Prompted by a question from Mike I updated the Changelog to explain why
we need to keep TIF_NEED_RESCHED.
---
Subject: sched: Add NEED_RESCHED to the preempt_count
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Wed Aug 14 14:55:31 CEST 2013
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@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
include/linux/preempt.h | 42 +++++++++++++++++++++++++++++++++++++-----
include/linux/sched.h | 2 +-
include/linux/thread_info.h | 1 +
kernel/context_tracking.c | 2 +-
kernel/cpu/idle.c | 10 ++++++++++
kernel/sched/core.c | 18 ++++++++++++++----
6 files changed, 64 insertions(+), 11 deletions(-)
--- 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/hardirq.h> for
+ * the other bits.
+ */
+#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)
@@ -20,6 +30,30 @@ static __always_inline int *preempt_coun
return ¤t_thread_info()->preempt_count;
}
+/*
+ * 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);
@@ -37,7 +71,7 @@ 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)
@@ -47,7 +81,7 @@ 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
@@ -83,7 +117,6 @@ do { \
#define preempt_enable() \
do { \
preempt_enable_no_resched(); \
- barrier(); \
preempt_check_resched(); \
} while (0)
@@ -111,7 +144,6 @@ do { \
#define preempt_enable_notrace() \
do { \
preempt_enable_no_resched_notrace(); \
- barrier(); \
preempt_check_resched_context(); \
} while (0)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2405,7 +2405,7 @@ static inline int signal_pending_state(l
static inline int need_resched(void)
{
- return unlikely(test_thread_flag(TIF_NEED_RESCHED));
+ return unlikely(test_preempt_need_resched());
}
/*
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -104,6 +104,7 @@ static inline int test_ti_thread_flag(st
#define test_thread_flag(flag) \
test_ti_thread_flag(current_thread_info(), flag)
+#define test_need_resched() test_thread_flag(TIF_NEED_RESCHED)
#define set_need_resched() set_thread_flag(TIF_NEED_RESCHED)
#define clear_need_resched() clear_thread_flag(TIF_NEED_RESCHED)
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -115,7 +115,7 @@ void __sched notrace preempt_schedule_co
{
enum ctx_state prev_ctx;
- if (likely(!preemptible()))
+ if (likely(preempt_count() || irqs_disabled()))
return;
/*
--- a/kernel/cpu/idle.c
+++ b/kernel/cpu/idle.c
@@ -106,6 +106,13 @@ 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 (test_need_resched())
+ set_preempt_need_resched();
}
tick_nohz_idle_exit();
schedule_preempt_disabled();
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -526,8 +526,10 @@ 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();
@@ -1411,6 +1413,14 @@ 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 (test_need_resched())
+ set_preempt_need_resched();
+
if (llist_empty(&this_rq()->wake_list)
&& !tick_nohz_full_cpu(smp_processor_id())
&& !got_nohz_idle_kick())
@@ -2445,6 +2455,7 @@ static void __sched __schedule(void)
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)) {
@@ -2531,7 +2542,7 @@ asmlinkage void __sched notrace preempt_
* If there is a non-zero preempt_count or interrupts are disabled,
* we do not want to preempt the current task. Just return..
*/
- if (likely(!preemptible()))
+ if (likely(preempt_count() || irqs_disabled()))
return;
do {
@@ -2556,11 +2567,10 @@ 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();
--
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