linux-kernel - Re: [patch 04/11] Introduce virtual debug register in thread_struct and wrapper-routines around process related functions

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Message-ID: <20090310170607.GB22897@elte.hu>
Date:	Tue, 10 Mar 2009 18:06:07 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	prasad@...ux.vnet.ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Roland McGrath <roland@...hat.com>
Subject: Re: [patch 04/11] Introduce virtual debug register in
	thread_struct and wrapper-routines around process related functions


* Alan Stern <stern@...land.harvard.edu> wrote:

> On Tue, 10 Mar 2009, Ingo Molnar wrote:
> 
> > and why is this:
> > 
> > > @@ -427,13 +427,9 @@ struct thread_struct {
> > >  	unsigned long		ip;
> > >  	unsigned long		fs;
> > >  	unsigned long		gs;
> > > -	/* Hardware debugging registers: */
> > > -	unsigned long		debugreg0;
> > > -	unsigned long		debugreg1;
> > > -	unsigned long		debugreg2;
> > > -	unsigned long		debugreg3;
> > > -	unsigned long		debugreg6;
> > > -	unsigned long		debugreg7;
> > > +	/* Hardware breakpoint info */
> > > +	unsigned long	vdr6;
> > > +	struct thread_hw_breakpoint	*hw_breakpoint_info;
> > 
> > detached from thread_struct? There's a lot of complications 
> > (alloc/free, locking, etc.) from this for no good reason - the 
> > hardware-breakpoints info structure is alway per thread and is 
> > quite small, so there's no reason not to embedd it directly 
> > inside thread_struct.
> 
> The only reason for separating it out was to avoid bogging 
> down the vast majority of threads which aren't debugged.  If 
> you think the extra overhead isn't worth worrying about then 
> the hw-breakpoint info structure can be embedded.

yeah. This new facility is barely used, and such things should 
always strive for 100% dumb simplicity. If the overhead of that 
structure is ever a problem we can allocate it dynamically. (but 
generally it's just not worth the pain)

	Ingo
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