2002-08-07 17:02:06

by Luca Barbieri

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Subject: [PATCH] [2.5 i386] Swap TLS and TSS entries to improve spatial locality

---

GDT entries are usually accessed with this pattern:
* Interrupt: read TSS (for ss0:esp0) and kernel CS/DS
* Schedule: write TLS, LDT, load FS and GS (either TLS, user DS
or LDT entry)
* Return: read user CS/DS

Swapping the TLS and TSS entries causes the GDT entries that are read
during interrupt and schedule to be in the same cacheline.


diff --exclude-from=/home/ldb/src/linux-exclude -urNd a/arch/i386/kernel/head.S b/arch/i386/kernel/head.S
--- a/arch/i386/kernel/head.S 2002-07-29 04:18:06.000000000 +0200
+++ b/arch/i386/kernel/head.S 2002-08-07 19:03:24.000000000 +0200
@@ -416,12 +416,12 @@
*/
ENTRY(cpu_gdt_table)
.quad 0x0000000000000000 /* NULL descriptor */
- .quad 0x0000000000000000 /* TLS descriptor */
+ .quad 0x0000000000000000 /* TSS descriptor */
.quad 0x00cf9a000000ffff /* 0x10 kernel 4GB code at 0x00000000 */
.quad 0x00cf92000000ffff /* 0x18 kernel 4GB data at 0x00000000 */
.quad 0x00cffa000000ffff /* 0x23 user 4GB code at 0x00000000 */
.quad 0x00cff2000000ffff /* 0x2b user 4GB data at 0x00000000 */
- .quad 0x0000000000000000 /* TSS descriptor */
+ .quad 0x0000000000000000 /* TLS descriptor */
.quad 0x0000000000000000 /* LDT descriptor */
/*
* The APM segments have byte granularity and their bases
diff --exclude-from=/home/ldb/src/linux-exclude -urNd a/include/asm-i386/desc.h b/include/asm-i386/desc.h
--- a/include/asm-i386/desc.h 2002-08-02 01:19:14.000000000 +0200
+++ b/include/asm-i386/desc.h 2002-08-07 18:56:12.000000000 +0200
@@ -7,12 +7,12 @@
* The layout of the per-CPU GDT under Linux:
*
* 0 - null
- * 1 - Thread-Local Storage (TLS) segment
+ * 1 - TSS
* 2 - kernel code segment
* 3 - kernel data segment
* 4 - user code segment <==== new cacheline
* 5 - user data segment
- * 6 - TSS
+ * 6 - Thread-Local Storage (TLS) segment
* 7 - LDT
* 8 - APM BIOS support <==== new cacheline
* 9 - APM BIOS support
@@ -27,8 +27,8 @@
* 18 - not used
* 19 - not used
*/
-#define TLS_ENTRY 1
-#define TSS_ENTRY 6
+#define TSS_ENTRY 1
+#define TLS_ENTRY 6
#define LDT_ENTRY 7
/*
* The interrupt descriptor table has room for 256 idt's,

---

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2002-08-07 17:05:30

by Ingo Molnar

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Subject: Re: [PATCH] [2.5 i386] Swap TLS and TSS entries to improve spatial locality

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On 7 Aug 2002, Luca Barbieri wrote:

> GDT entries are usually accessed with this pattern:
> * Interrupt: read TSS (for ss0:esp0) and kernel CS/DS
> * Schedule: write TLS, LDT, load FS and GS (either TLS, user DS
> or LDT entry)
> * Return: read user CS/DS
>
> Swapping the TLS and TSS entries causes the GDT entries that are read
> during interrupt and schedule to be in the same cacheline.

yes, this makes perfect sense.

Ingo