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Subject[RFC PATCH 08/11] sched input interactivity-driven next buddy
[ Impact: implement INTERACTIVE feature to increase Xorg responsiveness. ]

Apply next buddy logic to interactivity-driven wakeups. Don't pass the
interactivity flag across forks to defuse interactivity-based fork-bombs. The
goal of this patch is to ensure that Xorg keeps a good interactivity level by
ensuring that Xorg and its related threads quickly respond to wakeups caused by
user inputs.

Derived from a patch from Peter Zijlstra.

* This patch also makes sure that as soon as an iowait is perceived, the
interactivity chain is stopped.
* This patch removes the previously available "NEXT_BUDDY" scheduler feature
altogether.

On my 2.0GHz uniprocessor desktop, enabling the INTERACTIVE feature makes
firefox very responsive even if I overcommit my CPU with a make -j5 kernel
build.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
drivers/input/evdev.c | 2 ++
include/linux/sched.h | 31 +++++++++++++++++++++++--------
kernel/sched.c | 12 +++++++++++-
kernel/sched_fair.c | 26 +++++++++++++++++++-------
kernel/sched_features.h | 11 ++++-------
5 files changed, 59 insertions(+), 23 deletions(-)

Index: linux-2.6-lttng.laptop/drivers/input/evdev.c
===================================================================
--- linux-2.6-lttng.laptop.orig/drivers/input/evdev.c
+++ linux-2.6-lttng.laptop/drivers/input/evdev.c
@@ -78,6 +78,7 @@ static void evdev_event(struct input_han
event.code = code;
event.value = value;

+ sched_wake_interactive_enable();
rcu_read_lock();

client = rcu_dereference(evdev->grab);
@@ -90,6 +91,7 @@ static void evdev_event(struct input_han
rcu_read_unlock();

wake_up_interruptible(&evdev->wait);
+ sched_wake_interactive_disable();
}

static int evdev_fasync(int fd, struct file *file, int on)
Index: linux-2.6-lttng.laptop/include/linux/sched.h
===================================================================
--- linux-2.6-lttng.laptop.orig/include/linux/sched.h
+++ linux-2.6-lttng.laptop/include/linux/sched.h
@@ -1024,14 +1024,17 @@ struct sched_domain;
/*
* wake flags
*/
-#define WF_SYNC 0x01 /* waker goes to sleep after wakup */
-#define WF_FORK 0x02 /* child wakeup after fork */
+#define WF_SYNC (1 << 0) /* waker goes to sleep after wakup */
+#define WF_FORK (1 << 1) /* child wakeup after fork */
+#define WF_INTERACTIVE (1 << 2) /* interactivity-driven wakeup */
+
+#define ENQUEUE_WAKEUP (1 << 0)
+#define ENQUEUE_WAKING (1 << 1)
+#define ENQUEUE_HEAD (1 << 2)
+#define ENQUEUE_IO (1 << 3)
+#define ENQUEUE_LATENCY (1 << 4)

-#define ENQUEUE_WAKEUP 1
-#define ENQUEUE_WAKING 2
-#define ENQUEUE_HEAD 4
-
-#define DEQUEUE_SLEEP 1
+#define DEQUEUE_SLEEP (1 << 0)

struct sched_class {
const struct sched_class *next;
@@ -1124,7 +1127,8 @@ struct sched_entity {
struct load_weight load; /* for load-balancing */
struct rb_node run_node;
struct list_head group_node;
- unsigned int on_rq;
+ unsigned int on_rq:1,
+ interactive:1;

u64 exec_start;
u64 sum_exec_runtime;
@@ -1237,6 +1241,7 @@ struct task_struct {
unsigned sched_in_iowait:1; /* Called io_schedule() */
unsigned sched_reset_on_fork:1; /* Revert to default
* priority/policy on fork */
+ unsigned sched_wake_interactive:4; /* User-driven wakeup */

pid_t pid;
pid_t tgid;
@@ -1502,6 +1507,16 @@ struct task_struct {
#endif
};

+static inline void sched_wake_interactive_enable(void)
+{
+ current->sched_wake_interactive++;
+}
+
+static inline void sched_wake_interactive_disable(void)
+{
+ current->sched_wake_interactive--;
+}
+
/* Future-safe accessor for struct task_struct's cpus_allowed. */
#define tsk_cpus_allowed(tsk) (&(tsk)->cpus_allowed)

Index: linux-2.6-lttng.laptop/kernel/sched.c
===================================================================
--- linux-2.6-lttng.laptop.orig/kernel/sched.c
+++ linux-2.6-lttng.laptop/kernel/sched.c
@@ -2288,6 +2288,13 @@ static int try_to_wake_up(struct task_st
unsigned long en_flags = ENQUEUE_WAKEUP;
struct rq *rq;

