linux-kernel - Re: [PATCH v4.15-rc9] sched, cgroup: Don't reject lower cpu.max on ancestors

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Date:   Thu, 1 Feb 2018 17:49:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH v4.15-rc9] sched, cgroup: Don't reject lower cpu.max on
 ancestors

On Tue, Jan 30, 2018 at 06:56:39AM -0800, Tejun Heo wrote:
> Hello,
> 
> On Tue, Jan 30, 2018 at 11:21:56AM +0100, Peter Zijlstra wrote:
> > afaiu the existing code does exactly the opposite, it forces the
> > descendants to configure less than the parent allows.
> > 
> > You're taking out an error condition and silently allowing descentant
> > misconfiguration. How does that make sense?
> 
> Well, they're upper limits, not strict allocations.  The current
> behavior implemented by cpu isn't either a strict allocation or upper
> limits.  It disallows a child from having a value higher than the
> parent (allocation-ish) but the sum of the children is allowed to
> exceed the parent's (limit-ish).

True; but its still weird to have the parent 'promise' something and
then retract that 'promise' later.

> The combination is rather arbitrary and makes it impossible to
> delegate safely (a delegatee can block the delegator from reducing the
> amount resource allocated to the delegatee) while not really
> protecting against overcommit from descendants either.
> 
> We had this sort of input validations in different controllers all in
> their own ways.  In most cases, these aren't well thought out and we
> can't support things like delegation without aligning controller
> behaviors.

I suppose.. 

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>