[英]What is the default behavior of perf record?
我很清楚perf
总是记录一个或多个事件,并且采样可以是基于计数器或基于时间的。 但是当没有给出-e
和-F
开关时, perf record
的默认行为是什么? perf-record
的手册页没有告诉您它在这种情况下的作用。
性能perf record
中的默认事件选择是在用户空间性能工具中完成的,该工具通常作为 linux kernel 的一部分分发。 With make perf-src-tar-gz
from linux kernel source dir we can make tar gz for quick rebuild or download such tar from https://mirrors.edge.kernel.org/pub/linux/kernel/tools/perf . linux kernel 源代码也有几个在线“LXR”交叉参考查看器,可以像 grep 一样使用来了解性能内部结构。
性能记录有 function 到 select 默认事件列表(evlist): __perf_evlist__add_default
of tools/perf/util/evlist.Z4A8A08F09D37B73795364903B 文件:
int __perf_evlist__add_default(struct evlist *evlist, bool precise)
{
struct evsel *evsel = perf_evsel__new_cycles(precise);
evlist__add(evlist, evsel);
return 0;
}
如果从选项解析的事件为零,则从 perf 记录实现调用: tools/perf/builtin-record.c: int cmd_record()
rec->evlist->core.nr_entries == 0 &&
__perf_evlist__add_default(rec->evlist, !record.opts.no_samples)
并且perf_evsel__new_cycles
将询问硬件事件周期(PERF_TYPE_HARDWARE + PERF_COUNT_HW_CPU_CYCLES),可选 kernel 采样和最大精确度( 检查 man perf-list 中的修饰符,它是使用 PEBS 或 IBS 的 EIP 采样防滑解决方法):
struct evsel *perf_evsel__new_cycles(bool precise)
{
struct perf_event_attr attr = {
.type = PERF_TYPE_HARDWARE,
.config = PERF_COUNT_HW_CPU_CYCLES,
.exclude_kernel = !perf_event_can_profile_kernel(),
};
struct evsel *evsel;
/*
* Now let the usual logic to set up the perf_event_attr defaults
* to kick in when we return and before perf_evsel__open() is called.
*/
evsel = evsel__new(&attr);
evsel->precise_max = true;
/* use asprintf() because free(evsel) assumes name is allocated */
if (asprintf(&evsel->name, "cycles%s%s%.*s",
(attr.precise_ip || attr.exclude_kernel) ? ":" : "",
attr.exclude_kernel ? "u" : "",
attr.precise_ip ? attr.precise_ip + 1 : 0, "ppp") < 0)
return evsel;
}
如果 perf_event_open 失败(无法访问硬件周期采样,例如在没有虚拟化 PMU 的虚拟化环境中),则在tools/perf/builtin-record.c: int record__open()
中故障回复到软件 cpu 时钟采样perf_evsel__fallback()
工具/perf/util/evsel.c :
bool perf_evsel__fallback(struct evsel *evsel, int err,
char *msg, size_t msgsize)
{
if ((err == ENOENT || err == ENXIO || err == ENODEV) &&
evsel->core.attr.type == PERF_TYPE_HARDWARE &&
evsel->core.attr.config == PERF_COUNT_HW_CPU_CYCLES) {
/*
* If it's cycles then fall back to hrtimer based
* cpu-clock-tick sw counter, which is always available even if
* no PMU support.
*/
scnprintf(msg, msgsize, "%s", "The cycles event is not supported, trying to fall back to cpu-clock-ticks");
evsel->core.attr.type = PERF_TYPE_SOFTWARE;
evsel->core.attr.config = PERF_COUNT_SW_CPU_CLOCK;
return true;
} ...
}
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