sched/core: Fix task and run queue sched_info::run_delay inconsistencies

Mike Meyer reported the following bug:

> During evaluation of some performance data, it was discovered thread
> and run queue run_delay accounting data was inconsistent with the other
> accounting data that was collected.  Further investigation found under
> certain circumstances execution time was leaking into the task and
> run queue accounting of run_delay.
>
> Consider the following sequence:
>
>     a. thread is running.
>     b. thread moves beween cgroups, changes scheduling class or priority.
>     c. thread sleeps OR
>     d. thread involuntarily gives up cpu.
>
> a. implies:
>
>     thread->sched_info.last_queued = 0
>
> a. and b. results in the following:
>
>     1. dequeue_task(rq, thread)
>
>            sched_info_dequeued(rq, thread)
>                delta = 0
>
>                sched_info_reset_dequeued(thread)
>                    thread->sched_info.last_queued = 0
>
>                thread->sched_info.run_delay += delta
>
>     2. enqueue_task(rq, thread)
>
>            sched_info_queued(rq, thread)
>
>                /* thread is still on cpu at this point. */
>                thread->sched_info.last_queued = task_rq(thread)->clock;
>
> c. results in:
>
>     dequeue_task(rq, thread)
>
>         sched_info_dequeued(rq, thread)
>
>             /* delta is execution time not run_delay. */
>             delta = task_rq(thread)->clock - thread->sched_info.last_queued
>
>         sched_info_reset_dequeued(thread)
>             thread->sched_info.last_queued = 0
>
>         thread->sched_info.run_delay += delta
>
>     Since thread was running between enqueue_task(rq, thread) and
>     dequeue_task(rq, thread), the delta above is really execution
>     time and not run_delay.
>
> d. results in:
>
>     __sched_info_switch(thread, next_thread)
>
>         sched_info_depart(rq, thread)
>
>             sched_info_queued(rq, thread)
>
>                 /* last_queued not updated due to being non-zero */
>                 return
>
>     Since thread was running between enqueue_task(rq, thread) and
>     __sched_info_switch(thread, next_thread), the execution time
>     between enqueue_task(rq, thread) and
>     __sched_info_switch(thread, next_thread) now will become
>     associated with run_delay due to when last_queued was last updated.
>

This alternative patch solves the problem by not calling
sched_info_{de,}queued() in {de,en}queue_task(). Therefore the
sched_info state is preserved and things work as expected.

By inlining the {de,en}queue_task() functions the new condition
becomes (mostly) a compile-time constant and we'll not emit any new
branch instructions.

It even shrinks the code (due to inlining {en,de}queue_task()):

$ size defconfig-build/kernel/sched/core.o defconfig-build/kernel/sched/core.o.orig
   text    data     bss     dec     hex filename
  64019   23378    2344   89741   15e8d defconfig-build/kernel/sched/core.o
  64149   23378    2344   89871   15f0f defconfig-build/kernel/sched/core.o.orig

Reported-by: Mike Meyer <Mike.Meyer@Teradata.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/20150930154413.GO3604@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4554cde2f6f96293be4636fb6e191f3dc0d98096..fb14a010426f55bd39d775c2cf2cce44f2221a4b 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -827,17 +827,19 @@ static void set_load_weight(struct task_struct *p)
        load->inv_weight = prio_to_wmult[prio];
 }
 
-static void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
+static inline void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
 {
        update_rq_clock(rq);
-       sched_info_queued(rq, p);
+       if (!(flags & ENQUEUE_RESTORE))
+               sched_info_queued(rq, p);
        p->sched_class->enqueue_task(rq, p, flags);
 }
 
-static void dequeue_task(struct rq *rq, struct task_struct *p, int flags)
+static inline void dequeue_task(struct rq *rq, struct task_struct *p, int flags)
 {
        update_rq_clock(rq);
-       sched_info_dequeued(rq, p);
+       if (!(flags & DEQUEUE_SAVE))
+               sched_info_dequeued(rq, p);
        p->sched_class->dequeue_task(rq, p, flags);
 }
 
@@ -1178,7 +1180,7 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
                 * holding rq->lock.
                 */
                lockdep_assert_held(&rq->lock);
-               dequeue_task(rq, p, 0);
+               dequeue_task(rq, p, DEQUEUE_SAVE);
        }
        if (running)
                put_prev_task(rq, p);
@@ -1188,7 +1190,7 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
        if (running)
                p->sched_class->set_curr_task(rq);
        if (queued)
-               enqueue_task(rq, p, 0);
+               enqueue_task(rq, p, ENQUEUE_RESTORE);
 }
 
 /*
@@ -1692,7 +1694,7 @@ ttwu_stat(struct task_struct *p, int cpu, int wake_flags)
 #endif /* CONFIG_SCHEDSTATS */
 }
 
-static void ttwu_activate(struct rq *rq, struct task_struct *p, int en_flags)
+static inline void ttwu_activate(struct rq *rq, struct task_struct *p, int en_flags)
 {
        activate_task(rq, p, en_flags);
        p->on_rq = TASK_ON_RQ_QUEUED;
@@ -3325,7 +3327,7 @@ EXPORT_SYMBOL(default_wake_function);
  */
 void rt_mutex_setprio(struct task_struct *p, int prio)
 {
-       int oldprio, queued, running, enqueue_flag = 0;
+       int oldprio, queued, running, enqueue_flag = ENQUEUE_RESTORE;
        struct rq *rq;
        const struct sched_class *prev_class;
 