+ if (sched_feat(INTERACTIVE) && !(wake_flags & WF_FORK)) {
+ if (current->sched_wake_interactive ||
+ wake_flags & WF_INTERACTIVE ||
+ current->se.interactive)
+ en_flags |= ENQUEUE_LATENCY;
+ }
+
this_cpu = get_cpu();

smp_wmb();
@@ -3613,8 +3620,11 @@ need_resched_nonpreemptible:
if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
if (unlikely(signal_pending_state(prev->state, prev)))
prev->state = TASK_RUNNING;
- else
+ else {
+ if (sched_feat(INTERACTIVE))
+ prev->se.interactive = 0;
deactivate_task(rq, prev, DEQUEUE_SLEEP);
+ }
switch_count = &prev->nvcsw;
}

Index: linux-2.6-lttng.laptop/kernel/sched_fair.c
===================================================================
--- linux-2.6-lttng.laptop.orig/kernel/sched_fair.c
+++ linux-2.6-lttng.laptop/kernel/sched_fair.c
@@ -774,6 +774,9 @@ enqueue_entity(struct cfs_rq *cfs_rq, st
account_entity_enqueue(cfs_rq, se);

if (flags & ENQUEUE_WAKEUP) {
+ if (sched_feat(INTERACTIVE)
+ && flags & ENQUEUE_LATENCY && !(flags & ENQUEUE_IO))
+ se->interactive = 1;
place_entity(cfs_rq, se, 0);
enqueue_sleeper(cfs_rq, se);
}
@@ -916,14 +919,14 @@ static struct sched_entity *pick_next_en
struct sched_entity *se = __pick_next_entity(cfs_rq);
struct sched_entity *left = se;

- if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
- se = cfs_rq->next;
+ if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
+ se = cfs_rq->last;

/*
- * Prefer last buddy, try to return the CPU to a preempted task.
+ * Prefer the next buddy, only set through the interactivity logic.
*/
- if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
- se = cfs_rq->last;
+ if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
+ se = cfs_rq->next;

clear_buddies(cfs_rq, se);

@@ -1046,6 +1049,9 @@ enqueue_task_fair(struct rq *rq, struct
struct cfs_rq *cfs_rq;
struct sched_entity *se = &p->se;

+ if (p->sched_in_iowait)
+ flags |= ENQUEUE_IO;
+
for_each_sched_entity(se) {
if (se->on_rq)
break;
@@ -1657,6 +1663,7 @@ static void check_preempt_wakeup(struct
* tasks for there to be buddies.
*/
int buddies = (cfs_rq->nr_running >= 2);
+ int preempt = 0;

if (unlikely(rt_prio(p->prio)))
goto preempt;
@@ -1667,8 +1674,13 @@ static void check_preempt_wakeup(struct
if (unlikely(se == pse))
return;

- if (sched_feat(NEXT_BUDDY) && buddies && !(wake_flags & WF_FORK))
+ if (sched_feat(INTERACTIVE)
+ && !(wake_flags & WF_FORK) && pse->interactive) {
+ clear_buddies(cfs_rq, NULL);
set_next_buddy(pse);
+ preempt = 1;
+ buddies = 0;
+ }

/*
* We can come here with TIF_NEED_RESCHED already set from new task
@@ -1694,7 +1706,7 @@ static void check_preempt_wakeup(struct
update_curr(cfs_rq);
find_matching_se(&se, &pse);
BUG_ON(!pse);
- if (wakeup_preempt_entity(se, pse) == 1)
+ if (preempt || wakeup_preempt_entity(se, pse) == 1)
goto preempt;

return;
Index: linux-2.6-lttng.laptop/kernel/sched_features.h
===================================================================
--- linux-2.6-lttng.laptop.orig/kernel/sched_features.h
+++ linux-2.6-lttng.laptop/kernel/sched_features.h
@@ -26,13 +26,6 @@ SCHED_FEAT(WAKEUP_PREEMPT, 1)
SCHED_FEAT(AFFINE_WAKEUPS, 1)

/*
- * Prefer to schedule the task we woke last (assuming it failed
- * wakeup-preemption), since its likely going to consume data we
- * touched, increases cache locality.
- */
-SCHED_FEAT(NEXT_BUDDY, 0)
-
-/*
* Prefer to schedule the task that ran last (when we did
* wake-preempt) as that likely will touch the same data, increases
* cache locality.
@@ -61,6 +54,10 @@ SCHED_FEAT(ASYM_EFF_LOAD, 1)
* ensures the spread does not grow beyond control.
*/
SCHED_FEAT(DYN_MIN_VRUNTIME, 0)
+/*
+ * Input subsystem next buddy affinity. Not transitive across new task wakeups.
+ */
+SCHED_FEAT(INTERACTIVE, 0)

/*
* Spin-wait on mutex acquisition when the mutex owner is running on


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