Date: Wed, 6 Jun 2007 20:31:28 -0500
From: Matt Mackall <mpm@selenic.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: markh@compro.net, linux-kernel@vger.kernel.org,
       Oleg Nesterov <oleg@tv-sign.ru>, Mark Hounschell <dmarkh@cfl.rr.com>,
       Ingo Molnar <mingo@elte.hu>
Subject: Re: floppy.c soft lockup
Message-ID: <20070607013128.GW11166@waste.org>
References: <20070601151605.GA108@tv-sign.ru> <4660534E.6050903@cfl.rr.com> <20070601183642.GA92@tv-sign.ru> <466078FF.2080508@cfl.rr.com> <20070602123030.GA719@tv-sign.ru> <4661D698.5040009@cfl.rr.com> <20070603081417.GA81@tv-sign.ru> <46641B16.9090701@compro.net> <4666B2A4.7090603@compro.net> <20070606102828.752bf955.akpm@linux-foundation.org>
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On Wed, Jun 06, 2007 at 10:28:28AM -0700, Andrew Morton wrote:
> On Wed, 06 Jun 2007 09:12:04 -0400 Mark Hounschell <markh@compro.net> wrote:
> 
> > > 
> > > As far as a 100% CPU bound task being a valid thing to do, it has been 
> > > done for many years on SMP machines. Any kernel limitation on this 
> > > surely must be considered a bug? 
> > > 
> > 
> > Could someone authoritatively comment on this? Is a SCHED_RR/SCHED_FIFO
> > 100% Cpu bound process supported in an SMP env on Linux? (vanilla or -rt)
> 
> It will kill the kernel, sorry.
> 
> The only way in which we can fix that is to allow kernel threads to preempt
> rt-priority userspace threads.  But if we were to do that (to benefit the
> few) it would cause _all_ people's rt-prio processes to experience glitches
> due to kernel activity, which we believe to be worse.
> 
> So we're between a rock and a hard place here.
> 
> If we really did want to solve this then I guess the kernel would need some
> new code to detect a 100%-busy rt-prio process and to then start premitting
> preemption of it for kernel thread activity.  That detector would need to
> be smart enough to detect a number of 100%-busy rt-prio processes which are
> yielding to each other, and one rt-prio process which keeps forking others,
> etc.  It might get tricky.

The usual alternative is to manually chrt the relevant kernel threads
to RT priority and adjust the priority scheme of their processes appropriately.

-- 
Mathematics is the supreme nostalgia of our time.
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