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From: Rusty Russell <rusty@rustcorp.com.au>
To: Richard Henderson <rth@twiddle.net>
Cc: Andrew Morton <akpm@zip.com.au>, linux-kernel@vger.kernel.org,
        torvalds@transmeta.com, kuznet@ms2.inr.ac.ru
Subject: Re: [PATCH] per-cpu areas for 2.5.3-pre6 
In-Reply-To: Your message of "Wed, 30 Jan 2002 21:49:35 -0800."
             <20020130214935.A7479@are.twiddle.net> 
Date: Fri, 01 Feb 2002 19:56:24 +1100
Message-Id: <E16WZUe-0001Xw-00@wagner.rustcorp.com.au>
Sender: linux-kernel-owner@vger.kernel.org
X-Mailing-List: linux-kernel@vger.kernel.org

In message <20020130214935.A7479@are.twiddle.net> you write:
> On Thu, Jan 31, 2002 at 09:45:45AM +1100, Rusty Russell wrote:
> > "better".  Believe me, I was fully aware, but I refuse to write such
> > crap unless *proven* to be required.
> 
> You're going to wait until the compiler generates incorrect
> code for you after knowing that it *probably* will?  Nice.

It's a judgement call: the obvious code "might" fail, the obfuscated
code "might" fail.  However, your judgment >> mine.

> "Small" variables may be positioned by the compiler such that
> they are addressable via a 16-bit relocation from some GP register.
> If that variable isn't actually located in the small data area,
> then the 16-bit relocation may overflow, resulting in link errors.

Ahhh... Thanks for the clue injection.

This better?
Rusty.
--
  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.

diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.3/arch/i386/vmlinux.lds working-2.5.3-percpu/arch/i386/vmlinux.lds
--- linux-2.5.3/arch/i386/vmlinux.lds	Thu Jan 31 08:58:52 2002
+++ working-2.5.3-percpu/arch/i386/vmlinux.lds	Fri Feb  1 09:11:18 2002
@@ -58,6 +58,10 @@
 	*(.initcall7.init)
   }
   __initcall_end = .;
+  . = ALIGN(32);
+  __per_cpu_start = .;
+  .data.percpu  : { *(.data.percpu) }
+  __per_cpu_end = .;
   . = ALIGN(4096);
   __init_end = .;
 
diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.3/arch/ppc/vmlinux.lds working-2.5.3-percpu/arch/ppc/vmlinux.lds
--- linux-2.5.3/arch/ppc/vmlinux.lds	Thu Jan 31 08:58:55 2002
+++ working-2.5.3-percpu/arch/ppc/vmlinux.lds	Fri Feb  1 09:11:18 2002
@@ -104,6 +104,10 @@
 	*(.initcall7.init)
   }
   __initcall_end = .;
+  . = ALIGN(32);
+  __per_cpu_start = .;
+  .data.percpu  : { *(.data.percpu) }
+  __per_cpu_end = .;
   . = ALIGN(4096);
   __init_end = .;
 
diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.3/include/linux/cache.h working-2.5.3-percpu/include/linux/cache.h
--- linux-2.5.3/include/linux/cache.h	Fri Feb  1 19:21:37 2002
+++ working-2.5.3-percpu/include/linux/cache.h	Fri Feb  1 19:17:09 2002
@@ -4,8 +4,10 @@
 #include <linux/config.h>
 #include <asm/cache.h>
 
+#define ALIGN(x,a) (((x)+(a)-1)&~((a)-1))
+
 #ifndef L1_CACHE_ALIGN
-#define L1_CACHE_ALIGN(x) (((x)+(L1_CACHE_BYTES-1))&~(L1_CACHE_BYTES-1))
+#define L1_CACHE_ALIGN(x) ALIGN(x, L1_CACHE_BYTES)
 #endif
 
 #ifndef SMP_CACHE_BYTES
diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.3/include/linux/compiler.h working-2.5.3-percpu/include/linux/compiler.h
--- linux-2.5.3/include/linux/compiler.h	Wed Sep 19 07:12:45 2001
+++ working-2.5.3-percpu/include/linux/compiler.h	Fri Feb  1 09:46:02 2002
@@ -13,4 +13,12 @@
 #define likely(x)	__builtin_expect((x),1)
 #define unlikely(x)	__builtin_expect((x),0)
 
