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

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Date:	Thu, 03 Dec 2015 10:36:18 -0800
From:	"Shi, Yang" <yang.shi@...aro.org>
To:	Steven Rostedt <rostedt@...dmis.org>,
	Dave Hansen <dave.hansen@...el.com>
CC:	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 12/2/2015 8:13 PM, Steven Rostedt wrote:
> 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

Yes. My preliminary test just could cover the success case. gup could 
return errno from different a few code path.

> called directly that calls these functions internally and the tracepoint
> can trap the return value.

This will incur more changes in other subsystems (futex, kvm, etc), I'm 
not sure if it is worth making such changes to get 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.

If the return value requirement is not limited to gup, the approach 
sounds more reasonable.

Thanks,
Yang

>
> -- Steve
>

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