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Subject[patch] faster vgetcpu using sidt
below is a patch which improves vgetcpu latency on all x86_64 
implementations i've tested.

Nathan Laredo pointed out the sgdt/sidt/sldt instructions are
userland-accessible and we could use their limit fields to tuck away a few
bits of per-cpu information.

vgetcpu generally uses lsl at present, but all of sgdt/sidt/sldt are
faster than lsl on all x86_64 processors i've tested. on p4 processers
lsl tends to be 150 cycles whereas the s*dt instructions are 15 cycles or
less. lsl requires microcoded permission testing whereas s*dt are free
of any such hassle.

sldt is the least expensive of the three instructions however it's a
hassle to use because processes may want to adjust their ldt. sidt/sgdt
have essentially the same performance across all the major architectures
-- however sidt has the advantage that its limit field is 16-bits, yet any
value >= 0xfff is essentially "infinite" because there are only 256 (16
byte) descriptors. so sidt is probably the best choice of the three.

in benchmarking i've discovered the rdtscp implementation of vgetcpu is
slower than even the lsl-based implementation on opteron revF. so i've
dropped the rdtscp implementation in this patch. however i've left the
rdtscp_aux register initialized because i'm sure it's the right choice for
various proposed vgettimeofday / per-cpu tsc state improvements which need
the atomic nature of the rdtscp instruction and i hope it'll be used in
those situations.

at compile time this patch detects if 0x1000 +
(CONFIG_NR_CPUS<<CONFIG_NODES_SHIFT) will fit in the idt limit field and
selects the lsl method otherwise. i've further added a test for the 20
bit limit of the lsl method and #error in the event it doesn't fit (we
could fall all the way back to cpuid method if someone has a box with that
many cpus*nodes, but i'll let someone else handle that case ;).

given this is a compile-time choice, and rdtscp is always slower than
sidt, i've dropped the vgetcpu_mode variable.

i've also dropped the cache support in the sidt case -- depending on the
compiler and cpu i found it to be 1 cycle slower than the uncached case,
and it just doesn't seem worth the potential extra L1 traffic (besides if
you add in the implied __thread overhead it's definitely a loss).

here are the before/after results:

baseline patched
no cache cache
k8 pre-revF 21 14 16
k8 revF 31 14 17
core2 38 12 17

sorry i don't have a handy EMT p4 on which i can install a 2.6.20-rc3
kernel... but based on userland-only comparisons of the sidt/lsl
instructions i'll be amazed if this isn't a huge win on p4.

timing tools and test case can be found at
<http://arctic.org/~dean/vgetcpu/>

-dean

Signed-off-by: dean gaudet <dean@arctic.org>

Index: linux/arch/x86_64/kernel/time.c
===================================================================
--- linux.orig/arch/x86_64/kernel/time.c 2007-01-06 13:31:10.000000000 -0800
+++ linux/arch/x86_64/kernel/time.c 2007-01-06 16:04:01.000000000 -0800
@@ -957,11 +957,6 @@
if (unsynchronized_tsc())
notsc = 1;

- if (cpu_has(&boot_cpu_data, X86_FEATURE_RDTSCP))
- vgetcpu_mode = VGETCPU_RDTSCP;
- else
- vgetcpu_mode = VGETCPU_LSL;
-
if (vxtime.hpet_address && notsc) {
timetype = hpet_use_timer ? "HPET" : "PIT/HPET";
if (hpet_use_timer)
Index: linux/arch/x86_64/kernel/vsyscall.c
===================================================================
--- linux.orig/arch/x86_64/kernel/vsyscall.c 2007-01-06 13:31:10.000000000 -0800
+++ linux/arch/x86_64/kernel/vsyscall.c 2007-01-06 17:29:36.000000000 -0800
@@ -40,13 +40,18 @@
#include <asm/segment.h>
#include <asm/desc.h>
#include <asm/topology.h>
+#include <asm/desc.h>

#define __vsyscall(nr) __attribute__ ((unused,__section__(".vsyscall_" #nr)))
#define __syscall_clobber "r11","rcx","memory"

int __sysctl_vsyscall __section_sysctl_vsyscall = 1;
seqlock_t __xtime_lock __section_xtime_lock = SEQLOCK_UNLOCKED;
-int __vgetcpu_mode __section_vgetcpu_mode;
+
+/* is this necessary? */
+#ifndef CONFIG_NODES_SHIFT
+#define CONFIG_NODES_SHIFT 0
+#endif

#include <asm/unistd.h>

@@ -147,11 +152,21 @@
long __vsyscall(2)
vgetcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *tcache)
{
- unsigned int dummy, p;
+ unsigned int p;
+#ifdef VGETCPU_USE_SIDT
+ struct {
+ char pad[6]; /* avoid unaligned stores */
+ u16 size;
+ u64 address;
+ } idt;
+
+ asm("sidt %0" : "=m" (idt.size));
+ p = idt.size - 0x1000;
+#else
unsigned long j = 0;