+/* This macro obfuscates arithmetic on a variable address so that gcc
+   shouldn't recognize it. This prevents optimizations such as the
+   famous:
+	strcpy(s, "xxx"+X) => memcpy(s, "xxx"+X, 4-X) */
+#define rth(var, off)						\
+  ({ __typeof__(&(var)) __ptr;					\
+    __asm__ ("" : "=g"(__ptr) : "0"((void *)&(var) + (off)));	\
+    *__ptr; })
 #endif /* __LINUX_COMPILER_H */
diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.3/include/linux/interrupt.h working-2.5.3-percpu/include/linux/interrupt.h
--- linux-2.5.3/include/linux/interrupt.h	Fri Feb  1 19:21:37 2002
+++ working-2.5.3-percpu/include/linux/interrupt.h	Fri Feb  1 19:27:57 2002
@@ -124,14 +124,6 @@
 	TASKLET_STATE_RUN	/* Tasklet is running (SMP only) */
 };
 
-struct tasklet_head
-{
-	struct tasklet_struct *list;
-} __attribute__ ((__aligned__(SMP_CACHE_BYTES)));
-
-extern struct tasklet_head tasklet_vec[NR_CPUS];
-extern struct tasklet_head tasklet_hi_vec[NR_CPUS];
-
 #ifdef CONFIG_SMP
 static inline int tasklet_trylock(struct tasklet_struct *t)
 {
diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.3/include/linux/smp.h working-2.5.3-percpu/include/linux/smp.h
--- linux-2.5.3/include/linux/smp.h	Fri Feb  1 19:21:37 2002
+++ working-2.5.3-percpu/include/linux/smp.h	Fri Feb  1 19:17:09 2002
@@ -11,6 +11,7 @@
 #ifdef CONFIG_SMP
 
 #include <linux/kernel.h>
+#include <linux/compiler.h>
 #include <asm/smp.h>
 
 /*
@@ -71,7 +72,17 @@
 #define MSG_RESCHEDULE		0x0003	/* Reschedule request from master CPU*/
 #define MSG_CALL_FUNCTION       0x0004  /* Call function on all other CPUs */
 
-#else
+#define __per_cpu_data	__attribute__((section(".data.percpu")))
+
+#ifndef __HAVE_ARCH_PER_CPU
+extern unsigned long __per_cpu_offset[NR_CPUS];
+
+/* var is in discarded region: offset to particular copy we want */
+#define per_cpu(var, cpu) rth(var, per_cpu_offset(cpu))
+
+#define this_cpu(var) per_cpu(var, smp_processor_id())
+#endif /* !__HAVE_ARCH_PER_CPU */
+#else /* !SMP */
 
 /*
  *	These macros fold the SMP functionality into a single CPU system
@@ -88,6 +99,9 @@
 #define cpu_online_map				1
 static inline void smp_send_reschedule(int cpu) { }
 static inline void smp_send_reschedule_all(void) { }
+#define __per_cpu_data
+#define per_cpu(var, cpu)			var
+#define this_cpu(var)				var
 
 #endif
 #endif
diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.3/init/main.c working-2.5.3-percpu/init/main.c
--- linux-2.5.3/init/main.c	Thu Jan 31 08:59:17 2002
+++ working-2.5.3-percpu/init/main.c	Fri Feb  1 19:13:52 2002
@@ -272,8 +272,32 @@
 #define smp_init()	do { } while (0)
 #endif
 
+static inline void setup_per_cpu_areas(void)
+{
+}
 #else
 
+#ifndef __HAVE_ARCH_PER_CPU
+unsigned long __per_cpu_offset[NR_CPUS];
+
+static void __init setup_per_cpu_areas(void)
+{
+	unsigned long size, i;
+	char *ptr;
+	/* Created by linker magic */
+	extern char __per_cpu_start[], __per_cpu_end[];
+
+	/* Copy section for each CPU (we discard the original) */
+	size = ALIGN(__per_cpu_end - __per_cpu_start, SMP_CACHE_BYTES);
+	ptr = alloc_bootmem(size * NR_CPUS);
+
+	for (i = 0; i < NR_CPUS; i++, ptr += size) {
+		__per_cpu_offset[i] = ptr - __per_cpu_start;
+		memcpy(ptr, __per_cpu_start, size);
+	}
+}
+#endif /* !__HAVE_ARCH_PER_CPU */
+
 /* Called by boot processor to activate the rest. */
 static void __init smp_init(void)
 {
@@ -316,6 +340,7 @@
 	lock_kernel();
 	printk(linux_banner);
 	setup_arch(&command_line);
+	setup_per_cpu_areas();
 	printk("Kernel command line: %s\n", saved_command_line);
 	parse_options(command_line);
 	trap_init();
diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.3/kernel/ksyms.c working-2.5.3-percpu/kernel/ksyms.c
--- linux-2.5.3/kernel/ksyms.c	Thu Jan 31 08:59:17 2002
+++ working-2.5.3-percpu/kernel/ksyms.c	Fri Feb  1 09:11:18 2002
@@ -543,8 +543,6 @@
 EXPORT_SYMBOL(strsep);
 
