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

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Date:   Thu, 21 Mar 2019 20:31:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.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, Mar 21, 2019 at 02:55:51PM -0400, Steven Rostedt wrote:
> On Thu, 21 Mar 2019 19:28:30 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Thu, Mar 21, 2019 at 02:10:20PM -0400, Steven Rostedt wrote:
> > > On Thu, 21 Mar 2019 11:05:06 -0700
> > > Andy Lutomirski <luto@...capital.net> wrote:
> > >   
> > > > In the long run, I think the right solution is to rewrite even more of
> > > > this mess in C.  We really ought to be able to put the IRQ flag
> > > > tracing and the context tracking into C code.  
> > > 
> > > And once we do that, we can work on getting the irq tracing
> > > incorporated into a jump_label type that we could possibly enable
> > > lockdep at start up, and then disable it later, even on production
> > > systems! That is, to be able to turn it off and bring the system back
> > > up to full speed.  
> > 
> > You forget the stupid amount of data bloat that lockdep brings.
> 
> No I didn't. Some users only care about performance, but find memory
> cheap.

Because cache-misses are free?