linux-kernel - Re: [patch] rt: fix ipi kfree(), introduce IPI

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Date:	Thu, 12 Feb 2009 09:22:07 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Thomas Gleixner <tglx@...x.de>,
	LKML <linux-kernel@...r.kernel.org>,
	rt-users <linux-rt-users@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Carsten Emde <ce@...g.ch>, Clark Williams <williams@...hat.com>
Subject: Re: [patch] rt: fix ipi kfree(), introduce IPI_SOFTIRQ


* Ingo Molnar <mingo@...e.hu> wrote:

> ------------------>
> Subject: rt: fix ipi kfree(), introduce IPI_SOFTIRQ
> From: Ingo Molnar <mingo@...e.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
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