linux-kernel - Re: [PATCH 1/2] mm: page_alloc.c: Add tracepoints for slowpath

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Date:	Wed, 27 Jul 2016 11:23:03 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Janani Ravichandran <janani.rvchndrn@...il.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org, riel@...riel.com,
	akpm@...ux-foundation.org, hannes@...pxchg.org,
	vdavydov@...tuozzo.com, mhocko@...e.com, vbabka@...e.cz,
	mgorman@...hsingularity.net, kirill.shutemov@...ux.intel.com,
	bywxiaobai@....com
Subject: Re: [PATCH 1/2] mm: page_alloc.c: Add tracepoints for slowpath

On Wed, 27 Jul 2016 10:47:59 -0400
Janani Ravichandran <janani.rvchndrn@...il.com> wrote:

> Add tracepoints to the slowpath code to gather some information.
> The tracepoints can also be used to find out how much time was spent in
> the slowpath.
> 
> Signed-off-by: Janani Ravichandran <janani.rvchndrn@...il.com>
> ---
>  include/trace/events/kmem.h | 40 ++++++++++++++++++++++++++++++++++++++++
>  mm/page_alloc.c             |  5 +++++
>  2 files changed, 45 insertions(+)
> 
> diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
> index 6b2e154..c19ab9f 100644
> --- a/include/trace/events/kmem.h
> +++ b/include/trace/events/kmem.h
> @@ -169,6 +169,46 @@ TRACE_EVENT(mm_page_free,
>  			__entry->order)
>  );
>  
> +TRACE_EVENT(mm_slowpath_begin,
> +
> +	TP_PROTO(gfp_t gfp_mask, int order),
> +
> +	TP_ARGS(gfp_mask, order),
> +
> +	TP_STRUCT__entry(
> +		__field(gfp_t, gfp_mask)
> +		__field(int, order)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->gfp_mask = gfp_mask;
> +		__entry->order = order;
> +	),
> +
> +	TP_printk("gfp_mask:%s order=%d",
> +		show_gfp_flags(__entry->gfp_mask),
> +		__entry->order)
> +);
> +
> +TRACE_EVENT(mm_slowpath_end,
> +
> +	TP_PROTO(struct page *page),
> +
> +	TP_ARGS(page),
> +
> +	TP_STRUCT__entry(
> +		__field(unsigned long, pfn)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->pfn = page ? page_to_pfn(page) : -1UL;
> +	),
> +
> +	TP_printk("page=%p pfn=%lu",
> +		__entry->pfn != -1UL ? pfn_to_page(__entry->pfn) : NULL,

Note, userspace tools will not be able to do this conversion (like
trace-cmd or perf).

> +		__entry->pfn != -1UL ? __entry->pfn : 0)
> +);
> +
>  TRACE_EVENT(mm_page_free_batched,
>  
>  	TP_PROTO(struct page *page, int cold),
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8b3e134..be9c688 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3595,6 +3595,8 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  				(__GFP_ATOMIC|__GFP_DIRECT_RECLAIM)))
>  		gfp_mask &= ~__GFP_ATOMIC;
>  
> +	trace_mm_slowpath_begin(gfp_mask, order);
> +
>  retry:
>  	if (gfp_mask & __GFP_KSWAPD_RECLAIM)
>  		wake_all_kswapds(order, ac);
> @@ -3769,6 +3771,9 @@ noretry:
>  nopage:
>  	warn_alloc_failed(gfp_mask, order, NULL);
>  got_pg:
> +
> +	trace_mm_slowpath_end(page);
> +

I'm thinking you only need one tracepoint, and use function_graph
tracer for the length of the function call.

 # cd /sys/kernel/debug/tracing
 # echo __alloc_pages_nodemask > set_ftrace_filter
 # echo function_graph > current_tracer
 # echo 1 > events/kmem/trace_mm_slowpath/enable

-- Steve


>  	return page;
>  }
>