linux-kernel - Re: [RFC] scripts: Include postprocessing script for memory allocation tracing

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Date:   Thu, 20 Oct 2016 18:10:37 -0500
From:   Janani Ravichandran <janani.rvchndrn@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Janani Ravichandran <janani.rvchndrn@...il.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] scripts: Include postprocessing script for memory allocation tracing

Michal,

> On Oct 18, 2016, at 8:13 AM, Michal Hocko <mhocko@...nel.org> wrote:
> 
>> 
> 
> yes, function_graph tracer will give you _some_ information but it will
> not have the context you are looking for, right? See the following
> example
> 
> ------------------------------------------
> 0) x-www-b-22756  =>  x-termi-4083 
> ------------------------------------------
> 
> 0)               |  __alloc_pages_nodemask() {
> 0)               |  /* mm_page_alloc: page=ffffea000411b380 pfn=1066702 order=0 migratetype=0 gfp_flags=GFP_KERNEL */
> 0)   3.328 us    |  }
> 3)               |  __alloc_pages_nodemask() {
> 3)               |  /* mm_page_alloc: page=ffffea0008f1f6c0 pfn=2344923 order=0 migratetype=0 gfp_flags=GFP_KERNEL */
> 3)   1.011 us    |  }
> 0)               |  __alloc_pages_nodemask() {
> 0)               |  /* mm_page_alloc: page=ffffea000411b380 pfn=1066702 order=0 migratetype=0 gfp_flags=GFP_KERNEL */
> 0)   0.587 us    |  }
> 3)               |  __alloc_pages_nodemask() {
> 3)               |  /* mm_page_alloc: page=ffffea0008f1f6c0 pfn=2344923 order=0 migratetype=0 gfp_flags=GFP_KERNEL */
> 3)   1.125 us    |  }
> 
> How do I know which process has performed those allocations? I know that
> CPU0 should be running x-termi-4083 but what is running on other CPUs?
> 
> Let me explain my usecase I am very interested in. Say I that a usespace
> application is not performing well. I would like to see some statistics
> about memory allocations performed for that app - are there few outliers
> or the allocation stalls increase gradually? Where do we spend time during
> that allocation? Reclaim LRU pages? Compaction or the slab shrinkers?
> 
> To answer those questions I need to track particular events (alocation,
> reclaim, compaction) to the process and know how long each step
> took. Maybe we can reconstruct something from the above output but it is
> a major PITA.  If we either hard start/stop pairs for each step (which
> we already do have for reclaim, compaction AFAIR) then this is an easy
> scripting. Another option would be to have only a single tracepoint for
> each step with a timing information.
> 
> See my point?

Yes, if we want to know what processes are running on what CPUs,
echo funcgraph-proc > trace_options in the tracing directory should give us
what we want.

The bash script which is part of this patch does this kind of setup for you.
As a result, the output you get is something like what you see here:

https://github.com/Jananiravichandran/Analyzing-tracepoints/blob/master/no_tp_no_threshold.txt

Does this answer your question? Let me know if otherwise.

Janani.

> -- 
> Michal Hocko
> SUSE Labs