in the rather complicated dance of refcounting active_mm.

Something like so should work I suppose.

It keeps the field, but ensure that for all tasks tsk->active_mm ==
tsk->mm. The implication is that switch_mm(.prev == NULL) should work
(it does for x86, we don't care about @prev).


diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d84eafcf5e29..7bfd850c6bf7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2780,12 +2780,8 @@ static __always_inline struct rq *
context_switch(struct rq *rq, struct task_struct *prev,
struct task_struct *next, struct rq_flags *rf)
{
- struct mm_struct *mm, *oldmm;
-
prepare_task_switch(rq, prev, next);

- mm = next->mm;
- oldmm = prev->active_mm;
/*
* For paravirt, this is coupled with an exit in switch_to to
* combine the page table reload and the switch backend into
@@ -2800,16 +2796,32 @@ context_switch(struct rq *rq, struct task_struct *prev,
* membarrier after storing to rq->curr, before returning to
* user-space.
*/
- if (!mm) {
- next->active_mm = oldmm;
- mmgrab(oldmm);
- enter_lazy_tlb(oldmm, next);
- } else
- switch_mm_irqs_off(oldmm, mm, next);
-
- if (!prev->mm) {
- prev->active_mm = NULL;
- rq->prev_mm = oldmm;
+
+ /*
+ * kernel -> kernel lazy + transfer active
+ * user -> kernel lazy + mmgrab() active
+ *
+ * kernel -> user switch + mmdrop() active
+ * user -> user switch
+ */
+ if (!next->mm) { // to kernel
+ enter_lazy_tlb(prev->active_mm, next);
+
+#ifdef ARCH_NO_ACTIVE_MM
+ next->active_mm = prev->active_mm;
+ if (prev->mm) // from user
+ mmgrab(prev->active_mm);
+#endif
+ } else { // to user
+ switch_mm_irqs_off(prev->active_mm, next->mm, next);
+
+#ifdef ARCH_NO_ACTIVE_MM
+ if (!prev->mm) { // from kernel
+ /* will mmdrop() in finish_task_switch(). */
+ rq->prev_mm = prev->active_mm;
+ prev->active_mm = NULL;
+ }
+#endif
}

rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
diff --git a/mm/mmu_context.c b/mm/mmu_context.c
index 3e612ae748e9..770bb615d115 100644
--- a/mm/mmu_context.c
+++ b/mm/mmu_context.c
@@ -12,6 +12,10 @@

#include <asm/mmu_context.h>

+#ifndef finish_arch_post_lock_switch
+# define finish_arch_post_lock_switch() do { } while (0)
+#endif
+
/*
* use_mm
* Makes the calling kernel thread take on the specified
@@ -33,12 +37,14 @@ void use_mm(struct mm_struct *mm)
tsk->mm = mm;
switch_mm(active_mm, mm, tsk);
task_unlock(tsk);
-#ifdef finish_arch_post_lock_switch
finish_arch_post_lock_switch();
-#endif

+#ifdef ARCH_NO_ACTIVE_MM
+ WARN_ON_ONCE(active_mm != NULL);
+#else
if (active_mm != mm)
mmdrop(active_mm);
+#endif
}
EXPORT_SYMBOL_GPL(use_mm);

@@ -57,8 +63,22 @@ void unuse_mm(struct mm_struct *mm)
task_lock(tsk);
sync_mm_rss(mm);
tsk->mm = NULL;
+
+ WARN_ON_ONCE(tsk->active_mm != mm);
+
+#ifdef ARCH_NO_ACTIVE_MM
+ switch_mm(tsk->active_mm, &init_mm, tsk);
+ mmdrop(tsk->active_mm);
+ tsk->active_mm = NULL;
+#else
/* active_mm is still 'mm' */
enter_lazy_tlb(mm, tsk);
+#endif
+
task_unlock(tsk);
+
+#ifdef ARCH_NO_ACTIVE_MM
+ finish_arch_post_lock_switch();
+#endif
}
EXPORT_SYMBOL_GPL(unuse_mm);
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