Как понять «$ location» в скрипте Systemtap?

Question

Systemtap сценарий:

# Array to hold the list of drop points we find 
global locations 

# Note when we turn the monitor on and off 
probe begin { printf("Monitoring for dropped packets\n") } 
probe end { printf("Stopping dropped packet monitor\n") } 

# increment a drop counter for every location we drop at 
#probe kernel.trace("kfree_skb") { locations[$location] <<< 1 } 

# Every 5 seconds report our drop locations 
probe timer.sec(5) 
{ 
     printf("\n") 
     foreach (l in locations-) { 
       printf("%d packets dropped at location %p\n", 
          @count(locations[l]), l) 
     } 
     delete locations 
} 

и исходный код kfree_skb() является:

void kfree_skb(struct sk_buff *skb) 
{ 
    if (unlikely(!skb)) 
     return; 
    if (likely(atomic_read(&skb->users) == 1)) 
     smp_rmb(); 
    else if (likely(!atomic_dec_and_test(&skb->users))) 
     return; 
    trace_kfree_skb(skb, __builtin_return_address(0)); 
    __kfree_skb(skb); 
} 

Я просто хочу знать, где находится $location от? И
Какая связь между $location и kfree_skb()?
Спасибо.

Answer 1

Согласно странице stap.1 людей:

Many types of probe points provide context variables, which are 
    run-time values, safely extracted from the kernel or userspace 
    program being probed. These are pre‐ fixed with the $ 
    character. The CONTEXT VARIABLES section in stapprobes(3stap) 
    lists what is available for each type of probe point. 

Согласно странице stapprobes.3stap людей:

KERNEL TRACEPOINTS 

    This family of probe points hooks up to static probing 
    tracepoints inserted into the kernel or modules. [...] 

    Tracepoint probes look like: kernel.trace("name"). The 
    tracepoint name string, which may contain the usual wildcard 
    characters, is matched against the names defined by the kernel 
    developers in the tracepoint header files. 

    The handler associated with a tracepoint-based probe may read 
    the optional parame‐ ters specified at the macro call site. 
    [...] For example, the tracepoint probe kernel.trace("sched_switch") 
    provides the parameters $rq, $prev, and $next. [...] 

    The name of the tracepoint is available in $$name, and a string 
    of name=value pairs for all parameters of the tracepoint is 
    available in $$vars or $$parms. 

В соответствии с исходными кодами ядра Linux кода:

% cd net/core 
% git grep trace_kfree_skb 

dev.c: [...] 
drop_monitor.c: [...] 
skbuff.c: [...] 

% cd ../../include/trace/events 
% git grep -A5 'TRACE_EVENT.*kfree_skb' 

skb.h:TRACE_EVENT(kfree_skb, 
skb.h- 
skb.h- TP_PROTO(struct sk_buff *skb, void *location), 
skb.h- 
skb.h- TP_ARGS(skb, location), 
skb.h-