/* Fast cache - only recompute value once per jiffies and avoid
- relatively costly rdtscp/cpuid otherwise.
+ relatively costly lsl otherwise.
This works because the scheduler usually keeps the process
on the same CPU and this syscall doesn't guarantee its
results anyways.
@@ -160,21 +175,20 @@
If you don't like it pass NULL. */
if (tcache && tcache->blob[0] == (j = __jiffies)) {
p = tcache->blob[1];
- } else if (__vgetcpu_mode == VGETCPU_RDTSCP) {
- /* Load per CPU data from RDTSCP */
- rdtscp(dummy, dummy, p);
- } else {
+ }
+ else {
/* Load per CPU data from GDT */
asm("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
+ if (tcache) {
+ tcache->blob[0] = j;
+ tcache->blob[1] = p;
+ }
}
- if (tcache) {
- tcache->blob[0] = j;
- tcache->blob[1] = p;
- }
+#endif
if (cpu)
- *cpu = p & 0xfff;
+ *cpu = p >> CONFIG_NODES_SHIFT;
if (node)
- *node = p >> 12;
+ *node = p & ((1<<CONFIG_NODES_SHIFT) - 1);
return 0;
}

@@ -250,22 +264,37 @@
doesn't violate that. We'll find out if it does. */
static void __cpuinit vsyscall_set_cpu(int cpu)
{
- unsigned long *d;
- unsigned long node = 0;
+ unsigned long cpu_node_encoding = cpu << CONFIG_NODES_SHIFT;
+
#ifdef CONFIG_NUMA
- node = cpu_to_node[cpu];
+ cpu_node_encoding |= cpu_to_node[cpu];
#endif
+
+ /* Even though we never use rdtscp for vgetcpu we set up the rdtscp_aux
+ * register here for (future) use in vgettimeofday et al.
+ */
if (cpu_has(&cpu_data[cpu], X86_FEATURE_RDTSCP))
- write_rdtscp_aux((node << 12) | cpu);
+ write_rdtscp_aux(cpu_node_encoding);

+#ifdef VGETCPU_USE_SIDT
+ {
+ struct desc_ptr local_idt;
+
+ local_idt.size = 0x1000 + cpu_node_encoding;
+ local_idt.address = idt_descr.address;
+ asm("lidt %0" :: "m" (local_idt));
+ }
+#else
/* Store cpu number in limit so that it can be loaded quickly
- in user space in vgetcpu.
- 12 bits for the CPU and 8 bits for the node. */
- d = (unsigned long *)(cpu_gdt(cpu) + GDT_ENTRY_PER_CPU);
- *d = 0x0f40000000000ULL;
- *d |= cpu;
- *d |= (node & 0xf) << 12;
- *d |= (node >> 4) << 48;
+ in user space in vgetcpu. */
+ {
+ unsigned long *d;
+ d = (unsigned long *)(cpu_gdt(cpu) + GDT_ENTRY_PER_CPU);
+ *d = 0x0f40000000000ULL;
+ *d |= cpu_node_encoding & 0xffff;
+ *d |= (cpu_node_encoding >> 16) << 48;
+ };
+#endif
}

static void __cpuinit cpu_vsyscall_init(void *arg)
Index: linux/include/asm-x86_64/vsyscall.h
===================================================================
--- linux.orig/include/asm-x86_64/vsyscall.h 2007-01-06 13:31:10.000000000 -0800
+++ linux/include/asm-x86_64/vsyscall.h 2007-01-06 17:15:53.000000000 -0800
@@ -17,7 +17,6 @@
#include <linux/seqlock.h>

#define __section_vxtime __attribute__ ((unused, __section__ (".vxtime"), aligned(16)))
-#define __section_vgetcpu_mode __attribute__ ((unused, __section__ (".vgetcpu_mode"), aligned(16)))
#define __section_jiffies __attribute__ ((unused, __section__ (".jiffies"), aligned(16)))
#define __section_sys_tz __attribute__ ((unused, __section__ (".sys_tz"), aligned(16)))
#define __section_sysctl_vsyscall __attribute__ ((unused, __section__ (".sysctl_vsyscall"), aligned(16)))
@@ -28,9 +27,6 @@
#define VXTIME_HPET 2
#define VXTIME_PMTMR 3

-#define VGETCPU_RDTSCP 1
-#define VGETCPU_LSL 2
-
struct vxtime_data {
long hpet_address; /* HPET base address */
int last;
@@ -45,7 +41,6 @@

