Seeing %gs prefixes used now by i386 port, I recalled seeing strange oprofile
results on Opteron machines.
I really think %gs prefixes can be expensive in some (most ?) cases, even if
the Intel/AMD docs say they are free.
I wrote this trivial User program to benchmark vfs_read()/vfs_write() that
happens to use 'current' many times.
#include <unistd.h>
#include <errno.h>
int main()
{
int i, fd[2];
char c = 0;
pipe(fd);
for (i = 0; i < 10000000; i++) {
errno = 0; // glibc also use %gs
write(fd[1], &c, 1);
read(fd[0], &c, 1);
}
return 0;
}
The best elap time I got for this program on 10 runs was : 12.811 s
(Intel(R) Pentium(R) M processor 1.60GHz)
With the attached patch, I got 12.212 s, and a kernel text size reduction of
3400 bytes.
I wish Jeremy give us patches for UP machines so that %gs can be let untouched
in entry.S (syscall entry/exit). A lot of ia32 machines are still using one
CPU.
Note : I dont have a x86_64 machine here, but I suspect a similar patch could
be done for x86_64 too.
Thank you
[PATCH] i386-pda UP optimization
On a !CONFIG_SMP machine, there is only one PDA, (one CPU).
We can avoid %gs prefixes when reading/writing fields in PDA.
This reduce kernel text size and also give better performance.
Signed-off-by: Eric Dumazet <[email protected]>
--- linux-2.6.19-rc5-mm2/include/asm-i386/pda.h 2006-11-15 11:21:24.000000000 +0100
+++ linux-2.6.19-rc5-mm2-ed/include/asm-i386/pda.h 2006-11-15 11:23:49.000000000 +0100
@@ -91,10 +91,19 @@
((typeof(_proxy_pda.field) *)((unsigned char *)read_pda(_pda) + \
pda_offset(field)))
+#if defined(CONFIG_SMP)
#define read_pda(field) pda_from_op("mov",field)
#define write_pda(field,val) pda_to_op("mov",field,val)
#define add_pda(field,val) pda_to_op("add",field,val)
#define sub_pda(field,val) pda_to_op("sub",field,val)
#define or_pda(field,val) pda_to_op("or",field,val)
+#else
+extern struct i386_pda boot_pda;
+#define read_pda(field) boot_pda.field
+#define write_pda(field,val) do { boot_pda.field = (val);} while (0)
+#define add_pda(field,val) ) do { boot_pda.field += (val);} while (0)
+#define sub_pda(field,val) do { boot_pda.field -= (val);} while (0)
+#define or_pda(field,val) do { boot_pda.field |= (val);} while (0)
+#endif
#endif /* _I386_PDA_H */
--- linux-2.6.19-rc5-mm2/arch/i386/kernel/cpu/common.c 2006-11-15 11:21:25.000000000 +0100
+++ linux-2.6.19-rc5-mm2-ed/arch/i386/kernel/cpu/common.c 2006-11-15 11:45:09.000000000 +0100
@@ -609,6 +609,14 @@
return regs;
}
+/* Initial PDA used by boot CPU */
+struct i386_pda boot_pda = {
+ ._pda = &boot_pda,
+ .cpu_number = 0,
+ .pcurrent = &init_task,
+};
+EXPORT_SYMBOL(boot_pda);
+
static __cpuinit int alloc_gdt(int cpu)
{
struct Xgt_desc_struct *cpu_gdt_descr = &per_cpu(cpu_gdt_descr, cpu);
@@ -628,11 +636,10 @@
BUG_ON(gdt != NULL || pda != NULL);
gdt = alloc_bootmem_pages(PAGE_SIZE);
- pda = alloc_bootmem(sizeof(*pda));
+ pda = &boot_pda;
/* alloc_bootmem(_pages) panics on failure, so no check */
memset(gdt, 0, PAGE_SIZE);
- memset(pda, 0, sizeof(*pda));
} else {
/* GDT and PDA might already have been allocated if
this is a CPU hotplug re-insertion. */
@@ -655,13 +662,6 @@
return 1;
}
-/* Initial PDA used by boot CPU */
-struct i386_pda boot_pda = {
- ._pda = &boot_pda,
- .cpu_number = 0,
- .pcurrent = &init_task,
-};
-
static inline void set_kernel_gs(void)
{
/* Set %gs for this CPU's PDA. Memory clobber is to create a
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