Re: weird loadavg on idle machine post 5.7

Peter Zijlstra
2020-07-06T15:00:07Z

On Fri, Jul 03, 2020 at 04:51:53PM -0400, Dave Jones wrote:
> On Fri, Jul 03, 2020 at 12:40:33PM +0200, Peter Zijlstra wrote:
>
> > So ARM/Power/etc.. can speculate the load such that the
> > task_contributes_to_load() value is from before ->on_rq.
> >
> > The compiler might similar re-order things -- although I've not found it
> > doing so with the few builds I looked at.
> >
> > So I think at the very least we should do something like this. But i've
> > no idea how to reproduce this problem.
> >
> > Mel's patch placed it too far down, as the WF_ON_CPU path also relies on
> > this, and by not resetting p->sched_contributes_to_load it would skew
> > accounting even worse.
>
> looked promising the first few hours, but as soon as it hit four hours
> of uptime, loadavg spiked and is now pinned to at least 1.00

OK, lots of cursing later, I now have the below...

The TL;DR is that while schedule() doesn't change p->state once it
starts, it does read it quite a bit, and ttwu() will actually change it
to TASK_WAKING. So if ttwu() changes it to WAKING before schedule()
reads it to do loadavg accounting, things go sideways.

The below is extra complicated by the fact that I've had to scrounge up
a bunch of load-store ordering without actually adding barriers. It adds
yet another control dependency to ttwu(), so take that C standard :-)

I've booted it, and build a few kernels with it and checked loadavg
drops to 0 after each build, so from that pov all is well, but since
I'm not confident I can reproduce the issue, I can't tell this actually
fixes anything, except maybe phantoms of my imagination.

---
include/linux/sched.h | 4 ---
kernel/sched/core.c | 67 +++++++++++++++++++++++++++++++++++++--------------
2 files changed, 49 insertions(+), 22 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 9bd073a10224..e26c8bbeda00 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -114,10 +114,6 @@ struct task_group;

#define task_is_stopped_or_traced(task) ((task->state & (__TASK_STOPPED | __TASK_TRACED)) != 0)

-#define task_contributes_to_load(task) ((task->state & TASK_UNINTERRUPTIBLE) != 0 && \
- (task->flags & PF_FROZEN) == 0 && \
- (task->state & TASK_NOLOAD) == 0)
-
#ifdef CONFIG_DEBUG_ATOMIC_SLEEP

/*
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 1d3d2d67f398..f245444b4b15 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1313,9 +1313,6 @@ static inline void dequeue_task(struct rq *rq, struct task_struct *p, int flags)

void activate_task(struct rq *rq, struct task_struct *p, int flags)
{
- if (task_contributes_to_load(p))
- rq->nr_uninterruptible--;
-
enqueue_task(rq, p, flags);

p->on_rq = TASK_ON_RQ_QUEUED;
@@ -1325,9 +1322,6 @@ void deactivate_task(struct rq *rq, struct task_struct *p, int flags)
{
p->on_rq = (flags & DEQUEUE_SLEEP) ? 0 : TASK_ON_RQ_MIGRATING;

- if (task_contributes_to_load(p))
- rq->nr_uninterruptible++;
-
dequeue_task(rq, p, flags);
}

@@ -2228,10 +2222,10 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,

lockdep_assert_held(&rq->lock);

-#ifdef CONFIG_SMP
if (p->sched_contributes_to_load)
rq->nr_uninterruptible--;

+#ifdef CONFIG_SMP
if (wake_flags & WF_MIGRATED)
en_flags |= ENQUEUE_MIGRATED;
#endif
@@ -2575,7 +2569,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
* A similar smb_rmb() lives in try_invoke_on_locked_down_task().
*/
smp_rmb();
- if (p->on_rq && ttwu_remote(p, wake_flags))
+ if (READ_ONCE(p->on_rq) && ttwu_remote(p, wake_flags))
goto unlock;

if (p->in_iowait) {
@@ -2584,9 +2578,6 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
}

