Message-ID: <41F194DC.40603@kolivas.org>
Date: Sat, 22 Jan 2005 10:48:44 +1100
From: Con Kolivas <kernel@kolivas.org>
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To: utz lehmann <lkml@s2y4n2c.de>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
       rlrevell@joe-job.com, paul@linuxaudiosystems.com, joq@io.com,
       CK Kernel <ck@vds.kolivas.org>, Andrew Morton <akpm@osdl.org>,
       alexn@dsv.su.se
Subject: Re: [PATCH]sched: Isochronous class v2 for unprivileged soft rt	scheduling
References: <41EEE1B1.9080909@kolivas.org> <1106350245.4442.5.camel@segv.aura.of.mankind>
In-Reply-To: <1106350245.4442.5.camel@segv.aura.of.mankind>
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utz lehmann wrote:
> Hi
> 
> I dislike the behavior of the SCHED_ISO patch that iso tasks are
> degraded to SCHED_NORMAL if they exceed the limit.
> IMHO it's better to throttle them at the iso_cpu limit.
> 
> I have modified Con's iso2 patch to do this. If iso_cpu > 50 iso tasks
> only get stalled for 1 tick (1ms on x86).

Some tasks are so cache intensive they would make almost no forward 
progress running for only 1ms.

> Fortunately there is a currently unused task prio (MAX_RT_PRIO-1) [1]. I

Your implementation is not correct. The "prio" field of real time tasks 
is determined by MAX_RT_PRIO-1-rt_priority. Therefore you're limiting 
the best real time priority, not the other way around.

Throttling them for only 1ms will make it very easy to starve the system 
  with 1 or more short running (<1ms) SCHED_NORMAL tasks running. Lower 
priority tasks will never run.

Cheers,
Con

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