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SubjectRe: [PATCH] preemptive kernel behavior change: don't be rude
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Linus and company,

Attached find a final version of the new behavior patch. It meets
everyone's concerns and it compiles, boots, and survived a basic stress
test here.

The next step would be to push the preempt_count check back into
preempt_schedule and duplicate preempt_schedule in entry.S and then have
it call schedule directly. This would be both a space and time
optimization. I will do this, but I'd like to give this as it is some
testing first.

Linus, if this is how you want it, please apply. Patch is against
2.5.8-pre1.

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 20:44:40 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 20:44:40 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 20:44:40 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 21:56:11 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/include/linux/spinlock.h linux/include/linux/spinlock.h
--- linux-2.5.8-pre1/include/linux/spinlock.h Wed Apr 3 20:57:34 2002
+++ linux/include/linux/spinlock.h Thu Apr 4 21:56:11 2002
@@ -177,8 +177,9 @@
do { \
--current_thread_info()->preempt_count; \
barrier(); \
- if (unlikely(test_thread_flag(TIF_NEED_RESCHED))) \
- preempt_schedule(); \
+ if (unlikely(!(current_thread_info()->preempt_count) && \
+ test_thread_flag(TIF_NEED_RESCHED))) \
+ preempt_schedule(); \
} while (0)

#define spin_lock(lock) \
diff -urN linux-2.5.8-pre1/kernel/exit.c linux/kernel/exit.c
--- linux-2.5.8-pre1/kernel/exit.c Wed Apr 3 20:57:37 2002
+++ linux/kernel/exit.c Thu Apr 4 21:55:29 2002
@@ -499,6 +495,7 @@
acct_process(code);
__exit_mm(tsk);

+ preempt_disable();
sem_exit();
__exit_files(tsk);
__exit_fs(tsk);
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 20:54:17 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))) {
@@ -775,9 +782,7 @@
case TASK_RUNNING:
;
}
-#if CONFIG_SMP
pick_next_task:
-#endif
if (unlikely(!rq->nr_running)) {
#if CONFIG_SMP
load_balance(rq, 1);
@@ -841,10 +846,12 @@
*/
asmlinkage void preempt_schedule(void)
{
- if (unlikely(preempt_get_count()))
- return;
- current->state = TASK_RUNNING;
- schedule();
+ do {
+ current_thread_info()->preempt_count += PREEMPT_ACTIVE;
+ schedule();
+ current_thread_info()->preempt_count -= PREEMPT_ACTIVE;
+ barrier();
+ } while(test_thread_flag(TIF_NEED_RESCHED));
}
#endif /* CONFIG_PREEMPT */

-
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