使用systemtap调试Linux内核-爱开源

最近在公司看Linux内核的nmi死锁检测功能的实现机制，当然，是因为它变了，所以我才看的，简单来说就是在红帽的某牛提交了一个内核patch：new nmi_watchdog using perf events，这个patch已经被合入到内核主线2.6.38版本，所以使用自该版本开始后内核的Linux系统，其/proc/interrupts显示的中断数不再按每秒1000次的频率增长。关于new nmi_watchdog问题，本文不再多说，后续写专篇文章，下面看使用systemtap调试Linux内核的几个案例，因为我最近就是通过这个手段来理解new nmi_watchdog的实现机制，相比利用printk或kgdb而言，使用systemtap更为简单方便，效率也大大提高。

系统环境：

[root@localhost ~]# cat /etc/issue
CentOS release 6.2 (Final)
Kernel r on an m
[root@localhost ~]# uname -a
Linux localhost.lenkydomain 3.6.11 #1 SMP Wed Feb 20 21:26:16 CST 2013 x86_64 x86_64 x86_64 GNU/Linux
[root@localhost ~]# stap -V
Systemtap translator/driver (version 2.1/0.152, non-git sources)
Copyright (C) 2005-2012 Red Hat, Inc. and others
This is free software; see the source for copying conditions.
enabled features: LIBRPM LIBSQLITE3 NSS BOOST_SHARED_PTR TR1_UNORDERED_MAP NLS

案例一，判断函数的真实执行路径，比如这个函数：

static inline void x86_assign_hw_event(struct perf_event *event,
                struct cpu_hw_events *cpuc, int i)
{
    struct hw_perf_event *hwc = &event->hw;
    hwc->idx = cpuc->assign[i];
    hwc->last_cpu = smp_processor_id();
    hwc->last_tag = ++cpuc->tags[i];
    if (hwc->idx == INTEL_PMC_IDX_FIXED_BTS) {
a->      hwc->config_base = 0;
        hwc->event_base  = 0;
    } else if (hwc->idx >= INTEL_PMC_IDX_FIXED) {
b->      hwc->config_base = MSR_ARCH_PERFMON_FIXED_CTR_CTRL;
        hwc->event_base = MSR_ARCH_PERFMON_FIXED_CTR0 + (hwc->idx - INTEL_PMC_IDX_FIXED);
        hwc->event_base_rdpmc = (hwc->idx - INTEL_PMC_IDX_FIXED) | 1<<30;
    } else {
c->      hwc->config_base = x86_pmu_config_addr(hwc->idx);
        hwc->event_base  = x86_pmu_event_addr(hwc->idx);
        hwc->event_base_rdpmc = hwc->idx;
    }
}

我想知道nmi_watchdog的perf event走的是路径a？路径b？还是路径c？
以前利用printk的矬做法是修改这个函数，在a、b、c分别插入printk(“aaan”);、printk(“bbbn”);、printk(“cccn”);，然后需要重新编译内核，重启机器跑一次逻辑，再根据打印结果做判断。
利用systemtap的做法是在上面三个路径上分别下探测点，并设置执行语句为打印相应的字符串即可。
比如在上面的路径b处下探测点：

[root@localhost ~]# stap -ve 'probe kernel.statement("*@arch/x86/kernel/cpu/perf_event.c:824"){printf("bbbn")}'

在另一个终端触发nmi_watchdog设置逻辑：

[root@localhost ~]# echo 0 > /proc/sys/kernel/nmi_watchdog ; echo 1 > /proc/sys/kernel/nmi_watchdog ;

可以看到探测点被执行到，并且打印了相关信息，根据打印的信息来看，探测点被多次执行到：

[root@localhost ~]# stap -ve 'probe kernel.statement("*@arch/x86/kernel/cpu/perf_event.c:824"){printf("bbbn")}'
Pass 1: parsed user script and 90 library script(s) using 190140virt/24872res/2788shr/22676data kb, in 170usr/10sys/178real ms.
Pass 2: analyzed script: 2 probe(s), 0 function(s), 0 embed(s), 0 global(s) using 358304virt/79680res/18160shr/63552data kb, in 280usr/20sys/310real ms.
Pass 3: translated to C into "/tmp/stapGGJa26/stap_b1c9e6f6ba4bf7f4d0a8eb727add532b_1128_src.c" using 358304virt/79840res/18312shr/63552data kb, in 10usr/0sys/5real ms.
Pass 4: compiled C into "stap_b1c9e6f6ba4bf7f4d0a8eb727add532b_1128.ko" in 1210usr/290sys/1525real ms.
Pass 5: starting run.
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb

