Messages in this thread | | | Date | Wed, 13 Jul 2011 15:50:42 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 5/7] cgroups: Ability to stop res charge propagation on bounded ancestor |
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On Tue, Jul 12, 2011 at 09:11:31AM +0900, KAMEZAWA Hiroyuki wrote: > On Mon, 11 Jul 2011 16:15:04 +0200 > Frederic Weisbecker <fweisbec@gmail.com> wrote: > > > Moving a task from a cgroup to another may require to substract > > its resource charge from the old cgroup and add it to the new one. > > > > For this to happen, the uncharge/charge propagation can just stop > > when we reach the common ancestor for the two cgroups. Further > > the performance reasons, we also want to avoid to temporarily > > overload the common ancestors with a non-accurate resource > > counter usage if we charge first the new cgroup and uncharge the > > old one thereafter. This is going to be a requirement for the coming > > max number of task subsystem. > > > > To solve this, provide a pair of new API that can charge/uncharge > > a resource counter until we reach a given ancestor. > > > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> > > Cc: Paul Menage <menage@google.com> > > Cc: Li Zefan <lizf@cn.fujitsu.com> > > Cc: Johannes Weiner <hannes@cmpxchg.org> > > Cc: Aditya Kali <adityakali@google.com> > > > Hmm, do you have the number to show the benefit of this new function ? > And....tasks is moving among cgroups so frequently as to show the benefit > of this function in your environment ??
So the benefit is not really in the optimization, although that's a side effect.
Let me clarify the point in the changelog. Imagine we have these cgroups:
A (usage = 2, limit = 2) | / \ / \ / \ / \ / \ / \ B C (usage = 1, limit = 2) (usage = 1, limit = 2)
The usage in A is the accumulation of the usage in B and C. Imagine i want to move a task from C to B. This should work well. We need to first check if we can charge B and do it, and then later uncharge C.
But if we do:
err = res_counter_charge(B) if (err) exit res_counter_uncharge(C)
it is going to fail because charging B will also charge A. And A will refuse because it's already full. Ideally we should first uncharge C and then charge B, so that A doesn't reject:
res_counter_uncharge(C) err = res_countrer_charge(B) if (err) res_counter_charge(C)
The problem is that if charging B fails we need to rollback on C, but it might be too late as a fork might have happen inside C since we uncharged it, so we couldn't charge it back.
So the only solution is to first charge B but stop the charge propagation on A. And then uncharge on C but stop uncharge on A.
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