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

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Message-Id: <93BEB5B5-321C-429B-9B87-40F8B499E45D@gmail.com>
Date:	Sun, 7 Aug 2016 16:06:18 +0530
From:	Janani Ravichandran <janani.rvchndrn@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Janani Ravichandran <janani.rvchndrn@...il.com>,
	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 Aug 5, 2016, at 10:00 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> 
>> 
> 
> You probably want to clear the trace here, or set function_graph here
> first. Because the function graph starts writing to the buffer
> immediately.
> 

I did that, just didn’t include it here :)
>> 
> 
> When threshold is set, the entry is not recorded, because it is only
> showing the exit and the time it took in that function:
> 
> 0) kswapd0-52 | + 54.141 us | } /* shrink_zone */
> 
> shrink_zone() took 54.141us.
> 
> The reason it doesn't record the entry is because it would fill the
> entire buffer, if the threshold is never hit. One can't predict the
> time in a function when you first enter that function.

Right!
> 
>> 
> 
> You need your own interpreter here. Perhaps a module that either reads
> the tracepoints directly and registers a function graph tracer itself.
> The trace events and function tracers are plugable. You don't need to
> use the tracing system to use them. Just hook into them directly.
> 
> Things like the wakeup latency tracer does this. Look at
> kernel/trace/trace_sched_wakeup.c for an example. It hooks into the
> sched_wakeup and sched_switch tracepoints, and also has a way to use
> function and function_graph tracing.
> 
> 

I will look at that file. Thanks!
> 
>> 
> 
> Great! And note, even if you add extra tracepoints, you can hook
> directly into them too. Again, see the trace_sched_wakeup.c for
> examples.

Alright. Thanks for all the help! :)

Janani.
> 
> -- Steve