On Tue, Jun 26, 2012 at 12:14:50PM -0700, Tejun Heo wrote:

Hello, Johannes.

On Tue, Jun 26, 2012 at 08:55:42PM +0200, Johannes Weiner wrote:

quoted

quoted

2. Mark flat hierarchy deprecated and produce a warning message if
   memcg is mounted w/o hierarchy option for a year or two.

I think most of us assume that the common case is either not nesting
directories or still working with hierarchy support actually enabled.

How do we know that?  At least from what I see from blkcg usage,
people do crazy stuff when given crazy interface and we've been
providing completely crazy interface for years.  We cannot switch that
implicitly - the changed default behavior is drastically different and
even could be difficult to chase down.  Transitions towards good
behavior are good but they have to be explicit.

I can't really argue against this because this can't be anything but a
handwaving contest.  We simply have no data.

Backward compatibility is a really good idea but I think it depends on
the situation:

  I don't expect such a configuration to happen by accident because
  this behaviour is against fundamental expectations users have toward
  a filesystem, which is the hierarchical nature of nested entities.

  I also don't expect it to happen intentionnaly, simply because there
  is nothing to gain from a more complicated nested setup.

  And because there is nothing to gain, it is in addition really
  trivial to fix the insane setups by simply undoing the nesting,
  there is no downside for them.

The only point where I agree with you is that it may indeed be
non-obvious to detect in case you were relying on the filesystem
hierarchy not being reflected in the controller hierarchy.  But even
that depends on the usecase, whether it's a subtle performance
regression or a total failure to execute a previously supported
workload, which would be pretty damn obvious.

quoted

I would hate if people had to jump through hoops to get the only
behaviour we want to end up supporting and to not get yelled at, it
sends all the wrong signals.

It is inconvenient but that's the price that we have to pay for having
been stupid.  Kernel flipping behavior implicitly is far worse than
any such inconvenience.

That's a matter of taste.  It's not inconvenient for us, but for every
normal person trying to use the feature in a sane way, just to support
hypothetical crazy people when there is a really trivial solution for
the crazy people's problem.

These default behavior flips are something is better handled by
distros / admins than kernel itself.  They can orchestrate the
userland infrastructure and handle and communicate these flips far
better than kernel alone can do.  We can't send out mails to flat
hierarchy users after all.  That's the reason why I'm suggesting mount
option which can't be flipped (sans remount but that's going away too)
once the system is configured by the distro or admin.

The kernel should nudge mainline users towards new behavior while
providing distros / admins a way to move to the new behavior.  The
kernel itself can't flip it like that.

I thought it was the kernel's job to provide sensible defaults and the
distro's job to ease transitions...

quoted

quoted

3. After the existing users had enough chance to move away from flat
   hierarchy, rip out flat hierarchy code and error if hierarchy
   option is not specified.

This description sounds much more sane than what we are actually
trying to ban, which is not a flat structure, but treating groups with
nested directories as equal siblings.

Ah... yeah, flat hierarchy probably is the wrong way to describe it.
I don't know.  Superficial hierarchy?

Not sure there really is an appropriate two-word name for this
artifact.  How about, pardon my German,
Cgroupfilesystemcontrollerhierarchysemanticsdiscrepancy?

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