linux-kernel - Re: [PATCH 1/5] ftrace: Synchronize variable setting with breakpoints

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Date:	Thu, 31 May 2012 22:40:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"H. Peter Anvin" <hpa@...or.com>, Dave Jones <davej@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>, acme <acme@...stprotocols.net>
Subject: Re: [PATCH 1/5] ftrace: Synchronize variable setting with
 breakpoints

On Thu, 2012-05-31 at 16:37 -0400, Steven Rostedt wrote:
> On Thu, 2012-05-31 at 22:26 +0200, Peter Zijlstra wrote:
> 
> > Right, but when you loose stop-machine you could simply do 30k
> > kmap_atomic/kunmap_atomic's consecutively since you're not holding
> > anybody up.
> 
> It requires 3 IPIs per update too. Thus that's 90,000 IPIs you are
> blasting^Wsending to all CPUs.

Uhm, no. 

for_each() {
  kmap_atomic()
  frob int3
  kunmap_atomic();
}

sync-ipi-broadcast();

for_each() {
  kmap_atomic();
  frob tail
  kunmap_atomic();
}

sync-ipi-broadcast();

for_each() {
  kmap_atomic();
  frob head
  kunmap_atomic();
};

sync-ipi-broadcast();

How is that sending 90k ipis?
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