linux-kernel - Re: [PATCH V2 2/7] mm/gup: add gup trace points

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Date:	Wed, 2 Dec 2015 23:13:48 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	Yang Shi <yang.shi@...aro.org>, akpm@...ux-foundation.org,
	mingo@...hat.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH V2 2/7] mm/gup: add gup trace points

On Wed, 2 Dec 2015 15:36:50 -0800
Dave Hansen <dave.hansen@...el.com> wrote:

> On 12/02/2015 02:53 PM, Yang Shi wrote:
> > diff --git a/mm/gup.c b/mm/gup.c
> > index deafa2c..10245a4 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -13,6 +13,9 @@
> >  #include <linux/rwsem.h>
> >  #include <linux/hugetlb.h>
> >  
> > +#define CREATE_TRACE_POINTS
> > +#include <trace/events/gup.h>
> > +
> >  #include <asm/pgtable.h>
> >  #include <asm/tlbflush.h>  
> 
> This needs to be _the_ last thing that gets #included.  Otherwise, you
> risk colliding with any other trace header that gets implicitly included
> below.

Agreed.

> 
> > @@ -1340,6 +1346,8 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> >  					start, len)))
> >  		return 0;
> >  
> > +	trace_gup_get_user_pages_fast(start, nr_pages, write, pages);
> > +
> >  	/*
> >  	 * Disable interrupts.  We use the nested form as we can already have
> >  	 * interrupts disabled by get_futex_key.  
> 
> It would be _really_ nice to be able to see return values from the
> various gup calls as well.  Is that feasible?

Only if you rewrite the functions to have a single return code path
that we can add a tracepoint too. Or have a wrapper function that gets
called directly that calls these functions internally and the tracepoint
can trap the return value.

I can probably make function_graph tracer give return values, although
it will give a return value for void functions as well. And it may give
long long returns for int returns that may have bogus data in the
higher bits.

-- Steve

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