linux-kernel - Re: [RFC][PATCH] tracing/x86: Save CR2 before tracing irqsoff on error

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Date:   Thu, 21 Mar 2019 15:50:06 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Juergen Gross <jgross@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Joel Fernandes <joel@...lfernandes.org>,
        He Zhe <zhe.he@...driver.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Clark Williams <williams@...hat.com>
Subject: Re: [RFC][PATCH] tracing/x86: Save CR2 before tracing irqsoff on
 error_entry

On Thu, 21 Mar 2019 20:31:52 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> > 
> > No I didn't. Some users only care about performance, but find memory
> > cheap.  
> 
> Because cache-misses are free?

If I ever did implement this, I would try to get all the data out of
line as much as possible, where only a nop would be inserted:

	jmp lockdep_code
	raw_locking
1:

[..]

lockdep_code:
	do all that lockdep needs
	raw_locking
	jmp 1b

and have that converted to:


	nop // jmp lockdep_code
	raw_locking
1:

when disabled. So the only extra pressure on the icache is the nop.

-- Steve