Messages in this thread | | | Date | Mon, 22 Oct 2007 23:06:54 -0700 | From | "Paul Menage" <> | Subject | Re: [PATCH 2/2] CFS CGroup: Report usage |
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On 10/22/07, Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com> wrote: > On Mon, Oct 22, 2007 at 05:49:39PM -0700, Paul Menage wrote: > > +static u64 cpu_usage_read(struct cgroup *cgrp, struct cftype *cft) > > +{ > > + struct task_group *tg = cgroup_tg(cgrp); > > + int i; > > + u64 res = 0; > > + for_each_possible_cpu(i) { > > + unsigned long flags; > > + spin_lock_irqsave(&tg->cfs_rq[i]->rq->lock, flags); > > Is the lock absolutely required here?
I'm not sure, I was hoping you or Ingo could comment on this. But some kind of locking seems to required at least on 32-bit platforms, since sum_exec_runtime is a 64-bit number.
> > Hmm .. I hope the cgroup code prevents a task group from being destroyed while > we are still reading a task group's cpu usage. Is that so?
Good point - cgroups certainly prevents a cgroup itself from being freed while a control file is being read in an RCU section, and prevents a task group from being destroyed when that task group has been read via a task's cgroups pointer and the reader is still in an RCU section, but we need a generic protection for subsystem state objects being accessed via control files too.
Using cgroup_mutex is certainly possible for now, although more heavy-weight than I'd like long term. Using css_get isn't the right approach, I think - we shouldn't be able to cause an rmdir to fail due to a concurrent read.
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