lkml.org 
[lkml]   [2013]   [Sep]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[tip:sched/core] sched: Add NEED_RESCHED to the preempt_count
Commit-ID:  f27dde8deef33c9e58027df11ceab2198601d6a6
Gitweb: http://git.kernel.org/tip/f27dde8deef33c9e58027df11ceab2198601d6a6
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Wed, 14 Aug 2013 14:55:31 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 25 Sep 2013 14:07:49 +0200

sched: Add NEED_RESCHED to the preempt_count

In order to combine the preemption and need_resched test we need to
fold the need_resched information into the preempt_count value.

Since the NEED_RESCHED flag is set across CPUs this needs to be an
atomic operation, however we very much want to avoid making
preempt_count atomic, therefore we keep the existing TIF_NEED_RESCHED
infrastructure in place but at 3 sites test it and fold its value into
preempt_count; namely:

- resched_task() when setting TIF_NEED_RESCHED on the current task
- scheduler_ipi() when resched_task() sets TIF_NEED_RESCHED on a
remote task it follows it up with a reschedule IPI
and we can modify the cpu local preempt_count from
there.
- cpu_idle_loop() for when resched_task() found tsk_is_polling().

We use an inverted bitmask to indicate need_resched so that a 0 means
both need_resched and !atomic.

Also remove the barrier() in preempt_enable() between
preempt_enable_no_resched() and preempt_check_resched() to avoid
having to reload the preemption value and allow the compiler to use
the flags of the previuos decrement. I couldn't come up with any sane
reason for this barrier() to be there as preempt_enable_no_resched()
already has a barrier() before doing the decrement.

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-7a7m5qqbn5pmwnd4wko9u6da@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
include/linux/preempt.h | 47 ++++++++++++++++++++++++++++++++++++++++++-----
include/linux/sched.h | 7 +++++--
kernel/cpu/idle.c | 7 +++++++
kernel/sched/core.c | 20 +++++++++++++++-----
4 files changed, 69 insertions(+), 12 deletions(-)

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index eaac52a..92e3418 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -10,9 +10,19 @@
#include <linux/linkage.h>
#include <linux/list.h>

+/*
+ * We use the MSB mostly because its available; see <linux/preempt_mask.h> for
+ * the other bits -- can't include that header due to inclusion hell.
+ */
+#define PREEMPT_NEED_RESCHED 0x80000000
+
+/*
+ * We mask the PREEMPT_NEED_RESCHED bit so as not to confuse all current users
+ * that think a non-zero value indicates we cannot preempt.
+ */
static __always_inline int preempt_count(void)
{
- return current_thread_info()->preempt_count;
+ return current_thread_info()->preempt_count & ~PREEMPT_NEED_RESCHED;
}

static __always_inline int *preempt_count_ptr(void)
@@ -20,11 +30,40 @@ static __always_inline int *preempt_count_ptr(void)
return &current_thread_info()->preempt_count;
}

+/*
+ * We now loose PREEMPT_NEED_RESCHED and cause an extra reschedule; however the
+ * alternative is loosing a reschedule. Better schedule too often -- also this
+ * should be a very rare operation.
+ */
static __always_inline void preempt_count_set(int pc)
{
*preempt_count_ptr() = pc;
}

+/*
+ * We fold the NEED_RESCHED bit into the preempt count such that
+ * preempt_enable() can decrement and test for needing to reschedule with a
+ * single instruction.
+ *
+ * We invert the actual bit, so that when the decrement hits 0 we know we both
+ * need to resched (the bit is cleared) and can resched (no preempt count).
+ */
+
+static __always_inline void set_preempt_need_resched(void)
+{
+ *preempt_count_ptr() &= ~PREEMPT_NEED_RESCHED;
+}
+
+static __always_inline void clear_preempt_need_resched(void)
+{
+ *preempt_count_ptr() |= PREEMPT_NEED_RESCHED;
+}
+
+static __always_inline bool test_preempt_need_resched(void)
+{
+ return !(*preempt_count_ptr() & PREEMPT_NEED_RESCHED);
+}
+
#if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_PREEMPT_TRACER)
extern void add_preempt_count(int val);
extern void sub_preempt_count(int val);
@@ -42,7 +81,7 @@ asmlinkage void preempt_schedule(void);

#define preempt_check_resched() \
do { \
- if (unlikely(test_thread_flag(TIF_NEED_RESCHED))) \
+ if (unlikely(!*preempt_count_ptr())) \
preempt_schedule(); \
} while (0)

@@ -52,7 +91,7 @@ void preempt_schedule_context(void);

#define preempt_check_resched_context() \
do { \
- if (unlikely(test_thread_flag(TIF_NEED_RESCHED))) \
+ if (unlikely(!*preempt_count_ptr())) \
preempt_schedule_context(); \
} while (0)
#else
@@ -88,7 +127,6 @@ do { \
#define preempt_enable() \
do { \
preempt_enable_no_resched(); \
- barrier(); \
preempt_check_resched(); \
} while (0)

