2.5: preemptive kernel on UP

During 2.5.5-pre, an optimization was made that removed schedule_tail from UP kernels. This causes the initial preempt_count of a new task, which starts at 1, to never decrement to zero and thus never become preemptible.

Thanks to everyone who pointed out the lousy performance - it took awhile to find but it should be fixed now. Please comment if not.

Patch is against 2.5.6-pre2 and is critical for all UP+preempt users.

Robert Love

diff -urN linux-2.5.6-pre2/arch/alpha/kernel/entry.S linux/arch/alpha/kernel/entry.S

arch/alpha/kernel/entry.S line 498

Quoted 9 lines

diff -urN linux-2.5.6-pre2/arch/alpha/kernel/entry.S linux/arch/alpha/kernel/entry.S

arch/alpha/kernel/entry.S +1 -1

@@ -495,7 +495,7 @@
 	ret	$31,($26),1
 .end alpha_switch_to
 
-#ifdef CONFIG_SMP
+#ifdef CONFIG_SMP || CONFIG_PREEMPT

Surely you really don't mean this?

Quoted 7 lines · Russell King

Quoted 7 lines

diff -urN linux-2.5.6-pre2/arch/alpha/kernel/entry.S linux/arch/alpha/kernel/entry.S

arch/alpha/kernel/entry.S +0 -0

@@ -495,7 +495,7 @@
 	ret	$31,($26),1
 .end alpha_switch_to
 

Quoted 4 lines

arch/alpha/kernel/entry.S line 498

Quoted 2 lines

arch/alpha/kernel/entry.S +1 -1

-#ifdef CONFIG_SMP
+#ifdef CONFIG_SMP || CONFIG_PREEMPT

Surely you really don't mean this?

Yes, why? We need to call schedule_tail ...

It's a #ifdef, not a #if

Better?

+#if CONFIG_SMP || CONFIG_PREEMPT

try

#if defined(a) || defined(b)

Interesting that the difference pre-empt makes is so large you didnt notice you hadn't re-enabled it ;)

Patch [...] is critical for all UP+preempt users.

The above fix isn't needed for i386

"Nothing would please me more than being able to hire ten programmers
and deluge the hobby market with good software." -- Bill Gates 1976

We are still waiting ....

During 2.5.5-pre, an optimization was made that removed schedule_tail from UP kernels. This causes the initial preempt_count of a new task, which starts at 1, to never decrement to zero and thus never become preemptible.

Thanks to everyone who pointed out the lousy performance - it took awhile to find but it should be fixed now. Please comment if not.

Patch is against 2.5.6-pre2 and is critical for all UP+preempt users.

Robert Love

diff -urN linux-2.5.6-pre2/arch/alpha/kernel/entry.S linux/arch/alpha/kernel/entry.S

arch/alpha/kernel/entry.S Mon Mar 4 17:49:27 2002 renamed from arch/alpha/kernel/entry.S Fri Mar 1 17:21:14 2002 +1 -1

@@ -495,7 +495,7 @@
        ret     $31,($26),1
 .end alpha_switch_to
 
-#ifdef CONFIG_SMP
+#ifdef CONFIG_SMP || CONFIG_PREEMPT
 .globl  ret_from_fork
 .align 3
 .ent ret_from_fork

diff -urN linux-2.5.6-pre2/arch/i386/kernel/entry.S linux/arch/i386/kernel/entry.S

arch/i386/kernel/entry.S Mon Mar 4 17:48:32 2002 renamed from arch/i386/kernel/entry.S Tue Feb 19 21:10:58 2002 +1 -1

@@ -195,7 +195,7 @@
 
 
 ENTRY(ret_from_fork)
-#if CONFIG_SMP
+#if CONFIG_SMP || CONFIG_PREEMPT
        call SYMBOL_NAME(schedule_tail)
 #endif
        GET_THREAD_INFO(%ebx)

diff -urN linux-2.5.6-pre2/arch/ppc/kernel/entry.S linux/arch/ppc/kernel/entry.S

arch/ppc/kernel/entry.S Mon Mar 4 17:48:41 2002 renamed from arch/ppc/kernel/entry.S Tue Feb 19 21:10:59 2002 +1 -1

@@ -343,7 +343,7 @@
 
        .globl  ret_from_fork
 ret_from_fork:
-#ifdef CONFIG_SMP
+#ifdef CONFIG_SMP || CONFIG_PREEMPT
        bl      schedule_tail
 #endif
        rlwinm  r3,r1,0,0,18

diff -urN linux-2.5.6-pre2/arch/ppc64/kernel/entry.S linux/arch/ppc64/kernel/entry.S

arch/ppc64/kernel/entry.S Mon Mar 4 17:52:16 2002 renamed from arch/ppc64/kernel/entry.S Tue Feb 19 21:10:53 2002 +1 -1

@@ -311,7 +311,7 @@
        blr
 
 _GLOBAL(ret_from_fork)
-#ifdef CONFIG_SMP
+#ifdef CONFIG_SMP || CONFIG_PREEMPT
        bl      .schedule_tail
 #endif
        clrrdi  r4,r1,THREAD_SHIFT

diff -urN linux-2.5.6-pre2/arch/s390/kernel/entry.S linux/arch/s390/kernel/entry.S

arch/s390/kernel/entry.S Mon Mar 4 17:48:12 2002 renamed from arch/s390/kernel/entry.S Tue Feb 19 21:10:57 2002 +2 -2

@@ -295,7 +295,7 @@
         stosm   24(%r15),0x03     # reenable interrupts
         sr      %r0,%r0           # child returns 0
         st      %r0,SP_R2(%r15)   # store return value (change R2 on stack)
-#ifdef CONFIG_SMP
+#ifdef CONFIG_SMP || CONFIG_PREEMPT
         l       %r1,BASED(.Lschedtail)
        la      %r14,BASED(sysc_return)
         br      %r1               # call schedule_tail, return to sysc_return

@@ -896,7 +896,7 @@
 #error .Ltrace:       .long  syscall_trace
 .Lvfork:       .long  sys_vfork
 
-#ifdef CONFIG_SMP
+#ifdef CONFIG_SMP || CONFIG_PREEMPT
 .Lschedtail:   .long  schedule_tail
 #endif
 

diff -urN linux-2.5.6-pre2/arch/s390x/kernel/entry.S linux/arch/s390x/kernel/entry.S

arch/s390x/kernel/entry.S Mon Mar 4 17:53:31 2002 renamed from arch/s390x/kernel/entry.S Tue Feb 19 21:10:58 2002 +1 -1

@@ -280,7 +280,7 @@
         GET_CURRENT               # load pointer to task_struct to R9
         stosm   48(%r15),0x03     # reenable interrupts
        xc      SP_R2(8,%r15),SP_R2(%r15) # child returns 0
-#ifdef CONFIG_SMP
+#ifdef CONFIG_SMP || CONFIG_PREEMPT
        larl    %r14,sysc_return
         jg      schedule_tail     # return to sysc_return
 #else

diff -urN linux-2.5.6-pre2/kernel/sched.c linux/kernel/sched.c

kernel/sched.c Mon Mar 4 17:54:00 2002 renamed from kernel/sched.c Fri Mar 1 17:21:16 2002 +1 -2

@@ -397,7 +397,7 @@
                        p->sleep_avg) / (EXIT_WEIGHT + 1);
 }
 
-#if CONFIG_SMP
+#if CONFIG_SMP || CONFIG_PREEMPT
 asmlinkage void schedule_tail(void)
 {
        spin_unlock_irq(&this_rq()->lock);
 
-

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With out a lot of looking, wouldn't it be easier to just change fork?