Re: [PATCH] perf parse events: Fix invalid precise_ip handling

From: zhangmengting
Date: Tue Nov 21 2017 - 03:30:39 EST

On 2017/11/20 15:33, Jiri Olsa wrote:
On Wed, Nov 15, 2017 at 09:00:03AM +0800, zhangmengting wrote:
Hi Jiri, thanks for your detailed review, please see my comments inline.

On 2017/11/10 18:39, Jiri Olsa wrote:
On Fri, Nov 10, 2017 at 04:28:37PM +0800, Mengting Zhang wrote:


diff --git a/tools/perf/util/parse-events.c b/tools/perf/util/parse-events.c
index 39b1596..25225f4 100644
--- a/tools/perf/util/parse-events.c
+++ b/tools/perf/util/parse-events.c
@@ -1369,6 +1369,32 @@ struct event_modifier {
int pinned;
+static int perf_get_max_precise_ip(void)
+ int max_precise_ip = 0;
+ struct perf_event_attr attr = {
+ };
+ event_attr_init(&attr);
+ attr.precise_ip = 3;
+ attr.sample_period = 1;
+ while (attr.precise_ip != 0) {
+ int fd = sys_perf_event_open(&attr, 0, -1, -1, 0);
+ if (fd != -1){
+ close(fd);
+ break;
+ }
+ --attr.precise_ip;
+ }
+ max_precise_ip = attr.precise_ip;
+ return max_precise_ip;
we already have a function for that, please check perf_event_attr__set_max_precise_ip
Yeah, I've checked that function. But perf_event_attr__set_max_precise_ip()
will change attr.precise_ip
into the max precise ip available.

In this case, perf should only check whether the user-specified precise_ip
is greater than the max
precise_ip without changing it into maximum. Here, introduce
perf_get_max_precise_ip() to return
the max precise ip and do not change attr.precise_ip.

But you reminds me that perf_get_max_precise_ip() can be simplied.
well both do the same.. probe kernel for max precise level,
so we can keep just one function for that

OKay, I will just keep that function for probing max precise level.

also I think the precise level is not generic for all the events,
so you should check it for specific perf_event_attr later, when
the attr is ready, not in modifier parsing
You are right, and I would check it for specific perf_event_attr.

BTW, I have a question. If the user-specified precise_ip is greater than the
max precise_ip, I wonder
whether it is better to adjust the user-specified precise_ip to the maximum
no, I think that user defined precise level should stay the
way the user wants it.. we don't want more angry users ;-)

Humm, I am sorry for being unclear.
If the user defined precise level is greater than the max precise level,
I think there are two ways to deal with it.
1. return EINVAL to indicate the invalid precise_ip setting;
2. adjust to the max precise level available and give message to indicate the adjustment.

Since we should check user-defined precise level in perf_evsel__config(),
when the attr is ready, I think there is a problem with method 1, if we keep the
user defined precise level stay the way the user wants it.

With method 1, we have to let perf_evsel__config() return value and show errno.
And this change will affect many related functions, such as perf_evlist__config(), and files.

With method 2, we don't need to change the return type of perf_evsel__config().

Am I right?