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

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Message-ID: <20131127141334.GA13532@twins.programming.kicks-ass.net>
Date:	Wed, 27 Nov 2013 15:13:34 +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 03:01:43PM +0100, Ingo Molnar wrote:
> > 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.
> 
> There are two approaches to this that I can see:
> 
> 1)
> 
> allow partial information to be returned to user-space, for existing 
> input parameters. The new fields won't be displayed, but the tool 
> doesn't know about them anyway so it's OK. The tool can still display 
> all the other existing parameters.

But suppose a task simply wants to lower/raise its static (FIFO)
priority and does:

sched_getparam(&params);
params.prio += 1;
sched_setparam(&params);

If anything outside of the known param fields was set, we just silently
lost it, for the setparam() call will fill out 0s for the unprovided
fields.

> 2)
> 
> Return -ENOSYS if the 'extra' fields are nonzero. In this case the 
> usual case of old tooling + new kernel will still work just fine, 
> because old tooling won't set the new fields to any non-default 
> (nonzero) values. In the 'mixed' case old tooling will not be able to 
> change/display those fields.
> 
> I tend to lean towards #1. What do you think?

As per the above that can result in silent unexpected behavioural
changes.

I'd much rather be explicit and break hard; so 2).

So mixing new tools (schedtool, chrt etc) and old apps will give pain,
but at least not silent surprises.
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