On Thu, 2002-04-04 at 18:55, Linus Torvalds wrote:
> Fair enough. Send me a patch to look at.
Linus et all,
Here we go:
- do not manually set task->state
- instead, in preempt_schedule, set a flag in preempt_count that
denotes that this task is entering schedule off a kernel preemption.
- use this flag in schedule to jump to pick_next_task
- in preempt_schedule, upon return from schedule, unset the flag
- have entry.S just call preempt_schedule and not duplicate this work,
as Linus suggested. I agree. Note this makes debugging easier as
we keep a single point of entry for kernel preemptions.
The result: we can safely preempt non-TASK_RUNNING tasks. If one is
preempted, we can safely survive schedule because we won't handle the
special casing of non-TASK_RUNNING at the top of schedule. Thus other
tasks can run as desired and our non-TASK_RUNNING task will eventually
be rescheduled, in its original state, and complete happily.
This is the behavior we have in the 2.4 patches and 2.5 until
~2.5.6-pre. This works. It requires no other changes elsewhere (it
actually removes some special-casing Ingo did in the signal code).
This patch works in theory and compiles, but received minimal testing.
Robert Love
diff -urN linux-2.5.8-pre1/arch/i386/kernel/entry.S linux/arch/i386/kernel/entry.S
--- linux-2.5.8-pre1/arch/i386/kernel/entry.S Wed Apr 3 20:57:24 2002
+++ linux/arch/i386/kernel/entry.S Thu Apr 4 18:32:03 2002
@@ -240,9 +240,7 @@
jnz restore_all
incl TI_PRE_COUNT(%ebx)
sti
- movl TI_TASK(%ebx), %ecx # ti->task
- movl $0,(%ecx) # current->state = TASK_RUNNING
- call SYMBOL_NAME(schedule)
+ call SYMBOL_NAME(preempt_schedule)
jmp ret_from_intr
#endif
diff -urN linux-2.5.8-pre1/arch/i386/kernel/ptrace.c linux/arch/i386/kernel/ptrace.c
--- linux-2.5.8-pre1/arch/i386/kernel/ptrace.c Wed Apr 3 20:57:24 2002
+++ linux/arch/i386/kernel/ptrace.c Thu Apr 4 18:31:17 2002
@@ -455,11 +455,9 @@
between a syscall stop and SIGTRAP delivery */
current->exit_code = SIGTRAP | ((current->ptrace & PT_TRACESYSGOOD)
? 0x80 : 0);
- preempt_disable();
current->state = TASK_STOPPED;
notify_parent(current, SIGCHLD);
schedule();
- preempt_enable();
/*
* this isn't the same as continuing with a signal, but it will do
* for normal use. strace only continues with a signal if the
diff -urN linux-2.5.8-pre1/arch/i386/kernel/signal.c linux/arch/i386/kernel/signal.c
--- linux-2.5.8-pre1/arch/i386/kernel/signal.c Wed Apr 3 20:57:24 2002
+++ linux/arch/i386/kernel/signal.c Thu Apr 4 18:31:00 2002
@@ -610,11 +610,9 @@
if ((current->ptrace & PT_PTRACED) && signr != SIGKILL) {
/* Let the debugger run. */
current->exit_code = signr;
- preempt_disable();
current->state = TASK_STOPPED;
notify_parent(current, SIGCHLD);
schedule();
- preempt_enable();
/* We're back. Did the debugger cancel the sig? */
if (!(signr = current->exit_code))
@@ -669,14 +667,12 @@
case SIGSTOP: {
struct signal_struct *sig;
+ current->state = TASK_STOPPED;
current->exit_code = signr;
sig = current->parent->sig;
- preempt_disable();
- current->state = TASK_STOPPED;
if (sig && !(sig->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDSTOP))
notify_parent(current, SIGCHLD);
schedule();
- preempt_enable();
continue;
}
diff -urN linux-2.5.8-pre1/include/linux/sched.h linux/include/linux/sched.h
--- linux-2.5.8-pre1/include/linux/sched.h Wed Apr 3 20:57:27 2002
+++ linux/include/linux/sched.h Thu Apr 4 18:29:53 2002
@@ -91,6 +91,7 @@
#define TASK_UNINTERRUPTIBLE 2
#define TASK_ZOMBIE 4
#define TASK_STOPPED 8
+#define PREEMPT_ACTIVE 0x4000000
#define __set_task_state(tsk, state_value) \
do { (tsk)->state = (state_value); } while (0)
diff -urN linux-2.5.8-pre1/kernel/sched.c linux/kernel/sched.c
--- linux-2.5.8-pre1/kernel/sched.c Wed Apr 3 20:57:37 2002
+++ linux/kernel/sched.c Thu Apr 4 18:29:24 2002
@@ -764,6 +764,13 @@
prev->sleep_timestamp = jiffies;
spin_lock_irq(&rq->lock);
+ /*
+ * if entering from preempt_schedule, off a kernel preemption,
+ * go straight to picking the next task.
+ */
+ if (unlikely(preempt_get_count() & PREEMPT_ACTIVE))
+ goto pick_next_task;
+
switch (prev->state) {
case TASK_INTERRUPTIBLE:
if (unlikely(signal_pending(prev))) {
@@ -775,7 +782,7 @@
case TASK_RUNNING:
;
}
-#if CONFIG_SMP
+#if CONFIG_SMP || CONFIG_PREEMPT
pick_next_task:
#endif
if (unlikely(!rq->nr_running)) {
@@ -843,8 +850,11 @@
{
if (unlikely(preempt_get_count()))
return;
- current->state = TASK_RUNNING;
+
+ current_thread_info()->preempt_count += PREEMPT_ACTIVE;
schedule();
+ current_thread_info()->preempt_count -= PREEMPT_ACTIVE;
+ barrier();
}
#endif /* CONFIG_PREEMPT */
-
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