cli/sti vs local_cmpxchg and local_add_return

Mathieu Desnoyers
2009-03-17T01:32:40Z

Hi,

I am trying to get access to some non-x86 hardware to run some atomic
primitive benchmarks for a paper on LTTng I am preparing. That should be
useful to argue about performance benefit of per-cpu atomic operations
vs interrupt disabling. I would like to run the following benchmark
module on CONFIG_SMP :

- PowerPC
- MIPS
- ia64
- alpha

usage :
make
insmod test-cmpxchg-nolock.ko
insmod: error inserting 'test-cmpxchg-nolock.ko': -1 Resource temporarily unavailable
dmesg (see dmesg output)

If some of you would be kind enough to run my test module provided below
and provide the results of these tests on a recent kernel (2.6.26~2.6.29
should be good) along with their cpuinfo, I would greatly appreciate.

Here are the CAS results for various Intel-based architectures :

Architecture | Speedup | CAS | Interrupts |
| (cli + sti) / local cmpxchg | local | sync | Enable (sti) | Disable (cli)
-------------------------------------------------------------------------------------------------
Intel Pentium 4 | 5.24 | 25 | 81 | 70 | 61 |
AMD Athlon(tm)64 X2 | 4.57 | 7 | 17 | 17 | 15 |
Intel Core2 | 6.33 | 6 | 30 | 20 | 18 |
Intel Xeon E5405 | 5.25 | 8 | 24 | 20 | 22 |

The benefit expected on PowerPC, ia64 and alpha should principally come
from removed memory barriers in the local primitives.

Thanks,

Mathieu

P.S. please forgive the coding style and hackish interface. :)

/* 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/math64.h>
#include <asm/timex.h>
#include <asm/system.h>

#define NR_LOOPS 20000

int test_val;

static void do_testbaseline(void)
{
unsigned long flags;
unsigned int i;
cycles_t time1, time2, time;
u32 rem;

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

printk(KERN_ALERT "test results: time for baseline\n");
printk(KERN_ALERT "number of loops: %d\n", NR_LOOPS);
printk(KERN_ALERT "total time: %llu\n", time);
time = div_u64_rem(time, NR_LOOPS, &rem);
printk(KERN_ALERT "-> baseline takes %llu cycles\n", time);
printk(KERN_ALERT "test end\n");
}

static void do_test_sync_cmpxchg(void)
{
int ret;
unsigned long flags;
unsigned int i;
cycles_t time1, time2, time;
u32 rem;

local_irq_save(flags);
preempt_disable();
time1 = get_cycles();
for (i = 0; i < NR_LOOPS; i++) {
#ifdef CONFIG_X86_32
ret = sync_cmpxchg(&test_val, 0, 0);
#else
ret = cmpxchg(&test_val, 0, 0);
#endif
}
time2 = get_cycles();
local_irq_restore(flags);
preempt_enable();
time = time2 - time1;

printk(KERN_ALERT "test results: time for locked cmpxchg\n");
printk(KERN_ALERT "number of loops: %d\n", NR_LOOPS);
printk(KERN_ALERT "total time: %llu\n", time);
time = div_u64_rem(time, NR_LOOPS, &rem);
printk(KERN_ALERT "-> locked cmpxchg takes %llu cycles\n", time);
printk(KERN_ALERT "test end\n");
}

static void do_test_cmpxchg(void)
{
int ret;
unsigned long flags;
unsigned int i;
cycles_t time1, time2, time;
u32 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_u64_rem(time, NR_LOOPS, &rem);
printk(KERN_ALERT "-> non locked cmpxchg takes %llu cycles\n", time);
printk(KERN_ALERT "test end\n");
}
static void do_test_sync_inc(void)
{
int ret;
unsigned long flags;
unsigned int i;
cycles_t time1, time2, time;
u32 rem;
atomic_t val;

local_irq_save(flags);
preempt_disable();
time1 = get_cycles();
for (i = 0; i < NR_LOOPS; i++) {
ret = atomic_add_return(10, &val);
}
time2 = get_cycles();
local_irq_restore(flags);
preempt_enable();
time = time2 - time1;

printk(KERN_ALERT "test results: time for locked add return\n");
printk(KERN_ALERT "number of loops: %d\n", NR_LOOPS);
printk(KERN_ALERT "total time: %llu\n", time);
time = div_u64_rem(time, NR_LOOPS, &rem);
printk(KERN_ALERT "-> locked add return takes %llu cycles\n", time);
printk(KERN_ALERT "test end\n");
}

static void do_test_inc(void)
{
int ret;
unsigned long flags;
unsigned int i;
cycles_t time1, time2, time;
u32 rem;
local_t loc_val;

local_irq_save(flags);
preempt_disable();
time1 = get_cycles();
for (i = 0; i < NR_LOOPS; i++) {
ret = local_add_return(10, &loc_val);
}
time2 = get_cycles();
local_irq_restore(flags);
preempt_enable();
time = time2 - time1;

printk(KERN_ALERT "test results: time for non locked add return\n");
printk(KERN_ALERT "number of loops: %d\n", NR_LOOPS);
printk(KERN_ALERT "total time: %llu\n", time);
time = div_u64_rem(time, NR_LOOPS, &rem);
printk(KERN_ALERT "-> non locked add return 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)
{
unsigned long flags;
unsigned int i;
cycles_t time1, time2, time;
u32 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_u64_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;
u32 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_u64_rem(time, NR_LOOPS, &rem);
printk(KERN_ALERT "-> disabling interrupts (CLI) takes %llu cycles\n",
time);
printk(KERN_ALERT "test end\n");
}

static void do_test_int(void)
{
unsigned long flags;
unsigned int i;
cycles_t time1, time2, time;
u32 rem;

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

printk(KERN_ALERT "test results: time for disabling/enabling interrupts (STI/CLI)\n");
printk(KERN_ALERT "number of loops: %d\n", NR_LOOPS);
printk(KERN_ALERT "total time: %llu\n", time);
time = div_u64_rem(time, NR_LOOPS, &rem);
printk(KERN_ALERT "-> enabling/disabling interrupts (STI/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_testbaseline();
do_test_sync_cmpxchg();
do_test_cmpxchg();
do_test_sync_inc();
do_test_inc();
do_test_enable_int();
do_test_disable_int();
do_test_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");

* Makefile

ifneq ($(KERNELRELEASE),)
obj-m += test-cmpxchg-nolock.o
else
KERNELDIR ?= /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)
KERNELRELEASE = $(shell cat $(KERNELDIR)/$(KBUILD_OUTPUT)/include/linux/version.h | sed -n 's/.*UTS_RELEASE.*\"\(.*\)\".*/\1/p')
ifneq ($(INSTALL_MOD_PATH),)
DEPMOD_OPT := -b $(INSTALL_MOD_PATH)
endif

default:
$(MAKE) -C $(KERNELDIR) M=$(PWD) modules

modules_install:
$(MAKE) -C $(KERNELDIR) M=$(PWD) modules_install
if [ -f $(KERNELDIR)/$(KBUILD_OUTPUT)/System.map ] ; then /sbin/depmod -ae -F $(KERNELDIR)/$(KBUILD_OUTPUT)/System.map $(DEPMOD_OPT) $(KERNELRELEASE) ; fi

clean:
$(MAKE) -C $(KERNELDIR) M=$(PWD) clean
endif

--
Mathieu Desnoyers
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