Re: [patch 04/15] sched: validate CFS quota hierarchies

From: Peter Zijlstra
Date: Tue May 17 2011 - 11:27:19 EST

On Mon, 2011-05-16 at 05:32 -0700, Paul Turner wrote:
> On Mon, May 16, 2011 at 2:43 AM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> >
> > On Tue, 2011-05-03 at 02:28 -0700, Paul Turner wrote:
> > > This behavior may be disabled (allowing child bandwidth to exceed parent) via
> > > kernel.sched_cfs_bandwidth_consistent=0
> >
> > why? this needs very good justification.
> I think it was lost in other discussion before, but I think there are
> two useful use-cases for it:
> Posting (condensed) relevant snippet:

Such stuff should really live in the changelog

> -----------------------------------------------------------
> Consider:
> - I have some application that I want to limit to 3 cpus
> I have a 2 workers in that application, across a period I would like
> those workers to use a maximum of say 2.5 cpus each (suppose they
> serve some sort of co-processor request per user and we want to
> prevent a single user eating our entire limit and starving out
> everything else).
> The goal in this case is not preventing increasing availability within a
> given limit, while not destroying the (relatively) work-conserving aspect of
> its performance in general.
> (...)
> - There's also the case of managing an abusive user, use cases such
> as the above means that users can usefully be given write permission
> to their relevant sub-hierarchy.
> If the system size changes, or a user becomes newly abusive then being
> able to set non-conformant constraint avoids the adversarial problem of having
> to find and bring all of their set (possibly maliciously large) limits
> within the global limit.
> -----------------------------------------------------------

But what about those where they want both behaviours on the same machine
but for different sub-trees?

Also, without the constraints, what does the hierarchy mean?
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