It was found that a dying mm_struct where the owning task has exited
can stay on as active_mm of kernel threads as long as no other user
tasks run on those CPUs that use it as active_mm. This prolongs the
life time of dying mm holding up some resources that cannot be freed
on a mostly idle system.
Fix that by forcing the kernel threads to use init_mm as the active_mm
during a kernel thread to kernel thread transition if the previous
active_mm is dying (!mm_users). This will allows the freeing of resources
associated with the dying mm ASAP.
The presence of a kernel-to-kernel thread transition indicates that
the cpu is probably idling with no higher priority user task to run.
So the overhead of loading the mm_users cacheline should not really
matter in this case.
My testing on an x86 system showed that the mm_struct was freed within
seconds after the task exited instead of staying alive for minutes or
even longer on a mostly idle system before this patch.
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
kernel/sched/core.c | 21 +++++++++++++++++++--
1 file changed, 19 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 795077af4f1a..41997e676251 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3214,6 +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 *next_mm = next->mm;
+
prepare_task_switch(rq, prev, next);
/*
@@ -3229,8 +3231,22 @@ context_switch(struct rq *rq, struct task_struct *prev,
*
* kernel -> user switch + mmdrop() active
* user -> user switch
+ *
+ * kernel -> kernel and !prev->active_mm->mm_users:
+ * switch to init_mm + mmgrab() + mmdrop()
*/
- if (!next->mm) { // to kernel
+ if (!next_mm) { // to kernel
+ /*
+ * Checking is only done on kernel -> kernel transition
+ * to avoid any performance overhead while user tasks
+ * are running.
+ */
+ if (unlikely(!prev->mm &&
+ !atomic_read(&prev->active_mm->mm_users))) {
+ next_mm = next->active_mm = &init_mm;
+ mmgrab(next_mm);
+ goto mm_switch;
+ }
enter_lazy_tlb(prev->active_mm, next);
next->active_mm = prev->active_mm;
@@ -3239,6 +3255,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
else
prev->active_mm = NULL;
} else { // to user
+mm_switch:
/*
* sys_membarrier() requires an smp_mb() between setting
* rq->curr and returning to userspace.
@@ -3248,7 +3265,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
* finish_task_switch()'s mmdrop().
*/
- switch_mm_irqs_off(prev->active_mm, next->mm, next);
+ switch_mm_irqs_off(prev->active_mm, next_mm, next);
if (!prev->mm) { // from kernel
/* will mmdrop() in finish_task_switch(). */
--
2.18.1