 /* software interrupts */
-EXPORT_SYMBOL(tasklet_hi_vec);
-EXPORT_SYMBOL(tasklet_vec);
 EXPORT_SYMBOL(bh_task_vec);
 EXPORT_SYMBOL(init_bh);
 EXPORT_SYMBOL(remove_bh);
diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.3/kernel/softirq.c working-2.5.3-percpu/kernel/softirq.c
--- linux-2.5.3/kernel/softirq.c	Thu Jan 31 08:59:17 2002
+++ working-2.5.3-percpu/kernel/softirq.c	Fri Feb  1 09:11:18 2002
@@ -145,9 +145,13 @@
 
 
 /* Tasklets */
+struct tasklet_head
+{
+	struct tasklet_struct *list;
+};
 
-struct tasklet_head tasklet_vec[NR_CPUS] __cacheline_aligned_in_smp;
-struct tasklet_head tasklet_hi_vec[NR_CPUS] __cacheline_aligned_in_smp;
+static struct tasklet_head tasklet_vec __per_cpu_data;
+static struct tasklet_head tasklet_hi_vec __per_cpu_data;
 
 void __tasklet_schedule(struct tasklet_struct *t)
 {
@@ -155,8 +159,8 @@
 	unsigned long flags;
 
 	local_irq_save(flags);
-	t->next = tasklet_vec[cpu].list;
-	tasklet_vec[cpu].list = t;
+	t->next = per_cpu(tasklet_vec, cpu).list;
+	per_cpu(tasklet_vec, cpu).list = t;
 	cpu_raise_softirq(cpu, TASKLET_SOFTIRQ);
 	local_irq_restore(flags);
 }
@@ -167,8 +171,8 @@
 	unsigned long flags;
 
 	local_irq_save(flags);
-	t->next = tasklet_hi_vec[cpu].list;
-	tasklet_hi_vec[cpu].list = t;
+	t->next = per_cpu(tasklet_hi_vec, cpu).list;
+	per_cpu(tasklet_hi_vec, cpu).list = t;
 	cpu_raise_softirq(cpu, HI_SOFTIRQ);
 	local_irq_restore(flags);
 }
@@ -179,8 +183,8 @@
 	struct tasklet_struct *list;
 
 	local_irq_disable();
-	list = tasklet_vec[cpu].list;
-	tasklet_vec[cpu].list = NULL;
+	list = per_cpu(tasklet_vec, cpu).list;
+	per_cpu(tasklet_vec, cpu).list = NULL;
 	local_irq_enable();
 
 	while (list) {
@@ -200,8 +204,8 @@
 		}
 
 		local_irq_disable();
-		t->next = tasklet_vec[cpu].list;
-		tasklet_vec[cpu].list = t;
+		t->next = per_cpu(tasklet_vec, cpu).list;
+		per_cpu(tasklet_vec, cpu).list = t;
 		__cpu_raise_softirq(cpu, TASKLET_SOFTIRQ);
 		local_irq_enable();
 	}
@@ -213,8 +217,8 @@
 	struct tasklet_struct *list;
 
 	local_irq_disable();
-	list = tasklet_hi_vec[cpu].list;
-	tasklet_hi_vec[cpu].list = NULL;
+	list = per_cpu(tasklet_hi_vec, cpu).list;
+	per_cpu(tasklet_hi_vec, cpu).list = NULL;
 	local_irq_enable();
 
 	while (list) {
@@ -234,8 +238,8 @@
 		}
 
 		local_irq_disable();
-		t->next = tasklet_hi_vec[cpu].list;
-		tasklet_hi_vec[cpu].list = t;
+		t->next = per_cpu(tasklet_hi_vec, cpu).list;
+		per_cpu(tasklet_hi_vec, cpu).list = t;
 		__cpu_raise_softirq(cpu, HI_SOFTIRQ);
 		local_irq_enable();
 	}
-
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