Re: [PATCH 05/10] Make total_forks per-cgroup

[Glauber Costa]

On 10/12/2011 04:59 PM, Frederic Weisbecker wrote:
> On Wed, Oct 12, 2011 at 11:35:50AM +0400, Glauber Costa wrote:
> > On 10/12/2011 03:45 AM, Frederic Weisbecker wrote:
> > > On Wed, Oct 05, 2011 at 04:12:00PM +0400, Glauber Costa wrote:
> > > > On 10/05/2011 01:05 PM, Peter Zijlstra wrote:
> > > > > On Sun, 2011-10-02 at 23:21 +0400, Glauber Costa wrote:
> > > > > > This patch counts the total number of forks per-cgroup.
> > > > > > The information is propagated to the parent, so the total
> > > > > > number of forks in the system, is the parent cgroup's one.
> > > > > >
> > > > > > To achieve that, total_forks is made per-cpu. There is no
> > > > > > particular reason to do that, but by doing this, we are
> > > > > > able to bundle it inside the cpustat structure already
> > > > > > present.
> > > > >
> > > > > I think fweisbec is also doing something with forks and cgroups.
> > > >
> > > > I am all ears...
> > > >
> > > > Frederic, does it conflict with what you're doing ?
> > >
> > > I don't know if that really conflicts but I'm working
> > > on a cgroup subsystem that is able to control the number
> > > of tasks running in a subsystem.
> > >
> > > It consists in two new files added:
> > >
> > > * tasks.usage
> > > * tasks.limit
> > >
> > > The subsystem rejects any new fork or migration into the
> > > cgroup when tasks.usage>   tasks.limit
> > >
> > > So tasks.usage can inform you about the number of tasks
> > > running into the cgroup. It's not strictly the number
> > > of forks because it also counts the tasks that have been
> > > attached to the cgroup.
> > >
> > > But something like a tasks.fork file could be implemented
> > > in that subsystem as well.
> > >
> > > It depends on what you need.
> >
> > So the specific piece I am working on, is to display /proc/stat
> > information per-cgroup. One of the many fields it has, is
> > total_forks.
> > (it is actually just a small part of the series)
> > So instead of tracking how many forks the system has in total, I'll
> > track it per-cpucgroup.
> >
> > So I don't think we conflict at all. At the very least, IIUC, you
> > are planning to account and check *before* a fork happens, right?
> > This particular stat is incremented after it already succeeded.
>
> That doesn't make much difference since the accounting is cancelled
> in case the fork is finally rejected.
>
> But probably having a simple accouting like you do involves less
> overhead than the whole task counter subsystem.
>
> Is your counting propagated to the parents in a hierarchy?
> For example if A is parent cgroup of B and C, does A account the
> forks happening in B and C?

Yes.
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