Message-ID: <41677CBC.7050705@kolivas.org>
Date: Sat, 09 Oct 2004 15:53:00 +1000
From: Con Kolivas <kernel@kolivas.org>
User-Agent: Mozilla Thunderbird 0.8 (X11/20040913)
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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: Preemption model (was 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> <41677862.2020806@kolivas.org> <1097300995.1442.156.camel@krustophenia.net>
In-Reply-To: <1097300995.1442.156.camel@krustophenia.net>
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Lee Revell wrote:
> On Sat, 2004-10-09 at 01:34, Con Kolivas wrote:
> 
>>Lee Revell wrote:
>>
>>>>>>>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.
> 
> 
> Oh, I was not going to post a patch, I don't know the code nearly well
> enough at this point :-).  But it looks pretty straightforward.

I was talking about me ;-)

Cheers,
Con


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