Re: [RFC, patch] i386: vgetcpu() for NUMA, take 2

Chuck Ebbert wrote:

This is attempt #2 at vgetcpu() NUMA support for i386.  It uses a
GDT entry to hold cpu and node number for fast userspace access.

changes since #1:
   proper function prototype (same as x86_64)
   changed alignment of vsyscall functions to 16 bytes
        (sigreturn needs to stay fixed, others can move)

to-do:
   CFI annotations
   test NUMA on real NUMA hardware (someone please test)


What was the point of returning wrong info to userspace really quickly?
;-) (ie you could have migrated CPUs, so it's totally unreliable)

Is this just for some statistical monitoring thing?

M.

Test program:

/* vgetcpu.c: test how fast vgetcpu runs
 * boot kernel with vgetcpu patch first, then:
 *  gcc -O3 -o vgetcpu vgetcpu.c <srcpath>/arch/i386/kernel/vsyscall-int80.so
 * (don't forget the optimization (-O3))
 */
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>

extern int __attribute__ ((regparm(2))) __vgetcpu(int *cpu, int *node);

#define rdtscll(t)	asm("rdtsc" : "=A" (t))

int main(int argc, char * const argv[])
{
	long long tsc1, tsc2;
	int i, cpu = 999, node = 999, iters = 99999;
	
	if (__vgetcpu(&cpu, &node) || node == 999 || cpu == 999) {
		printf("vgetcpu failed!\n");
		_exit(1);
	}
	printf("node: %d, cpu: %d\n", node, cpu);

	rdtscll(tsc1);
	for (i = 0; i < iters; i++)
		__vgetcpu(&cpu, &node);
	rdtscll(tsc2);

	printf("vgetcpu took %llu clocks per call\n", (tsc2 - tsc1) / iters);

	return 0;
}


Signed-off-by: Chuck Ebbert <[email protected]>

 arch/i386/kernel/cpu/common.c         |    3 ++
 arch/i386/kernel/head.S               |   11 +++++++-
 arch/i386/kernel/smpboot.c            |    2 +
 arch/i386/kernel/vsyscall-getcpu.S    |   42 ++++++++++++++++++++++++++++++++++
 arch/i386/kernel/vsyscall-int80.S     |    2 +
 arch/i386/kernel/vsyscall-sigreturn.S |    3 --
 arch/i386/kernel/vsyscall-sysenter.S  |    2 +

arch/i386/kernel/vsyscall.lds.S | 1include/asm-i386/segment.h | 4 ++-

 9 files changed, 65 insertions(+), 5 deletions(-)

--- 2.6.17-32.orig/arch/i386/kernel/cpu/common.c
+++ 2.6.17-32/arch/i386/kernel/cpu/common.c
@@ -642,6 +642,9 @@ void __cpuinit cpu_init(void)
 		((((__u64)stk16_off) << 32) & 0xff00000000000000ULL) |
 		(CPU_16BIT_STACK_SIZE - 1);

+ /* Set up GDT entry for per-cpu data */

+ 	gdt[GDT_ENTRY_VGETCPU].a |= cpu & 0xff;
+
 	cpu_gdt_descr->size = GDT_SIZE - 1;
  	cpu_gdt_descr->address = (unsigned long)gdt;

--- 2.6.17-32.orig/arch/i386/kernel/head.S

+++ 2.6.17-32/arch/i386/kernel/head.S
@@ -480,7 +480,7 @@ ENTRY(boot_gdt_table)
 	.quad 0x00cf92000000ffff	/* kernel 4GB data at 0x00000000 */

/*

- * The Global Descriptor Table contains 28 quadwords, per-CPU.
+ * The Global Descriptor Table contains 32 quadwords, per-CPU.
  */
 	.align L1_CACHE_BYTES
 ENTRY(cpu_gdt_table)
@@ -525,7 +525,14 @@ ENTRY(cpu_gdt_table)
 	.quad 0x004092000000ffff	/* 0xc8 APM DS    data */

