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

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Date:	Thu, 31 May 2012 16:49:20 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.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 22:40 +0200, Peter Zijlstra wrote:
> 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();		+--- Break points applied
> 				|
> for_each() {			|
>   kmap_atomic();		|
>   frob tail			|
>   kunmap_atomic();		|
> }				|
----------------------------------+

Note, for the above time, the entire kernel has breakpoints added, and
every function is taking a hit due to it. By slowing down this process,
the rest of the system *will* be impacted. Ideally, we want to finish it
as quick as possible. Not to mention, the kmap_atomics will be slowed
down by the breakpoints that are attached to them.

-- Steve

> 
> sync-ipi-broadcast();
> 
> for_each() {
>   kmap_atomic();
>   frob head
>   kunmap_atomic();
> };
> 
> sync-ipi-broadcast();
> 
> How is that sending 90k ipis?


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