Tejun Heo wrote: > Hello, > > On Mon, Jan 16, 2012 at 04:07:05PM +0800, Li Zefan wrote: >> This is one of the items in the plumber's wish list. >> >> For use cases: >> >>>> What would the use case be for this? >>> >>> Attaching meta information to services, in an easily discoverable >>> way. For example, in systemd we create one cgroup for each service, and >>> could then store data like the main pid of the specific service as an >>> xattr on the cgroup itself. That way we'd have almost all service state >>> in the cgroupfs, which would make it possible to terminate systemd and >>> later restart it without losing any state information. But there's more: >>> for example, some very peculiar services cannot be terminated on >>> shutdown (i.e. fakeraid DM stuff) and it would be really nice if the >>> services in question could just mark that on their cgroup, by setting an >>> xattr. On the more desktopy side of things there are other >>> possibilities: for example there are plans defining what an application >>> is along the lines of a cgroup (i.e. an app being a collection of >>> processes). With xattrs one could then attach an icon or human readable >>> program name on the cgroup. >>> >>> The key idea is that this would allow attaching runtime meta information >>> to cgroups and everything they model (services, apps, vms), that doesn't >>> need any complex userspace infrastructure, has good access control >>> (i.e. because the file system enforces that anyway, and there's the >>> "trusted." xattr namespace), notifications (inotify), and can easily be >>> shared among applications. >>> >>> Lennart >> >> Signed-off-by: Li Zefan <lizf@xxxxxxxxxxxxxx> > > Ummm... I don't feel too good about this. Why can't those extra > information be kept somewhere else? Overloading cgroupfs as storage > for essentially opaque userland information doesn't seem like a good > idea to me. > Long ago Paul M toyed with a patch that adds a control file for userspace to read/write per-cgroup user information, but there were no use cases. This patchset has a similar purpose, but this interface is more flexable and easier to use, and we do have systemd as a use case this time. I'll let Lennart answer if we can easily live without this. Furthermore, I noticed tmpfs also implemented xattr support, and we should be able to share most of the code, which makes the cost for having this feature smaller. -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