#ifdef CONFIG_SMP
- p->sched_contributes_to_load = !!task_contributes_to_load(p);
- p->state = TASK_WAKING;
-
/*
* Ensure we load p->on_cpu _after_ p->on_rq, otherwise it would be
* possible to, falsely, observe p->on_cpu == 0.
@@ -2605,8 +2596,20 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
*
* Pairs with the LOCK+smp_mb__after_spinlock() on rq->lock in
* __schedule(). See the comment for smp_mb__after_spinlock().
+ *
+ * Form a control-dep-acquire with p->on_rq == 0 above, to ensure
+ * schedule()'s deactivate_task() has 'happened' and p will no longer
+ * care about it's own p->state. See the comment in __schedule().
*/
- smp_rmb();
+ smp_acquire__after_ctrl_dep();
+
+ /*
+ * We're doing the wakeup (@success == 1), they did a dequeue (p->on_rq
+ * == 0), which means we need to do an enqueue, change p->state to
+ * TASK_WAKING such that we can unlock p->pi_lock before doing the
+ * enqueue, such as ttwu_queue_wakelist().
+ */
+ p->state = TASK_WAKING;

/*
* If the owning (remote) CPU is still in the middle of schedule() with
@@ -4088,6 +4091,7 @@ static void __sched notrace __schedule(bool preempt)
{
struct task_struct *prev, *next;
unsigned long *switch_count;
+ unsigned long prev_state;
struct rq_flags rf;
struct rq *rq;
int cpu;
@@ -4104,12 +4108,19 @@ static void __sched notrace __schedule(bool preempt)
local_irq_disable();
rcu_note_context_switch(preempt);

+ prev_state = prev->state;
+
/*
- * Make sure that signal_pending_state()->signal_pending() below
- * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
- * done by the caller to avoid the race with signal_wake_up().
+ * __set_current_state(@state)
+ * schedule() signal_wake_up()
+ * prev_state = p->state set_tsk_thread_flag(p, TIF_SIGPENDING)
+ * wake_up_state()
+ * LOCK rq->lock LOCK p->pi_state
+ * smp_mb__after_spinlock() smp_mb__after_spinlock()
+ * if (signal_pending_state() if (p->state & @state)
+ *
*
- * The membarrier system call requires a full memory barrier
+ * Also, the membarrier system call requires a full memory barrier
* after coming from user-space, before storing to rq->curr.
*/
rq_lock(rq, &rf);
@@ -4120,10 +4131,30 @@ static void __sched notrace __schedule(bool preempt)
update_rq_clock(rq);

switch_count = &prev->nivcsw;
- if (!preempt && prev->state) {
- if (signal_pending_state(prev->state, prev)) {
+ /*
+ * We must re-load p->state in case ttwu_runnable() changed it
+ * before we acquired rq->lock.
+ */
+ if (!preempt && prev_state && prev_state == prev->state) {
+ if (signal_pending_state(prev_state, prev)) {
prev->state = TASK_RUNNING;
} else {
+ prev->sched_contributes_to_load =
+ (prev_state & (TASK_UNINTERRUPTIBLE | TASK_NOLOAD)) == TASK_UNINTERRUPTIBLE &&
+ (prev->flags & PF_FROZEN) == 0;
+
+ if (prev->sched_contributes_to_load)
+ rq->nr_uninterruptible++;
+
+ /*
+ * __schedule() ttwu()
+ * prev_state = prev->state; if (READ_ONCE(p->on_rq) && ...)
+ * LOCK rq->lock goto out;
+ * smp_mb__after_spinlock(); smp_acquire__after_ctrl_dep();
+ * p->on_rq = 0; p->state = TASK_WAKING;
+ *
+ * After this, schedule() must not care about p->state any more.
+ */
deactivate_task(rq, prev, DEQUEUE_SLEEP | DEQUEUE_NOCLOCK);

if (prev->in_iowait) {