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Merge tag 'sched-urgent-2020-07-19' of git://git.kernel.org/pub/scm/l…
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…inux/kernel/git/tip/tip into master

Pull scheduler fixes from Thomas Gleixner:
 "A set of scheduler fixes:

   - Plug a load average accounting race which was introduced with a
     recent optimization casing load average to show bogus numbers.

   - Fix the rseq CPU id initialization for new tasks. sched_fork() does
     not update the rseq CPU id so the id is the stale id of the parent
     task, which can cause user space data corruption.

   - Handle a 0 return value of task_h_load() correctly in the load
     balancer, which does not decrease imbalance and therefore pulls
     until the maximum number of loops is reached, which might be all
     tasks just created by a fork bomb"

* tag 'sched-urgent-2020-07-19' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  sched/fair: handle case of task_h_load() returning 0
  sched: Fix unreliable rseq cpu_id for new tasks
  sched: Fix loadavg accounting race
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@torvalds
torvalds committed Jul 19, 2020
2 parents 9413cd7 + 01cfcde commit 43768f7
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Showing 3 changed files with 66 additions and 22 deletions.
4 changes: 0 additions & 4 deletions include/linux/sched.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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

/*
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69 changes: 53 additions & 16 deletions kernel/sched/core.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1311,9 +1311,6 @@ 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;
Expand All @@ -1323,9 +1320,6 @@ 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);
}

Expand Down Expand Up @@ -2236,10 +2230,10 @@ 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
Expand Down Expand Up @@ -2583,7 +2577,7 @@ 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) {
Expand All @@ -2592,9 +2586,6 @@ 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.
Expand All @@ -2613,8 +2604,20 @@ 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
Expand Down Expand Up @@ -2962,6 +2965,7 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
* Silence PROVE_RCU.
*/
raw_spin_lock_irqsave(&p->pi_lock, flags);
rseq_migrate(p);
/*
* We're setting the CPU for the first time, we don't migrate,
* so use __set_task_cpu().
Expand Down Expand Up @@ -3026,6 +3030,7 @@ void wake_up_new_task(struct task_struct *p)
* as we're not fully set-up yet.
*/
p->recent_used_cpu = task_cpu(p);
rseq_migrate(p);
__set_task_cpu(p, select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0));
#endif
rq = __task_rq_lock(p, &rf);
Expand Down Expand Up @@ -4097,6 +4102,7 @@ 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;
Expand All @@ -4113,12 +4119,22 @@ static void __sched notrace __schedule(bool preempt)
local_irq_disable();
rcu_note_context_switch(preempt);

/* See deactivate_task() below. */
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().
* done by the caller to avoid the race with signal_wake_up():
*
* __set_current_state(@state) signal_wake_up()
* schedule() set_tsk_thread_flag(p, TIF_SIGPENDING)
* wake_up_state(p, 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);
Expand All @@ -4129,10 +4145,31 @@ 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 prev->state in case ttwu_remote() 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) &&
!(prev_state & TASK_NOLOAD) &&
!(prev->flags & PF_FROZEN);

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) {
Expand Down
15 changes: 13 additions & 2 deletions kernel/sched/fair.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -4039,7 +4039,11 @@ static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
return;
}

rq->misfit_task_load = task_h_load(p);
/*
* Make sure that misfit_task_load will not be null even if
* task_h_load() returns 0.
*/
rq->misfit_task_load = max_t(unsigned long, task_h_load(p), 1);
}

#else /* CONFIG_SMP */
Expand Down Expand Up @@ -7638,7 +7642,14 @@ static int detach_tasks(struct lb_env *env)

switch (env->migration_type) {
case migrate_load:
load = task_h_load(p);
/*
* Depending of the number of CPUs and tasks and the
* cgroup hierarchy, task_h_load() can return a null
* value. Make sure that env->imbalance decreases
* otherwise detach_tasks() will stop only after
* detaching up to loop_max tasks.
*/
load = max_t(unsigned long, task_h_load(p), 1);

if (sched_feat(LB_MIN) &&
load < 16 && !env->sd->nr_balance_failed)
Expand Down

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