linux-kernel - Re: [sched-devel, patch-rfc] rework of "prioritize non-migratable tasks over migratable ones"

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Date:	Fri, 13 Jun 2008 15:08:04 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Dmitry Adamushko <dmitry.adamushko@...il.com>
Cc:	Gregory Haskins <ghaskins@...ell.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [sched-devel, patch-rfc] rework of "prioritize non-migratable
	tasks over migratable ones"

On Wed, 2008-06-11 at 12:05 +0200, Dmitry Adamushko wrote:
> 2008/6/11 Peter Zijlstra <a.p.zijlstra@...llo.nl>:
> > On Wed, 2008-06-11 at 00:58 +0200, Dmitry Adamushko wrote:
> >> Hi Gregory,
> >>
> >>
> >> regarding this commit: 45c01e824991b2dd0a332e19efc4901acb31209f
> >>
> >>
> >> I think we can do it simpler. Please take a look at the patch below.
> >>
> >> Instead of having 2 separate arrays (which is + ~800 bytes on x86_32 and twice so on x86_64),
> >> let's add "exclusive" (the ones that are bound to this CPU) tasks to the head of the queue
> >> and "shared" ones -- to the end.
> >>
> >> In case of a few newly woken up "exclusive" tasks, they are 'stacked' (not queued as now), meaning that
> >> a task {i+1} is being placed in front of the previously woken up task {i}. But I don't think that
> >> this behavior may cause any realistic problems.
> >
> > Doesn't this violate POSIX ?
> >
> 
> If so, then the idea of "prioritize non-migratable tasks over
> migratable ones" violates it, not just an artefact of this particular
> implementation.
> 
> No matter which implementation is used, we have a situation when a
> woken-up single-CPU-bound task (let's call it 'p') can preempt a
> current task with effects as follows:
> 
> - 'current' is not guaranteed to get another CPU;
> 
> - there might have been other pending tasks (of equal prio) on this
> queue. As a result, 'p' starts running before them violating currently
> used (explicitly requested by POSIX?) round-robin behavior.

> We may just consider dropping this idea completely.
> (my 0.02$)

If we cannot guarantee POSIX compliant scheduling I think we should get
rid of this.

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