linux-kernel - Re: ftrace performance impact with different configuration

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Date:	Thu, 29 Dec 2011 21:12:28 +0530
From:	Rabin Vincent <rabin@....in>
To:	Lei Wen <adrian.wenl@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	leiwen@...vell.com
Subject: Re: ftrace performance impact with different configuration

On Thu, Dec 29, 2011 at 14:08, Lei Wen <adrian.wenl@...il.com> wrote:
> 2. Seem dynamic ftrace also could involve some penalty for the running
> system, although it patching the running kernel with nop stub...
>
> For the second item, is there anyone done some research before that
> could zero the cost for the running system when the tracing is not
> enabled yet?

One thing that needs to be fixed (for ARM) is that for the new-style
mcounts, the nop that's currently being done is not really a nop -- it
removes the function call, but there is still an unnecessary push/pop
sequence.  This should be modified to have the push {lr} removed too.
(Two instructions replaced instead of one.)
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