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    SubjectRe: [PATCH] SLUB use cmpxchg_local
    * 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");



    --
    Mathieu Desnoyers
    Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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