Message-ID: <41677862.2020806@kolivas.org>
Date: Sat, 09 Oct 2004 15:34:26 +1000
From: Con Kolivas <kernel@kolivas.org>
User-Agent: Mozilla Thunderbird 0.8 (X11/20040913)
MIME-Version: 1.0
To: Lee Revell <rlrevell@joe-job.com>
Cc: Ingo Molnar <mingo@elte.hu>, linux-kernel <linux-kernel@vger.kernel.org>,
       "K.R. Foley" <kr@cybsft.com>, Rui Nuno Capela <rncbc@rncbc.org>,
       Florian Schmidt <mista.tapas@gmx.net>, Mark_H_Johnson@raytheon.com,
       Fernando Pablo Lopez-Lezcano <nando@ccrma.Stanford.EDU>
Subject: Re: voluntary-preempt-2.6.9-rc3-mm3-T3
References: <20040921071854.GA7604@elte.hu> <20040921074426.GA10477@elte.hu>	 <20040922103340.GA9683@elte.hu> <20040923122838.GA9252@elte.hu>	 <20040923211206.GA2366@elte.hu> <20040924074416.GA17924@elte.hu>	 <20040928000516.GA3096@elte.hu> <20041003210926.GA1267@elte.hu>	 <20041004215315.GA17707@elte.hu> <20041005134707.GA32033@elte.hu>	 <20041007105230.GA17411@elte.hu>	 <1097297824.1442.132.camel@krustophenia.net>	 <cone.1097298596.537768.1810.502@pc.kolivas.org>	 <1097299260.1442.142.camel@krustophenia.net>  <416775CD.70706@kolivas.org> <1097299886.1442.145.camel@krustophenia.net>
In-Reply-To: <1097299886.1442.145.camel@krustophenia.net>
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Lee Revell wrote:
> On Sat, 2004-10-09 at 01:23, Con Kolivas wrote:
> 
>>Lee Revell wrote:
>>
>>>On Sat, 2004-10-09 at 01:09, Con Kolivas wrote:
>>>
>>>
>>>>Lee Revell writes:
>>>>
>>>>
>>>>
>>>>>On Thu, 2004-10-07 at 06:52, Ingo Molnar wrote:
>>>>>
>>>>>
>>>>>>i've released the -T3 VP patch:
>>>>>>
>>>>>> http://redhat.com/~mingo/voluntary-preempt/voluntary-preempt-2.6.9-rc3-mm3-T3
>>>>>>
>>>>>
>>>>>With VP and PREEMPT in general, does the scheduler always run the
>>>>>highest priority process, or do we only preempt if a SCHED_FIFO process
>>>>>is runnable?
>>>>
>>>>Always the highest priority runnable.
>>>>
>>>
>>>
>>>Hmm, interesting.  Would there be any advantage to a mode where only
>>>SCHED_FIFO tasks can preempt?  This seems like a much lighter way to
>>>solve the realtime problem.
>>
>>No, the linux scheduler has always been preemptible. PREEMPT and VP just 
>>allows it to preempt kernel code paths as well. It could be modified to 
>>do such a thing but apart from real time applications it would perform 
>>very badly overall.
> 
> 
> I am talking about a mode where we only allow a SCHED_FIFO process to
> preempt a kernel code path.  In every other case it works like !PREEMPT.
> 
> This is apparently how kernel preemption worked on SVR4.

Yes it could. If you ask nicely, Ingo might even throw in yet another 
config option in the kernel. It gets messy if multiple people start 
hacking on the same thing when it's under heavy development.

Cheers,
Con

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