/* vsyscall space (readonly) */
extern struct vxtime_data __vxtime;
-extern int __vgetcpu_mode;
extern struct timespec __xtime;
extern volatile unsigned long __jiffies;
extern struct timezone __sys_tz;
@@ -53,7 +48,6 @@

/* kernel space (writeable) */
extern struct vxtime_data vxtime;
-extern int vgetcpu_mode;
extern struct timezone sys_tz;
extern int sysctl_vsyscall;
extern seqlock_t xtime_lock;
@@ -62,6 +56,28 @@

#define ARCH_HAVE_XTIME_LOCK 1

+/*
+ * To use the IDT limit for vgetcpu we encode things like so:
+ *
+ * 0x1000 + node + (cpu << CONFIG_NODES_SHIFT)
+ *
+ * this ensures a system using this method has an IDT limit other than
+ * 0xfff, while systems not using this method will have an IDT limit
+ * of 0xfff. (just in case anyone cares to have a test).
+ *
+ * This test verifies the various config options are in an appropriate
+ * range for the 16-bit limit field.
+ */
+#if 0x1000 + (CONFIG_NR_CPUS << CONFIG_NODES_SHIFT) <= 0x10000
+#define VGETCPU_USE_SIDT 1
+
+/* might as well test this somewhere -- the lsl method of vgetcpu has
+ * only 20 bits available to it.
+ */
+#elif (CONFIG_NR_CPUS << CONFIG_NODES_SHIFT) >= (1<<20)
+#error "(CONFIG_NR_CPUS << CONFIG_NODES_SHIFT) out of range for existing vgetcpu implementations"
+#endif
+
#endif /* __KERNEL__ */

#endif /* _ASM_X86_64_VSYSCALL_H_ */
Index: linux/arch/x86_64/kernel/vmlinux.lds
===================================================================
--- linux.orig/arch/x86_64/kernel/vmlinux.lds 2007-01-06 16:03:53.000000000 -0800
+++ linux/arch/x86_64/kernel/vmlinux.lds 2007-01-06 16:04:20.000000000 -0800
@@ -1372,8 +1372,6 @@
xtime_lock = (ADDR(.xtime_lock) - ((-10*1024*1024) - ((ADDR(.data.read_mostly) + SIZEOF(.data.read_mostly) + 4095) & ~(4095))));
.vxtime : AT((ADDR(.vxtime) - ((-10*1024*1024) - ((LOADADDR(.data.read_mostly) + SIZEOF(.data.read_mostly) + 4095) & ~(4095))))) { *(.vxtime) }
vxtime = (ADDR(.vxtime) - ((-10*1024*1024) - ((ADDR(.data.read_mostly) + SIZEOF(.data.read_mostly) + 4095) & ~(4095))));
- .vgetcpu_mode : AT((ADDR(.vgetcpu_mode) - ((-10*1024*1024) - ((LOADADDR(.data.read_mostly) + SIZEOF(.data.read_mostly) + 4095) & ~(4095))))) { *(.vgetcpu_mode) }
- vgetcpu_mode = (ADDR(.vgetcpu_mode) - ((-10*1024*1024) - ((ADDR(.data.read_mostly) + SIZEOF(.data.read_mostly) + 4095) & ~(4095))));
.sys_tz : AT((ADDR(.sys_tz) - ((-10*1024*1024) - ((LOADADDR(.data.read_mostly) + SIZEOF(.data.read_mostly) + 4095) & ~(4095))))) { *(.sys_tz) }
sys_tz = (ADDR(.sys_tz) - ((-10*1024*1024) - ((ADDR(.data.read_mostly) + SIZEOF(.data.read_mostly) + 4095) & ~(4095))));
.sysctl_vsyscall : AT((ADDR(.sysctl_vsyscall) - ((-10*1024*1024) - ((LOADADDR(.data.read_mostly) + SIZEOF(.data.read_mostly) + 4095) & ~(4095))))) { *(.sysctl_vsyscall) }
Index: linux/arch/x86_64/kernel/vmlinux.lds.S
===================================================================
--- linux.orig/arch/x86_64/kernel/vmlinux.lds.S 2007-01-06 16:03:53.000000000 -0800
+++ linux/arch/x86_64/kernel/vmlinux.lds.S 2007-01-06 16:04:07.000000000 -0800
@@ -94,9 +94,6 @@
.vxtime : AT(VLOAD(.vxtime)) { *(.vxtime) }
vxtime = VVIRT(.vxtime);

- .vgetcpu_mode : AT(VLOAD(.vgetcpu_mode)) { *(.vgetcpu_mode) }
- vgetcpu_mode = VVIRT(.vgetcpu_mode);
-
.sys_tz : AT(VLOAD(.sys_tz)) { *(.sys_tz) }
sys_tz = VVIRT(.sys_tz);

-
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