sched: Clean up active_mm reference counting

The current active_mm reference counting is confusing and sub-optimal.

Rewrite the code to explicitly consider the 4 separate cases:

    user -> user

	When switching between two user tasks, all we need to consider
	is switch_mm().

    user -> kernel

	When switching from a user task to a kernel task (which
	doesn't have an associated mm) we retain the last mm in our
	active_mm. Increment a reference count on active_mm.

  kernel -> kernel

	When switching between kernel threads, all we need to do is
	pass along the active_mm reference.

  kernel -> user

	When switching between a kernel and user task, we must switch
	from the last active_mm to the next mm, hoping of course that
	these are the same. Decrement a reference on the active_mm.

The code keeps a different order, because as you'll note, both 'to
user' cases require switch_mm().

And where the old code would increment/decrement for the 'kernel ->
kernel' case, the new code observes this is a neutral operation and
avoids touching the reference count.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Rik van Riel <riel@surriel.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: luto@kernel.org

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 46f3ca9e392a37d88d9c212ef55d7f6bda0acf20..b4a44bc8474906aa0abe63f2bb98ae98aeb80484 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3214,12 +3214,8 @@ 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
@@ -3228,22 +3224,37 @@ context_switch(struct rq *rq, struct task_struct *prev,
        arch_start_context_switch(prev);
 
        /*
-        * If mm is non-NULL, we pass through switch_mm(). If mm is
-        * NULL, we will pass through mmdrop() in finish_task_switch().
-        * Both of these contain the full memory barrier required by
-        * membarrier after storing to rq->curr, before returning to
-        * user-space.
+        * kernel -> kernel   lazy + transfer active
+        *   user -> kernel   lazy + mmgrab() active
+        *
+        * kernel ->   user   switch + mmdrop() active
+        *   user ->   user   switch
         */
-       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;
+       if (!next->mm) {                                // to kernel
+               enter_lazy_tlb(prev->active_mm, next);
+
+               next->active_mm = prev->active_mm;
+               if (prev->mm)                           // from user
+                       mmgrab(prev->active_mm);
+               else
+                       prev->active_mm = NULL;
+       } else {                                        // to user
+               /*
+                * sys_membarrier() requires an smp_mb() between setting
+                * rq->curr and returning to userspace.
+                *
+                * The below provides this either through switch_mm(), or in
+                * case 'prev->active_mm == next->mm' through
+                * finish_task_switch()'s mmdrop().
+                */
+
+               switch_mm_irqs_off(prev->active_mm, next->mm, next);
+
+               if (!prev->mm) {                        // from kernel
+                       /* will mmdrop() in finish_task_switch(). */
+                       rq->prev_mm = prev->active_mm;
+                       prev->active_mm = NULL;
+               }
        }
 
        rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);