Re: [PATCH v2] perf/core: Fix installing cgroup event into cpu

[Lin Xiulei]

2018-01-24 17:46 GMT+08:00 Peter Zijlstra <peterz@xxxxxxxxxxxxx>:
> On Wed, Jan 24, 2018 at 05:19:34PM +0800, Lin Xiulei wrote:
>> Sure, and I consider this "OK" works for "What goes wrong if we leave
>> it set?". : )
>
> It would be good if you inspect the code for the case of leaving
> cpuctx->cgrp set with no cgroup events left -- AND -- put a blurb about
> what you found in your new Changelog.
>


I have some test cases for this issue, I don't know if it's good to
put those in changelog


reproduction as below

Step 1

Create program for measuring, write below to the file d.py

```
while True:
    sumup = 0
    for i in range(10000000):
        sumup += i
```

Step 2

Create cgroup path and run relative program

```
mkdir /sys/fs/cgroup/perf_event/test1
mkdir /sys/fs/cgroup/perf_event/test2

python d.py &
echo $! > /sys/fs/cgroup/perf_event/test1/cgroup.procs

python d.py &
echo $! > /sys/fs/cgroup/perf_event/test2/cgroup.procs
```

Step 3

```
perf stat -e cycles -G test1 -e cycles -G test2  -a sleep 1
```

You would see output like below

```

 Performance counter stats for 'system wide':

     2,161,022,123      cycles                    test1
       138,626,073      cycles                    test2

       1.001858328 seconds time elapsed
```

The result of test2 is much less than test1, which happens commonly.
Just because of what I mentioned above that a second event couldn't be
activated immediately, that cases some loss

> I suspect it works out and something like perf_cgroup_switch() will fix
> things up for us later, but double check and test.

exactly, the case above wouldn't have any result if no
perf_cgroup_switch()  happened.