c-perf中用于计数器的格式说明符是什么

Question

有一个perf stat选项"间隔打印"；，给定为-I N，其中N是用于每N毫秒打印计数器的毫秒间隔(N>=10)，其输出如下：

./perf stat -I 1000 -e event_name
time                       counts unit   event_name
1.001177595                110           xyz

现在，我需要知道用于"；计数单位"；列，该列来自tools/perf/util/stat-display.c。

Answer 1

TL；DR：格式说明符%'18.0f，类型为double的变量。

---

ltrace+gdb的简单检查揭示了用于打印的格式说明符：

$ uname -r
4.19.0-20-amd64
$ sudo ltrace -e '*fprintf*' perf_4.19 stat -I 1000 -e branch-misses
#           timeperf_4.19->__fprintf_chk(0x7fcde97d1680, 1, 0x55a2b46652c0, 4             counts unit events
)                                                          = 32
1.001800886 perf_4.19->__fprintf_chk(0x7fcde97d1680, 1, 0x55a2b46596ab, 0x55a2b4770d89         2,536,128 )                                             = 19
perf_4.19->__fprintf_chk(0x7fcde97d1680, 1, 0x55a2b46596be, 4     )                                                          = 5
perf_4.19->__fprintf_chk(0x7fcde97d1680, 1, 0x55a2b46582ba, 25branch-misses            )                                                         = 25
perf_4.19->__fprintf_chk(0x7fcde97d1680, 1, 0x55a2b46582ba, 35
...

$ sudo gdb --pid $(pidof perf_4.19)
(gdb) b __fprintf_chk
Breakpoint 1 at 0x7fd72c20de60: file fprintf_chk.c, line 26.
(gdb) c
Continuing.
Breakpoint 1, ___fprintf_chk (fp=0x7fd72c2c1680 <_IO_2_1_stderr_>, flag=1, format=0x556d2f5cd6ab "%'18.0f%s") at fprintf_chk.c:26
26  fprintf_chk.c: No such file or directory.
(gdb) x/s 0x556d2f5cd6ab
0x556d2f5cd6ab: "%'18.0f%s"

$ cd linux_source
$ git describe --tags
v4.19
$ rg -g 'tools/perf/**/*' -F "%'18.0f"
tools/perf/builtin-stat.c
1081:           fmt = floor(sc) != sc ? "%'18.2f%s" : "%'18.0f%s";

这里是Linux v4.19的源代码，这里是最新的v5.18.12:

static void abs_printout(struct perf_stat_config *config,
struct aggr_cpu_id id, int nr, struct evsel *evsel, double avg)
{
FILE *output = config->output;
double sc =  evsel->scale;
const char *fmt;
if (config->csv_output) {
fmt = floor(sc) != sc ?  "%.2f%s" : "%.0f%s";
} else {
if (config->big_num)
fmt = floor(sc) != sc ? "%'18.2f%s" : "%'18.0f%s"; // <===
else
fmt = floor(sc) != sc ? "%18.2f%s" : "%18.0f%s";
}
aggr_printout(config, evsel, id, nr);
fprintf(output, fmt, avg, config->csv_sep);
// ...
}

使用的函数是fprintf，格式说明符是%'18.2f或%'18.0f，具体取决于是否有小数要打印。打印发生在此处，打印的变量为double avg。