三处都下探测点的完整执行情况：

[root@localhost perf_study]# cat x86_assign_hw_event.stp
probe kernel.statement("*@arch/x86/kernel/cpu/perf_event.c:821"){printf("aaan")}
probe kernel.statement("*@arch/x86/kernel/cpu/perf_event.c:824"){printf("bbbn")}
probe kernel.statement("*@arch/x86/kernel/cpu/perf_event.c:828"){printf("cccn")}
[root@localhost perf_study]# stap x86_assign_hw_event.stp
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb
bbb

另外一个常见的情况是要判断某个函数在某个逻辑中是否被跑到，以前的做法也是在函数的入口处加上打印语句，然后跑一遍功能逻辑，看是否有信息打印出来，现在利用systemtap可以这样：

[root@localhost perf_study]# stap -e 'probe kernel.function("x86_assign_hw_event"){printf("testn")}'
test
test
test
test
test
test
test
test

案例二，获取函数调用堆栈，示例：

[root@localhost perf_study]# cat bt.stp
#stap -v bt.stp schedule
probe kernel.function(@1){
    print("----------------START-------------------------n")
    printf("In process [%s]n", execname())
    print_regs()
    print_backtrace()
    print("----------------END-------------------------n")
    exit()
}
[root@localhost perf_study]# stap -v bt.stp x86_assign_hw_event
Pass 1: parsed user script and 90 library script(s) using 190148virt/24884res/2784shr/22684data kb, in 170usr/10sys/179real ms.
Pass 2: analyzed script: 1 probe(s), 4 function(s), 2 embed(s), 0 global(s) using 344464virt/50624res/3872shr/47224data kb, in 500usr/290sys/788real ms.
Pass 3: using cached /root/.systemtap/cache/13/stap_13ea16365226db9619f3f14ab2a27efc_2536.c
Pass 4: using cached /root/.systemtap/cache/13/stap_13ea16365226db9619f3f14ab2a27efc_2536.ko
Pass 5: starting run.
----------------START-------------------------
In process [swapper/0]
RIP: ffffffff81024f40
RSP: ffff88022fc03d18  EFLAGS: 00000086
RAX: 0000000000000021 RBX: ffff88022fc0c6e0 RCX: 0000000000000000
RDX: ffff88022fc169f8 RSI: ffff88022fc03d7c RDI: ffff8802215efc00
RBP: ffff88022fc03d58 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000003 R11: 0000000000000003 R12: 0000000000000000
R13: ffff88022fc0c6e0 R14: ffff88022fc0c6e0 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff88022fc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 0000000000419248 CR3: 0000000222468000 CR4: 00000000000007f0
 0xffffffff81024f40 : x86_pmu_enable+0x140/0x280 [kernel]
 0xffffffff8110c7eb : perf_pmu_enable+0x2b/0x40 [kernel]
 0xffffffff810247a9 : x86_pmu_commit_txn+0xa9/0xb0 [kernel]
 0xffffffff8110ee8a : group_sched_in+0x13a/0x170 [kernel]
 0xffffffff8110fb3d : __perf_event_enable+0x29d/0x2e0 [kernel]
 0xffffffff8110d038 : remote_function+0x48/0x60 [kernel]
 0xffffffff810b2371 : generic_smp_call_function_single_interrupt+0xa1/0x100 [kernel]
 0xffffffff810383f7 : smp_call_function_single_interrupt+0x27/0x40 [kernel]
 0xffffffff815276ca : call_function_single_interrupt+0x6a/0x70 [kernel]
----------------END-------------------------
Pass 5: run completed in 10usr/30sys/4564real ms.

案例三，打印某代码路径上的变量值：

[root@localhost perf_study]# stap -e 'probe kernel.statement("x86_assign_hw_event@arch/x86/kernel/cpu/perf_event.c:824"){printf("%dn", $hwc->idx)}'
33
33
33
33
33
33
33
33

如果有这样的提示错误：

[root@localhost perf_study]# stap -e 'probe kernel.statement("*@arch/x86/kernel/cpu/perf_event.c:824"){printf("%dn", $hwc->idx)}'
semantic error: not accessible at this address (0xffffffff81024f8e, dieoffset: 0x343357): identifier '$hwc' at <input>:1:81
        source: probe kernel.statement("*@arch/x86/kernel/cpu/perf_event.c:824"){printf("%dn", $hwc->idx)}
                                                                                                ^

Pass 2: analysis failed.  Try again with another '--vp 01' option.

