[Devel] Re: [PATCH 0/2] CGroups: cgroup member list enhancement/fix

[Balbir Singh]

* menage at google.com <menage at google.com> [2009-07-13 23:49:16]:

> On Mon, Jul 13, 2009 at 10:56 PM, Balbir Singh<balbir at linux.vnet.ibm.com> wrote:
> >>
> >> Waiting for the next scheduling point might be too long, since a
> >> thread can block for arbitrary amounts of time and keeping the marker
> >> around for arbitrary time (unless we add a new task_struct field)
> >> would be tricky. Marking the cgroup or tgid as being migrated which
> >> then triggers the extra synchronization in the fork path (but which
> >> isn't needed at other times) is probably where we'll end up.
> >
> >
> > Hmm... but we would not need that information till we schedule the
> > tasks, adding a field to task_struct is what I had in mind.
> 
> Waiting until schedule to move the threads would result in them still
> showing up in the old "tasks" file until they next ran, which would be
> confusing and misleading.
>

Yes, agreed, even first access to the data struture based lazy
migration might not be too helpful, since it can be very context
dependent.
 
> As a first cut, we were planning to add an rwsem that gets taken for
> read in cgroup_fork(), released in cgroup_post_fork(), and taken for
> write when moving an entire process to a new cgroup; not ideal
> performance-wise, but safe.
> 
> If adding a field to task_struct is an option, then the rwsem could be
> per thread-group leader, which would reduce contention.
>

We should also document that moving large processes with several
threads can be expensive.
 

-- 
	Balbir
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