Re: [PATCH] SLUB use cmpxchg_local

Mathieu Desnoyers
2007-08-22T00:38:45Z

* Andi Kleen (andi@firstfloor.org) wrote:
> Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> writes:
> >
> > The measurements I get (in cycles):
> > enable interrupts (STI) disable interrupts (CLI) local CMPXCHG
> > IA32 (P4) 112 82 26
> > x86_64 AMD64 125 102 19
>
> What exactly did you benchmark here? On K8 CLI/STI are only supposed
> to be a few cycles. pushf/popf might me more expensive, but not that much.
>

Hi Andi,

I benchmarked cmpxchg_local vs local_irq_save/local_irq_restore.
Details, and code, follow.

* cpuinfo:

processor : 0
vendor_id : AuthenticAMD
cpu family : 15
model : 35
model name : AMD Athlon(tm)64 X2 Dual Core Processor 3800+
stepping : 2
cpu MHz : 2009.204
cache size : 512 KB
physical id : 0
siblings : 2
core id : 0
cpu cores : 2
fpu : yes
fpu_exception : yes
cpuid level : 1
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt lm 3dnowext 3dnow pni lahf_lm cmp_legacy
bogomips : 4023.38
TLB size : 1024 4K pages
clflush size : 64
cache_alignment : 64
address sizes : 40 bits physical, 48 bits virtual
power management: ts fid vid ttp

processor : 1
vendor_id : AuthenticAMD
cpu family : 15
model : 35
model name : AMD Athlon(tm)64 X2 Dual Core Processor 3800+
stepping : 2
cpu MHz : 2009.204
cache size : 512 KB
physical id : 0
siblings : 2
core id : 1
cpu cores : 2
fpu : yes
fpu_exception : yes
cpuid level : 1
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt lm 3dnowext 3dnow pni lahf_lm cmp_legacy
bogomips : 4018.49
TLB size : 1024 4K pages
clflush size : 64
cache_alignment : 64
address sizes : 40 bits physical, 48 bits virtual
power management: ts fid vid ttp

* Test ran:

/* test-cmpxchg-nolock.c
*
* Compare local cmpxchg with irq disable / enable.
*/

#include <linux/jiffies.h>
#include <linux/compiler.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/calc64.h>
#include <asm/timex.h>
#include <asm/system.h>

#define NR_LOOPS 20000

int test_val;

static void do_test_cmpxchg(void)
{
int ret;
long flags;
unsigned int i;
cycles_t time1, time2, time;
long rem;

local_irq_save(flags);
preempt_disable();
time1 = get_cycles();
for (i = 0; i < NR_LOOPS; i++) {
ret = cmpxchg_local(&test_val, 0, 0);
}
time2 = get_cycles();
local_irq_restore(flags);
preempt_enable();
time = time2 - time1;

printk(KERN_ALERT "test results: time for non locked cmpxchg\n");
printk(KERN_ALERT "number of loops: %d\n", NR_LOOPS);
printk(KERN_ALERT "total time: %llu\n", time);
time = div_long_long_rem(time, NR_LOOPS, &rem);
printk(KERN_ALERT "-> non locked cmpxchg takes %llu cycles\n", time);
printk(KERN_ALERT "test end\n");
}

/*
* This test will have a higher standard deviation due to incoming interrupts.
*/
static void do_test_enable_int(void)
{
long flags;
unsigned int i;
cycles_t time1, time2, time;
long rem;

local_irq_save(flags);
preempt_disable();
time1 = get_cycles();
for (i = 0; i < NR_LOOPS; i++) {
local_irq_restore(flags);
}
time2 = get_cycles();
local_irq_restore(flags);
preempt_enable();
time = time2 - time1;

printk(KERN_ALERT "test results: time for enabling interrupts (STI)\n");
printk(KERN_ALERT "number of loops: %d\n", NR_LOOPS);
printk(KERN_ALERT "total time: %llu\n", time);
time = div_long_long_rem(time, NR_LOOPS, &rem);
printk(KERN_ALERT "-> enabling interrupts (STI) takes %llu cycles\n",
time);
printk(KERN_ALERT "test end\n");
}

static void do_test_disable_int(void)
{
unsigned long flags, flags2;
unsigned int i;
cycles_t time1, time2, time;
long rem;

local_irq_save(flags);
preempt_disable();
time1 = get_cycles();
for ( i = 0; i < NR_LOOPS; i++) {
local_irq_save(flags2);
}
time2 = get_cycles();
local_irq_restore(flags);
preempt_enable();
time = time2 - time1;

printk(KERN_ALERT "test results: time for disabling interrupts (CLI)\n");
printk(KERN_ALERT "number of loops: %d\n", NR_LOOPS);
printk(KERN_ALERT "total time: %llu\n", time);
time = div_long_long_rem(time, NR_LOOPS, &rem);
printk(KERN_ALERT "-> disabling interrupts (CLI) takes %llu cycles\n",
time);
printk(KERN_ALERT "test end\n");
}

static int ltt_test_init(void)
{
printk(KERN_ALERT "test init\n");

do_test_cmpxchg();
do_test_enable_int();
do_test_disable_int();
return -EAGAIN; /* Fail will directly unload the module */
}

static void ltt_test_exit(void)
{
printk(KERN_ALERT "test exit\n");
}

module_init(ltt_test_init)
module_exit(ltt_test_exit)

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Mathieu Desnoyers");
MODULE_DESCRIPTION("Cmpxchg vs int Test");