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

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Message-Id: <0AF03F78-AA34-4531-899A-EA1076B6B3A1@gmail.com>
Date:	Fri, 29 Jul 2016 01:41:20 +0530
From:	Janani Ravichandran <janani.rvchndrn@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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 Jul 27, 2016, at 8:53 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> 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

Thank you so much for your feedback! 

Actually, the goal is to only single out those cases with latencies higher than a given
threshold.

So, in accordance with this, I added those begin/end tracepoints and thought I 
could use a script to read the output of trace_pipe and only write to disk the trace 
information associated with latencies above the threshold. This would help prevent 
high disk I/O, especially when the threshold set is high.

I looked at function graph trace, as you’d suggested. I saw that I could set a threshold 
there using tracing_thresh. But the problem was that slowpath trace information was printed
for all the cases (even when __alloc_pages_nodemask latencies were below the threshold).
Is there a way to print tracepoint information only when __alloc_pages_nodemask
exceeds the threshold?

Thanks!

Janani.
> 
> -- Steve
> 
> 
>> 	return page;
>> }
>> 
>