linux-kernel - Re: [RFC PATCH v2 11/17] cgroup: Implement new thread mode semantics

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Date:   Sat, 3 Jun 2017 06:33:03 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, kernel-team@...com, pjt@...gle.com,
        luto@...capital.net, efault@....de
Subject: Re: [RFC PATCH v2 11/17] cgroup: Implement new thread mode semantics

Hello,

On Fri, Jun 02, 2017 at 04:36:22PM -0400, Waiman Long wrote:
> I wouldn't argue further on that if you insist. However, I still want to

Oh, please don't get me wrong.  I'm not trying to shut down the
discussion or anything.  It's just that whole-scope discussions can
get very meandering and time-consuming when these two issues can be
decoupled from each other without compromising on either.  Let's
approach these issues separately.

> relax the constraint somewhat by abandoning the no internal process
> constraint  when only threaded controllers (non-resource domains) are
> enabled even when thread mode has not been explicitly enabled. It is a
> modified version my second alternative. Now the question is which
> controllers are considered to be resource domains. I think memory and
> blkio are in the list. What else do you think should be considered
> resource domains?

And we're now a bit into repeating ourselves but for controlling of
any significant resources (mostly cpu, memory, io), there gotta be
significant portion of resource consumption which isn't tied to
spcific processes or threads that should be accounted for.  Both
memory and io already do this to a certain extent, but not completely.
cpu doesn't do it at all yet but we usually can't / shouldn't declare
a resource category to be domain-free.

There are exceptions - controllers which are only used for membership
identification (perf and the old net controllers), pids which is
explicitly tied to tasks (note that CPU cycles aren't), cpuset which
is an attribute propagating / restricting controller.

Out of those, the identification uses already aren't affected by the
constraint as they're now all either direct membership test against
the hierarchy or implicit controllers which aren't subject to the
constraint.  That leaves pids and cpuset.  We can exempt them from the
constraint but I'm not quite sure what that buys us given that neither
is affected by requiring explicit leaf nodes.  It'd just make the
rules more complicated without actual benefits.

That said, we can exempt those two.  I don't see much point in it but
we can definitely discuss the pros and cons, and it's likely that it's
not gonna make much difference wichever way we choose.

Thanks.

-- 
tejun