@@ -116,7 +154,6 @@ do { \
#define preempt_enable_notrace() \
do { \
preempt_enable_no_resched_notrace(); \
- barrier(); \
preempt_check_resched_context(); \
} while (0)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index e783ec5..9fa151f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -22,6 +22,7 @@ struct sched_param {
#include <linux/errno.h>
#include <linux/nodemask.h>
#include <linux/mm_types.h>
+#include <linux/preempt.h>

#include <asm/page.h>
#include <asm/ptrace.h>
@@ -434,7 +435,9 @@ struct task_cputime {
* We include PREEMPT_ACTIVE to avoid cond_resched() from working
* before the scheduler is active -- see should_resched().
*/
-#define INIT_PREEMPT_COUNT (1 + PREEMPT_ACTIVE)
+#define INIT_PREEMPT_COUNT (1 + PREEMPT_ACTIVE + PREEMPT_NEED_RESCHED)
+#define PREEMPT_ENABLED (PREEMPT_NEED_RESCHED)
+#define PREEMPT_DISABLED (1 + PREEMPT_NEED_RESCHED)

/**
* struct thread_group_cputimer - thread group interval timer counts
@@ -2408,7 +2411,7 @@ static inline int signal_pending_state(long state, struct task_struct *p)

static inline int need_resched(void)
{
- return unlikely(test_thread_flag(TIF_NEED_RESCHED));
+ return unlikely(test_preempt_need_resched());
}

/*
diff --git a/kernel/cpu/idle.c b/kernel/cpu/idle.c
index c261409..988573a 100644
--- a/kernel/cpu/idle.c
+++ b/kernel/cpu/idle.c
@@ -105,6 +105,13 @@ static void cpu_idle_loop(void)
__current_set_polling();
}
arch_cpu_idle_exit();
+ /*
+ * We need to test and propagate the TIF_NEED_RESCHED
+ * bit here because we might not have send the
+ * reschedule IPI to idle tasks.
+ */
+ if (tif_need_resched())
+ set_preempt_need_resched();
}
tick_nohz_idle_exit();
schedule_preempt_disabled();
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fe89afa..ee61f5a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -525,8 +525,10 @@ void resched_task(struct task_struct *p)
set_tsk_need_resched(p);

cpu = task_cpu(p);
- if (cpu == smp_processor_id())
+ if (cpu == smp_processor_id()) {
+ set_preempt_need_resched();
return;
+ }

/* NEED_RESCHED must be visible before we test polling */
smp_mb();
@@ -1391,6 +1393,14 @@ static void sched_ttwu_pending(void)

void scheduler_ipi(void)
{
+ /*
+ * Fold TIF_NEED_RESCHED into the preempt_count; anybody setting
+ * TIF_NEED_RESCHED remotely (for the first time) will also send
+ * this IPI.
+ */
+ if (tif_need_resched())
+ set_preempt_need_resched();
+
if (llist_empty(&this_rq()->wake_list)
&& !tick_nohz_full_cpu(smp_processor_id())
&& !got_nohz_idle_kick())
@@ -1714,7 +1724,7 @@ void sched_fork(struct task_struct *p)
#endif
#ifdef CONFIG_PREEMPT_COUNT
/* Want to start with kernel preemption disabled. */
- task_thread_info(p)->preempt_count = 1;
+ task_thread_info(p)->preempt_count = PREEMPT_DISABLED;
#endif
#ifdef CONFIG_SMP
plist_node_init(&p->pushable_tasks, MAX_PRIO);
@@ -2425,6 +2435,7 @@ need_resched:
put_prev_task(rq, prev);
next = pick_next_task(rq);
clear_tsk_need_resched(prev);
+ clear_preempt_need_resched();
rq->skip_clock_update = 0;

if (likely(prev != next)) {
@@ -2536,11 +2547,10 @@ EXPORT_SYMBOL(preempt_schedule);
*/
asmlinkage void __sched preempt_schedule_irq(void)
{
- struct thread_info *ti = current_thread_info();
enum ctx_state prev_state;

/* Catch callers which need to be fixed */
- BUG_ON(ti->preempt_count || !irqs_disabled());
+ BUG_ON(preempt_count() || !irqs_disabled());

prev_state = exception_enter();

@@ -4207,7 +4217,7 @@ void init_idle(struct task_struct *idle, int cpu)
raw_spin_unlock_irqrestore(&rq->lock, flags);

/* Set the preempt count _outside_ the spinlocks! */
- task_thread_info(idle)->preempt_count = 0;
+ task_thread_info(idle)->preempt_count = PREEMPT_ENABLED;

/*
* The idle tasks have their own, simple scheduling class:

\
 
 \ /
  Last update: 2013-09-25 19:01    [W:0.176 / U:0.032 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site