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Date: Thu, 12 Feb 2009 09:22:07 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Frederic Weisbecker <fweisbec@gmail.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@tglx.de>, LKML <linux-kernel@vger.kernel.org>,
       rt-users <linux-rt-users@vger.kernel.org>,
       Steven Rostedt <rostedt@goodmis.org>,
       Peter Zijlstra <peterz@infradead.org>, Carsten Emde <ce@ceag.ch>,
       Clark Williams <williams@redhat.com>
Subject: Re: [patch] rt: fix ipi kfree(), introduce IPI_SOFTIRQ
Message-ID: <20090212082207.GB26838@elte.hu>
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* Ingo Molnar <mingo@elte.hu> wrote:

> ------------------>
> Subject: rt: fix ipi kfree(), introduce IPI_SOFTIRQ
> From: Ingo Molnar <mingo@elte.hu>
> Date: Thu Feb 12 09:06:11 CET 2009
> 
> in 2.6.28 generic_smp_call_function_interrupt() grew a kfree(),
> which is a rather complex, sleepable method under -rt. But the
> IPI code runs as a hardirq - which cannot run such code.
> 
> So defer this work to a softirq context instead. It still stays
> on the same CPU so the percpu IPI assumptions are upheld.

On a second thought ...

I think we could eliminate the kfree() instead, and keep the
atomicity of IPI cross-calls. Linus expressed doubts about
the IPI kmalloc()/kfree() pair we do in the generic SMP IPI
code, suggesting that it probably does not help performance
all that much - so such a change might be upstream-able as well
and would keep -rt closer to mainline.

	Ingo