Re: [PATCH 2/9] perf/core: Add PERF_SAMPLE_CGROUP feature

[Song Liu]

On Mon, Sep 16, 2019 at 4:23 PM Tejun Heo <tj@xxxxxxxxxx> wrote:
>
> Hello, Song.
>
> On Sat, Sep 14, 2019 at 03:02:51PM +0100, Song Liu wrote:
> > I think we don't need a perfect identifier in this case. IIUC, the goal of
>
> I really don't want different versions of imperfect identifiers
> proliferating.
>
> > this patchset is to map each sample with a cgroup name (or full path).
> > To achieve this, we need
> >
> > 1. PERF_RECORD_CGROUP, that maps
> >            "64-bit number" => cgroup name/path
> > 2. PERF_SAMPLE_CGROUP, that adds "64-bit number" to each sample.
> >
> > I call the id a "64-bit number" because it is not required to be a globally
> > unique id. As long as it is consistent within the same perf-record session,
> > we won't get any confusion. Since we add PERF_RECORD_CGROUP
> > for each cgroup creation, we will map most of samples correctly even
> > when the  "64-bit number" is recycled within the same perf-record session.
> >
> > At the moment, I think ino is good enough for the "64-bit number" even
> > for 32-bit systems. If we don't call it "ino" (just call it "cgroup_tag" or
> > "cgroup_id", we can change it when kernfs provides a better 64-bit id.
>
> So, a firm nack on this direction.
>
> > About full path name: The user names the full path here. If the user gives
> > two different workloads the same name/path, we really cannot change that.
> > Reasonable users would be able to make sense from the full path.
>
> I don't see why we wanna be causing this avoidable problem to users.

Sharing some offline discussions with Tejun.

ino in current kernfs is not a good unique ID for cgroup, because it doesn't
increase monotonically. So we need to improve kernfs.

For 64-bit, we can make the ino monotonic, and use it as the ID.
For 32-bit, we need to make the ino monotonic. and use <ino> and <gen>
as the 64-bit ID.

Thanks,
Song