Re: cli/sti vs local_cmpxchg and local_add_return

From: Mathieu Desnoyers
Date: Tue Mar 17 2009 - 12:06:57 EST

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* Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) wrote:
> On Mon, Mar 16, 2009 at 09:32:20PM -0400, Mathieu Desnoyers wrote:
> > 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)
>
> Here you are on a 4.2GHz Power box:
>
> test init
> test results: time for baseline
> number of loops: 20000
> total time: 12490
> -> baseline takes 0 cycles
> test end
> test results: time for locked cmpxchg
> number of loops: 20000
> total time: 345748
> -> locked cmpxchg takes 17 cycles
> test end
> test results: time for non locked cmpxchg
> number of loops: 20000
> total time: 198304
> -> non locked cmpxchg takes 9 cycles
> test end
> test results: time for locked add return
> number of loops: 20000
> total time: 253977
> -> locked add return takes 12 cycles
> test end
> test results: time for non locked add return
> number of loops: 20000
> total time: 189837
> -> non locked add return takes 9 cycles
> test end
> test results: time for enabling interrupts (STI)
> number of loops: 20000
> total time: 298390
> -> enabling interrupts (STI) takes 14 cycles
> test end
> test results: time for disabling interrupts (CLI)
> number of loops: 20000
> total time: 43977
> -> disabling interrupts (CLI) takes 2 cycles
> test end
> test results: time for disabling/enabling interrupts (STI/CLI)
> number of loops: 20000
> total time: 298773
> -> enabling/disabling interrupts (STI/CLI) takes 14 cycles
> test end

Thanks !

So on powerpc64, we have :

local_cmpxchg + local_add_return : 9 + 9 = 18 cycles
irq off/on : ~14-16 cycles (this is without the write and increment
instructions performing the same work as the cmpxchg and add_return.
Imprecision of the measurement is probably due to pipeline effect).

But powerpc has non-maskable interrupts, so for less than 4 cycles, I
think it's better to stay with the local_t variant to be NMI-safe.

Mathieu

>
> Thanx, Paul
>
>
> > 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
> > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68

--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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