linux-kernel - Re: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this is for x86

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Date:	Fri, 18 Feb 2011 14:45:19 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Dominique Toupin <dominique.toupin@...csson.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"2nddept-manager@....hitachi.co.jp" 
	<2nddept-manager@....hitachi.co.jp>
Subject: Re: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this is
 for x86_64 right now)

On 02/18/2011 02:39 PM, Andi Kleen wrote:
> On Fri, Feb 18, 2011 at 03:10:18PM -0500, Dominique Toupin wrote:
>>
>> My understanding is stop_machine will stop all processors for many ms.
> 
> I haven't measured it recently, but as long as the callback inside stop
> machine is short it definitely shouldn't be "many ms". The latency
> is bound by how long each CPU needs to answer to an interrupt, so if
> you have some code that disables interrupts for a long time it will take
> long -- but then your realtime response will be already bad. 
> 
> The interrupts are also done in parallel, so the interrupt latencies
> don't add up.
> 
> If all the CPUs answer in a reasonable time it's still not a cheap
> operation, but nothing that takes "many ms". Most likely it's fine
> for most soft real time purposes.
> 

We should also be able to use the breakpoint hack to avoid holding all
the CPUs.  They still need to be interrupted, but that skips the
rendezvous operation.

	-hpa

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