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

[Michal Hocko <mhocko@kernel.org>]

On Mon 17-10-16 13:31:57, Janani Ravichandran wrote:
> 
> > On Oct 17, 2016, at 1:24 PM, Janani Ravichandran <janani.rvchndrn@gmail.com> wrote:
> > 
> > 
> > On Sun, Oct 16, 2016 at 3:33 AM, Michal Hocko <mhocko@kernel.org <mailto:mhocko@kernel.org>> wrote:
> > 
> > trace_mm_page_alloc will tell you details about the allocation, like
> > gfp mask, order but it doesn't tell you how long the allocation took at
> > its current form. So either you have to note jiffies at the allocation
> > start and then add the end-start in the trace point or we really need
> > another trace point to note the start. The later has an advantage that
> > we do not add unnecessary load for jiffies when the tracepoint is
> > disabled.
> 
> The function graph tracer can tell us how long alloc_pages_nodemask() took.
> Cana??t that, combined with the context information given by trace_mm_page_alloc
> give us what we want? Correct me if I am wrong.

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?
-- 
Michal Hocko
SUSE Labs

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