那么可以用-L选项看一下到底有哪些可用变量：

[root@localhost perf_study]# stap -L 'kernel.statement("*@arch/x86/kernel/cpu/perf_event.c:824")'

kernel.statement("x86_assign_hw_event@arch/x86/kernel/cpu/perf_event.c:824") $hwc:struct hw_perf_event* $i:int $cpuc:struct cpu_hw_events* $event:struct perf_event*

kernel.statement("x86_pmu_enable@arch/x86/kernel/cpu/perf_event.c:824") $n_running:int $cpuc:struct cpu_hw_events* $added:int

可以看到，是因为*匹配到两处（应该是宏的开启与否导致代码行号有移动，但个人暂不确定具体原因是否如此），因此可以明确使用“x86_assign_hw_event@arch/x86/kernel/cpu/perf_event.c:824”。

案例四，获知函数指针的具体指向，比如获取下面event_init函数指针的具体指向：

struct pmu *perf_init_event(struct perf_event *event)
{
    struct pmu *pmu = NULL;
    int idx;
    int ret;

    idx = srcu_read_lock(&pmus_srcu);

    rcu_read_lock();
    pmu = idr_find(&pmu_idr, event->attr.type);
    rcu_read_unlock();
    if (pmu) {
        event->pmu = pmu;
        ret = pmu->event_init(event);
        if (ret)
            pmu = ERR_PTR(ret);
        goto unlock;
    }

    list_for_each_entry_rcu(pmu, &pmus, entry) {
        event->pmu = pmu;
        ret = pmu->event_init(event);
        if (!ret)
            goto unlock;

        if (ret != -ENOENT) {
            pmu = ERR_PTR(ret);
            goto unlock;
        }
    }
    pmu = ERR_PTR(-ENOENT);
unlock:
    srcu_read_unlock(&pmus_srcu, idx);

    return pmu;
}

[root@localhost perf_study]# cat perf_init_event.stp
probe kernel.statement("perf_init_event@kernel/events/core.c:5892"){
    addr = sprintf("%p", $pmu->event_init);
    print("5892-Function name:n")
    print_stack(addr)
}

probe kernel.statement("perf_init_event@kernel/events/core.c:5900"){
    addr = sprintf("%p", $pmu->event_init);
    print("5900-Function name:n")
    print_stack(addr)
}
[root@localhost perf_study]# stap -v perf_init_event.stp
Pass 1: parsed user script and 90 library script(s) using 190092virt/24736res/2784shr/22628data kb, in 160usr/20sys/178real ms.
Pass 2: analyzed script: 2 probe(s), 7 function(s), 3 embed(s), 0 global(s) using 357076virt/62224res/3896shr/59836data kb, in 450usr/290sys/743real ms.
Pass 3: translated to C into "/tmp/stapsgSbwn/stap_8b6901af9c04b3e4907826ce793aca76_4839_src.c" using 354588virt/65020res/6820shr/59836data kb, in 170usr/10sys/175real ms.
Pass 4: compiled C into "stap_8b6901af9c04b3e4907826ce793aca76_4839.ko" in 2610usr/440sys/2892real ms.
Pass 5: starting run.
5900-Function name:
 0xffffffff81025510 : x86_pmu_event_init+0x0/0x220 [kernel]
5900-Function name:
 0xffffffff81025510 : x86_pmu_event_init+0x0/0x220 [kernel]
5900-Function name:
 0xffffffff81025510 : x86_pmu_event_init+0x0/0x220 [kernel]
5900-Function name:
 0xffffffff81025510 : x86_pmu_event_init+0x0/0x220 [kernel]
5900-Function name:
 0xffffffff81025510 : x86_pmu_event_init+0x0/0x220 [kernel]
5900-Function name:
 0xffffffff81025510 : x86_pmu_event_init+0x0/0x220 [kernel]
5900-Function name:
 0xffffffff81025510 : x86_pmu_event_init+0x0/0x220 [kernel]
5900-Function name:
 0xffffffff81025510 : x86_pmu_event_init+0x0/0x220 [kernel]

即指向的是函数x86_pmu_event_init()。

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使用systemtap调试Linux内核

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