.quad 0x0000920000000000 /* 0xd0 - ESPFIX 16-bit SS */

-	.quad 0x0000000000000000	/* 0xd8 - unused */
+
+	/*
+	 * Use GDT entries to store per-cpu data for user space (DPL 3.)
+	 * 32-bit data segment, byte granularity, base 0, limit set at runtime.
+	 * Userspace will use LSL to access this data, stored in the limit field.
+	 */
+	.quad 0x0040f20000000000	/* 0xd8 - nodeid and logical CPU number */
+
 	.quad 0x0000000000000000	/* 0xe0 - unused */
 	.quad 0x0000000000000000	/* 0xe8 - unused */
 	.quad 0x0000000000000000	/* 0xf0 - unused */
--- /dev/null
+++ 2.6.17-32/arch/i386/kernel/vsyscall-getcpu.S
@@ -0,0 +1,42 @@
+/*
+ * fastcall int __vgetcpu(int *cpu, int *node)
+ *
+ * This file is #include'd by vsyscall-*.S to place vgetcpu after the
+ * sigreturn code.
+ *
+ * Puts logical CPU number in *cpu, node ID in *node;
+ * returns 0 for success and -EFAULT on error.
+ *
+ * CPU number and node ID are 8 bits each, with 4 total bits available
+ * for future growth of either field.
+ */
+
+#include <linux/errno.h>
+#include <asm/segment.h>
+
+	.text
+	.balign 16
+	.globl __vgetcpu
+	.type __vgetcpu,@function
+__vgetcpu:
+.LSTART_vgetcpu:
+	mov $((GDT_ENTRY_VGETCPU<<3)|3),%cx
+	lsl %ecx,%ecx
+	jnz 1f
+	push %ecx
+	and $0xff,%ecx		/* 8-bit cpu number */
+	mov %ecx,(%eax)
+	pop %ecx
+	xor %eax,%eax
+	shr $8,%ecx		/* assume top 4 bits are zero */
+	mov %ecx,(%edx)
+	ret
+1:
+	push $-EFAULT		/* saves 2 bytes of .text */
+	pop %eax
+	ret
+.LEND_vgetcpu:
+	.size __vgetcpu,.-.LSTART_vgetcpu
+	.previous
+
+/* ZZZ: need CFI annotations here */
--- 2.6.17-32.orig/arch/i386/kernel/vsyscall-int80.S
+++ 2.6.17-32/arch/i386/kernel/vsyscall-int80.S
@@ -51,3 +51,5 @@ __kernel_vsyscall:
  * Get the common code for the sigreturn entry points.
  */
 #include "vsyscall-sigreturn.S"
+
+#include "vsyscall-getcpu.S"
--- 2.6.17-32.orig/arch/i386/kernel/vsyscall-sysenter.S
+++ 2.6.17-32/arch/i386/kernel/vsyscall-sysenter.S
@@ -120,3 +120,5 @@ SYSENTER_RETURN:
  * Get the common code for the sigreturn entry points.
  */
 #include "vsyscall-sigreturn.S"
+
+#include "vsyscall-getcpu.S"
--- 2.6.17-32.orig/arch/i386/kernel/vsyscall.lds.S
+++ 2.6.17-32/arch/i386/kernel/vsyscall.lds.S
@@ -57,6 +57,7 @@ VERSION
     	__kernel_vsyscall;
     	__kernel_sigreturn;
     	__kernel_rt_sigreturn;
+	__vgetcpu;

local: *;

   };
--- 2.6.17-32.orig/arch/i386/kernel/smpboot.c
+++ 2.6.17-32/arch/i386/kernel/smpboot.c
@@ -615,6 +615,7 @@ static inline void map_cpu_to_node(int c
 	printk("Mapping cpu %d to node %d\n", cpu, node);
 	cpu_set(cpu, node_2_cpu_mask[node]);
 	cpu_2_node[cpu] = node;
+ 	get_cpu_gdt_table(cpu)[GDT_ENTRY_VGETCPU].a |= (node & 0xff) << 8;
 }

/* undo a mapping between cpu and node. */

@@ -626,6 +627,7 @@ static inline void unmap_cpu_to_node(int
 	for (node = 0; node < MAX_NUMNODES; node ++)
 		cpu_clear(cpu, node_2_cpu_mask[node]);
 	cpu_2_node[cpu] = 0;
+ 	get_cpu_gdt_table(cpu)[GDT_ENTRY_VGETCPU].a &= ~(0xff << 8);
 }
 #else /* !CONFIG_NUMA */

--- 2.6.17-32.orig/include/asm-i386/segment.h

+++ 2.6.17-32/include/asm-i386/segment.h
@@ -39,7 +39,7 @@

* 25 - APM BIOS support*

  *  26 - ESPFIX small SS
- *  27 - unused
+ *  27 - vgetcpu() data
  *  28 - unused
  *  29 - unused
  *  30 - unused
@@ -74,6 +74,8 @@
 #define GDT_ENTRY_ESPFIX_SS		(GDT_ENTRY_KERNEL_BASE + 14)
 #define __ESPFIX_SS (GDT_ENTRY_ESPFIX_SS * 8)

+#define GDT_ENTRY_VGETCPU (GDT_ENTRY_KERNEL_BASE + 15)

+
 #define GDT_ENTRY_DOUBLEFAULT_TSS	31

/*

--- 2.6.17-32.orig/arch/i386/kernel/vsyscall-sigreturn.S
+++ 2.6.17-32/arch/i386/kernel/vsyscall-sigreturn.S
@@ -26,7 +26,7 @@ __kernel_sigreturn:
 .LEND_sigreturn:
 	.size __kernel_sigreturn,.-.LSTART_sigreturn

- .balign 32

+	.balign 16
 	.globl __kernel_rt_sigreturn
 	.type __kernel_rt_sigreturn,@function
 __kernel_rt_sigreturn:
@@ -35,7 +35,6 @@ __kernel_rt_sigreturn:
 	int $0x80
 .LEND_rt_sigreturn:
 	.size __kernel_rt_sigreturn,.-.LSTART_rt_sigreturn
-	.balign 32
 	.previous

.section .eh_frame,"a",@progbits


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References:
- [RFC, patch] i386: vgetcpu() for NUMA, take 2
  - From: Chuck Ebbert <[email protected]>

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