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

[Tejun Heo]

Hello,

On Sat, Aug 31, 2019 at 12:03:26PM +0900, Namhyung Kim wrote:
> Hmm.. it looks hard to use fhandle as the identifier since perf
> sampling is done in NMI context.  AFAICS the encode_fh part seems ok
> but getting dentry/inode from a kernfs_node seems not.
> 
> I assume kernfs_node_id's ino and gen are same to its inode's.  Then
> we might use kernfs_node for encoding but not sure you like it ;-)

Oh yeah, the whole cgroup id situation is kinda shitty and it's likely
that it needs to be cleaned up a bit for this to be used widely.  The
issues are...

* As identifiers, paths sucks.  It's too big and unwieldy and can be
  rapidly reused for different instances.

* ino is compact but can't be easily mapped to path from userland and
  also not unique.

* The fhandle identifier - currently ino+gen - is better in that it's
  finite sized and compact and can be efficiently mapped to path from
  userspace.  It's also mostly unique.  However, the way gen is
  currently generated still has some chance of the same ID getting
  reused and it isn't easily accessible from inside the kernel right
  now.

Eventually, where we wanna be at is having a single 64bit identifier
which can be easily used everywhere.  It should be pretty straight
forward on 64bit machines - we can just use monotonically increasing
id and use it for everything - ino, fhandle and internal cgroup id.
On 32bit, it gets a bit complicated because ino is 32bit, so it'll
need a custom allocator which bumps gen when the lower 32bit wraps and
skips in-use inos.  Once we have that, we can use that for cgrp->id
and fhandle and derive ino from it.

This is on the to-do list but obviously hasn't happened yet.  If you
wanna take on it, great, but, otherwise, what can be done now is
either moving gen+ino generation into cgroup and tell kernfs to use it
or copy gen+ino into cgroup for easier access.  The former likely is
the better approach given that it brings us closer to where we wanna
be eventually.

Thanks.

-- 
tejun