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

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Date:	Mon, 16 May 2011 05:32:31 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>, Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [patch 04/15] sched: validate CFS quota hierarchies

On Mon, May 16, 2011 at 2:43 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> 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:
-----------------------------------------------------------
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.
-----------------------------------------------------------
(Previously: https://lkml.org/lkml/2011/2/24/477)
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