Messages in this thread | | | Date | Sat, 27 Aug 2011 15:40:40 +0200 | From | Frederic Weisbecker <> | Subject | Re: [RFD] Task counter: cgroup core feature or cgroup subsystem? (was Re: [PATCH 0/8 v3] cgroups: Task counter subsystem) |
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On Fri, Aug 26, 2011 at 08:16:32AM -0700, Paul Menage wrote: > On Wed, Aug 24, 2011 at 10:54 AM, Frederic Weisbecker > <fweisbec@gmail.com> wrote: > > > > It seems your patch doesn't handle the ->fork() and ->exit() calls. > > We probably need a quick access to states of multi-subsystems from > > the task, some lists available from task->cgroups, I don't know yet. > > > > That state is available, but currently only while holding cgroup_mutex > - at least, that's what task_cgroup_from_root() requires. > > It might be the case that we could achieve the same effect by just > locking the task, so the pre-condition for task_cgroup_from_root() > would be either that cgroup_mutex is held or the task lock is held. > > We could extend the signature of cgroup_subsys.fork to include a > reference to the cgroup; for the singly-bindable subsystems this would > be trivially available via task->cgroups; for the multi-bindable > subsystems then for each hierarchy that the subsystem is mounted on > we'd call task_cgroup_from_root() to get the cgroup for that > hierarchy. So multi-bindable subsystems with fork/exit callbacks would > get called once for each mounted instance of the subsystem. > > This would still make the task counter subsystem a bit painful - it > would read_lock a global rwlock (css_set_lock) on every fork/exit in > order to find the cgroup to charge/uncharge. I'm not sure how painful > that would be on a big system. If that were a noticeable performance > problem, we could have a variable-length extension on the end of > css_set that contains a list of hierarchy_index/cgroup pairs for any > hierarchies that had multi-bindable subsystems (or maybe for all > hierarchies, for simplicity). This would make creating a css_set a > little bit more complicated, but overall shouldn't be too painful, and > would make the problem of finding a cgroup for a given hierarchy > trivial.
Oh you're right. My first idea was to reference multi-bindable subsystem states in cgroup_subsys_state, like it's done currently for singletons subsystems. But this indeed require cgroup_mutex or task_lock. And only the last one look sensible in fork/exit path. And if that becomes a scalability problem we can still have a dedicated lock for cgroup attach/detach on tasks.
Whatever we do, we need that lock. So we can pick your solution that references cgroups that belong to multi-bindable subsystems for a given task in css_set, or we can have tsk->cgroups->subsys[] a variable size array that references 1 * singletons and N * multi bindable subsystems, N beeing the number of hierarchies that use a given subsystem.
What do you think?
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