Re: [PATCH 1/2] perf_events: add cgroup support (v8)

[Li Zefan <lizf@cn.fujitsu.com>]

>>> while there is already cgroup monitoring active. In that case and if we do not
>>> want to wait until there is at least one ctxsw on all CPUs, then we have to
>>> check if the other threads are not already running on the other CPUs.If so,
>>> we need to do a cgroup switch on those CPUs. Otherwise, we have nothing to
>>> do. Am I getting this right?
>>
>> Right, so if any of those tasks is currently running, that cpu will be
>> monitoring their old cgroup, hence we send an IPI to flip cgroups.
>>
> I have built a test case where this would trigger. I launched a multi-threaded
> app, and then I move the pid into a cgroup via: echo PID >/cgroup/tests/tasks.
> I don't see any perf_cgroup move beyond the PID passed.
> 
> I looked at kernel/cgroup.c and I could not find a invocation of
> ss->attach() that
> would pass threadgroup = true. So I am confused here.
> 
> I wonder how the cgroupfs 'echo PID >tasks' interface would make the distinction
> between PID and TID. It seems possible to move one thread of a multi-threaded
> process into a cgroup but not the others.
> 

You can do this:

	# echo PID > cgroup.procs

When the patchset that implements the above feature is accepted. See:

	https://lkml.org/lkml/2011/2/7/418

The below commit that confused you is actually a part of the above patchset,
but it sneaked into the kernel accidentally:

commit be367d09927023d081f9199665c8500f69f14d22
Author: Ben Blum <bblum@google.com>
Date:   Wed Sep 23 15:56:31 2009 -0700

    cgroups: let ss->can_attach and ss->attach do whole threadgroups at a time