linux-kernel - Re: [PATCH 02/14] sched: add extended scheduling interface. (new ABI)

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Date:	Wed, 27 Nov 2013 14:30:27 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Juri Lelli <juri.lelli@...il.com>, tglx@...utronix.de,
	mingo@...hat.com, rostedt@...dmis.org, oleg@...hat.com,
	fweisbec@...il.com, darren@...art.com, johan.eker@...csson.com,
	p.faure@...tech.ch, linux-kernel@...r.kernel.org,
	claudio@...dence.eu.com, michael@...rulasolutions.com,
	fchecconi@...il.com, tommaso.cucinotta@...up.it,
	nicola.manica@...i.unitn.it, luca.abeni@...tn.it,
	dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...il.com, liming.wang@...driver.com, jkacur@...hat.com,
	harald.gustafsson@...csson.com, vincent.guittot@...aro.org,
	bruce.ashfield@...driver.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 02/14] sched: add extended scheduling interface. (new ABI)

On Wed, Nov 27, 2013 at 02:23:54PM +0100, Ingo Molnar wrote:
> There are 3 main compatibility cases:
> 
>  - the kernel's 'sizeof sched_attr' is equal to sched_attr:size: the 
>    kernel version and user-space version matches, it's a straight ABI 
>    in this case with full functionality.
> 
>  - the kernel's 'sizeof sched_attr' is larger than sched_attr::size 
>    [the kernel is newer than what user-space was built for], in this 
>    case the kernel assumes that all remaining values are zero and acts
>    accordingly.

It also needs to fail sched_getparam() when any of the fields that do
not fit in the smaller struct provided are !0.

>  - the kernel's 'sizeof sched_attr' is smaller than sched_attr::size 
>    [the kernel is older than what user-space was built for]. In this 
>    case the kernel should return -ENOSYS if any of the additional 
>    fields are nonzero. If those are all zero then it will work as if a 
>    smaller structure was passed in.

So the problem I see with this one is that because you're allowed to
call sched_setparam() or whatever it will be called next on another
task; a task can very easily fail its sched_getparam() call.

Suppose the application is 'old' and only supports a subset of the
fields; but its wants to get, modify and set its params. This will work
as long nothing will set anything it doesn't know about.

As soon as some external entity -- say a sysad using schedtool -- sets a
param field it doesn't support the get, modify, set routing completely
fails.

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