Hello, Peter.

On Tue, May 19, 2015 at 05:16:59PM +0200, Peter Zijlstra wrote:

.gitconfig:

[diff "default"]
        xfuncname = "^[[:alpha:]$_].*[^:]$"

Will avoid keying on labels like that and show us this is
__cgroup_procs_write().

Ah, nice trick.

So my only worry with this patch-set is that these operations will be
hugely expensive.

Now it looks like the cgroup_update_dfl_csses() thing is very rare, its
when you change which controllers are active in a given subtree under
the uber-l337-super-comount design.

The other one, __cgorup_procs_write() is every /procs, /tasks write to a
cgroup, and that does worry me, this could be a somewhat common thing.

The Changelog states task migration is a cold path, but is tens of
miliseconds per task really no problem?

The latency is bound by synchronize_sched_expedited().  Given the way
cgroups are used in majority of setups (process migration happening
only during service / session setups), I think this should be okay.

I agree that something which is closer to lglock in characteristics
would fit the workload better tho.  If this actually becomes a
problem, we can come up with a different percpu locking scheme which
puts a bit more overhead on the reader side to reduce the latency /
overhead on the writer side which shouldn't be that difficult but
let's see whether we need to get there at all.

Thanks.

-- 
tejun