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Date: Thu, 22 Jul 2004 09:07:44 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Lee Revell <rlrevell@joe-job.com>, Andrew Morton <akpm@osdl.org>,
       linux-audio-dev@music.columbia.edu, arjanv@redhat.com,
       linux-kernel <linux-kernel@vger.kernel.org>,
       "La Monte H.P. Yarroll" <piggy@timesys.com>
Subject: Re: [linux-audio-dev] Re: [announce] [patch] Voluntary Kernel Preemption Patch
Message-ID: <20040722070743.GA7553@elte.hu>
References: <20040719102954.GA5491@elte.hu> <1090380467.1212.3.camel@mindpipe> <20040721000348.39dd3716.akpm@osdl.org> <20040721053007.GA8376@elte.hu> <1090389791.901.31.camel@mindpipe> <20040721082218.GA19013@elte.hu> <20040721085246.GA19393@elte.hu> <40FE545E.3050300@yahoo.com.au> <20040721154428.GA24374@elte.hu> <40FF48F9.1020004@yahoo.com.au>
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* Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> Given that we're looking for something acceptable for 2.6, how about
> adding
>
> if (rt_task(current))
> 	kick ksoftirqd instead
> 
> Otherwise, what is the performance penalty of doing all softirq
> processing from ksoftirqd?

this is insufficient too. An RT task might be _waiting to run_ and
spending our time in a non-RT context (including the idle task) doing
softirq processing might delay it indefinitely.

what we could do is to add a rq->nr_running_rt and do the deferred
softirq processing unconditionally if (rq->nr_running_rt). I'd still add
a sysctl to make it unconditional for user processes too - if someone
really cares about latency and doesnt want to make all his tasks RT. 
I'll code this up for the next version of the patch.

	Ingo