@@ -3357,7 +3359,7 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
        queued = task_on_rq_queued(p);
        running = task_current(rq, p);
        if (queued)
-               dequeue_task(rq, p, 0);
+               dequeue_task(rq, p, DEQUEUE_SAVE);
        if (running)
                put_prev_task(rq, p);
 
@@ -3375,7 +3377,7 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
                if (!dl_prio(p->normal_prio) ||
                    (pi_task && dl_entity_preempt(&pi_task->dl, &p->dl))) {
                        p->dl.dl_boosted = 1;
-                       enqueue_flag = ENQUEUE_REPLENISH;
+                       enqueue_flag |= ENQUEUE_REPLENISH;
                } else
                        p->dl.dl_boosted = 0;
                p->sched_class = &dl_sched_class;
@@ -3383,7 +3385,7 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
                if (dl_prio(oldprio))
                        p->dl.dl_boosted = 0;
                if (oldprio < prio)
-                       enqueue_flag = ENQUEUE_HEAD;
+                       enqueue_flag |= ENQUEUE_HEAD;
                p->sched_class = &rt_sched_class;
        } else {
                if (dl_prio(oldprio))
@@ -3435,7 +3437,7 @@ void set_user_nice(struct task_struct *p, long nice)
        }
        queued = task_on_rq_queued(p);
        if (queued)
-               dequeue_task(rq, p, 0);
+               dequeue_task(rq, p, DEQUEUE_SAVE);
 
        p->static_prio = NICE_TO_PRIO(nice);
        set_load_weight(p);
@@ -3444,7 +3446,7 @@ void set_user_nice(struct task_struct *p, long nice)
        delta = p->prio - old_prio;
 
        if (queued) {
-               enqueue_task(rq, p, 0);
+               enqueue_task(rq, p, ENQUEUE_RESTORE);
                /*
                 * If the task increased its priority or is running and
                 * lowered its priority, then reschedule its CPU:
@@ -3946,7 +3948,7 @@ change:
        queued = task_on_rq_queued(p);
        running = task_current(rq, p);
        if (queued)
-               dequeue_task(rq, p, 0);
+               dequeue_task(rq, p, DEQUEUE_SAVE);
        if (running)
                put_prev_task(rq, p);
 
@@ -3956,11 +3958,15 @@ change:
        if (running)
                p->sched_class->set_curr_task(rq);
        if (queued) {
+               int enqueue_flags = ENQUEUE_RESTORE;
                /*
                 * We enqueue to tail when the priority of a task is
                 * increased (user space view).
                 */
-               enqueue_task(rq, p, oldprio <= p->prio ? ENQUEUE_HEAD : 0);
+               if (oldprio <= p->prio)
+                       enqueue_flags |= ENQUEUE_HEAD;
+
+               enqueue_task(rq, p, enqueue_flags);
        }
 
        check_class_changed(rq, p, prev_class, oldprio);
@@ -5109,7 +5115,7 @@ void sched_setnuma(struct task_struct *p, int nid)
        running = task_current(rq, p);
 
        if (queued)
-               dequeue_task(rq, p, 0);
+               dequeue_task(rq, p, DEQUEUE_SAVE);
        if (running)
                put_prev_task(rq, p);
 
@@ -5118,7 +5124,7 @@ void sched_setnuma(struct task_struct *p, int nid)
        if (running)
                p->sched_class->set_curr_task(rq);
        if (queued)
-               enqueue_task(rq, p, 0);
+               enqueue_task(rq, p, ENQUEUE_RESTORE);
        task_rq_unlock(rq, p, &flags);
 }
 #endif /* CONFIG_NUMA_BALANCING */
@@ -7737,7 +7743,7 @@ void sched_move_task(struct task_struct *tsk)
        queued = task_on_rq_queued(tsk);
 
        if (queued)
-               dequeue_task(rq, tsk, 0);
+               dequeue_task(rq, tsk, DEQUEUE_SAVE);
        if (unlikely(running))
                put_prev_task(rq, tsk);
 
@@ -7761,7 +7767,7 @@ void sched_move_task(struct task_struct *tsk)
        if (unlikely(running))
                tsk->sched_class->set_curr_task(rq);
        if (queued)
-               enqueue_task(rq, tsk, 0);
+               enqueue_task(rq, tsk, ENQUEUE_RESTORE);
 
        task_rq_unlock(rq, tsk, &flags);
 }

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 046242feff3a35552de27c3d575a06ee3f2a1df6..e08cc4cf68e25a9e8efa29b562e8c2c45abbb227 100644 (file)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1151,16 +1151,18 @@ static const u32 prio_to_wmult[40] = {
  /*  15 */ 119304647, 148102320, 186737708, 238609294, 286331153,
 };
 
-#define ENQUEUE_WAKEUP         1
-#define ENQUEUE_HEAD           2
+#define ENQUEUE_WAKEUP         0x01
+#define ENQUEUE_HEAD           0x02
 #ifdef CONFIG_SMP
-#define ENQUEUE_WAKING         4       /* sched_class::task_waking was called */
+#define ENQUEUE_WAKING         0x04    /* sched_class::task_waking was called */
 #else
-#define ENQUEUE_WAKING         0
+#define ENQUEUE_WAKING         0x00
 #endif
-#define ENQUEUE_REPLENISH      8
+#define ENQUEUE_REPLENISH      0x08
+#define ENQUEUE_RESTORE        0x10
 
-#define DEQUEUE_SLEEP          1
+#define DEQUEUE_SLEEP          0x01
+#define DEQUEUE_SAVE           0x02
 
 #define RETRY_TASK             ((void *)-1UL)