linux-kernel - Re: [RFC V5] Add gup trace points support

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Date:	Tue, 12 Jan 2016 12:00:54 -0800
From:	"Shi, Yang" <yang.shi@...aro.org>
To:	akpm@...ux-foundation.org, rostedt@...dmis.org, mingo@...hat.com
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linaro-kernel@...ts.linaro.org
Subject: Re: [RFC V5] Add gup trace points support

Hi Steven,

Any more comments on this series? How should I proceed it?

Thanks,
Yang


On 12/9/2015 1:22 PM, Yang Shi wrote:
> v5:
> * Fixed a typo introduced by v4 rebase
> * Removed redundant "#define CREATE_TRACE_POINTS" from architecture specifc
>    gup.c
>
> v4:
> * Adopted Steven's suggestion to use "unsigned int" for nr_pages to save
>    space in ring buffer since it is unlikely to have more than 0xffffffff
>    pages are touched by gup in one invoke
> * Remove unnecessray type cast
>
> v3:
> * Adopted suggestion from Dave Hansen to move the gup header include to the last
> * Adopted comments from Steven:
>    - Use DECLARE_EVENT_CLASS and DEFINE_EVENT
>    - Just keep necessary TP_ARGS
> * Moved archtichture specific fall-backable fast version trace point after the
>    do while loop since it may jump to the slow version.
> * Not implement recording return value since Steven plans to have it in generic
>    tracing code
>
> v2:
> * Adopted commetns from Steven
>    - remove all reference to tsk->comm since it is unnecessary for non-sched
>      trace points
>    - reduce arguments for __get_user_pages trace point and update mm/gup.c
>      accordingly
> * Added Ralf's acked-by for patch 4/7.
>
>
> Some background about why I think this might be useful.
>
> When I was profiling some hugetlb related program, I got page-faults event
> doubled when hugetlb is enabled. When I looked into the code, I found page-faults
> come from two places, do_page_fault and gup. So, I tried to figure out which
> play a role (or both) in my use case. But I can't find existing finer tracing
> event for sub page-faults in current mainline kernel.
>
> So, I added the gup trace points support to have finer tracing events for
> page-faults. The below events are added:
>
> __get_user_pages
> __get_user_pages_fast
> fixup_user_fault
>
> Both __get_user_pages and fixup_user_fault call handle_mm_fault.
>
> Just added trace points to raw version __get_user_pages since all variants
> will call it finally to do real work.
>
> Although __get_user_pages_fast doesn't call handle_mm_fault, it might be useful
> to have it to distinguish between slow and fast version.
>