Re: [PATCH 0/3] mm: cma: /proc/cmainfo

[Gioh Kim]

2014-12-30 ìí 1:47ì Minchan Kim ì(ê) ì ê:
> On Mon, Dec 29, 2014 at 11:52:58AM -0800, Laura Abbott wrote:
> > On 12/28/2014 6:36 PM, Minchan Kim wrote:
> > > Hello,
> > >
> > > On Fri, Dec 26, 2014 at 05:39:01PM +0300, Stefan I. Strogin wrote:
> > > > Hello all,
> > > >
> > > > Here is a patch set that adds /proc/cmainfo.
> > > >
> > > > When compiled with CONFIG_CMA_DEBUG /proc/cmainfo will contain information
> > > > about about total, used, maximum free contiguous chunk and all currently
> > > > allocated contiguous buffers in CMA regions. The information about allocated
> > > > CMA buffers includes pid, comm, allocation latency and stacktrace at the
> > > > moment of allocation.
> > >
> > > It just says what you are doing but you didn't say why we need it.
> > > I can guess but clear description(ie, the problem what you want to
> > > solve with this patchset) would help others to review, for instance,
> > > why we need latency, why we need callstack, why we need new wheel
> > > rather than ftrace and so on.
> > >
> > > Thanks.
> >
> >
> > I've been meaning to write something like this for a while so I'm
> > happy to see an attempt made to fix this. I can't speak for the
> > author's reasons for wanting this information but there are
> > several reasons why I was thinking of something similar.
> >
> > The most common bug reports seen internally on CMA are 1) CMA is
> > too slow and 2) CMA failed to allocate memory. For #1, not all
> > allocations may be slow so it's useful to be able to keep track
> > of which allocations are taking too long. For #2, migration
>
> Then, I don't think we could keep all of allocations. What we need
> is only slow allocations. I hope we can do that with ftrace.
>
> ex)
>
> # cd /sys/kernel/debug/tracing
> # echo 1 > options/stacktrace
> # echo cam_alloc > set_ftrace_filter
> # echo your_threshold > tracing_thresh
>
> I know it doesn't work now but I think it's more flexible
> and general way to handle such issues(ie, latency of some functions).
> So, I hope we could enhance ftrace rather than new wheel.
> Ccing ftrace people.

For CMA performance test or code flow check, ftrace is better.

ex)
echo cma_alloc > /sys/kernel/debug/tracing/set_graph_function
echo function_graph > /sys/kernel/debug/tracing/current_tracer
echo funcgraph-proc > /sys/kernel/debug/tracing/trace_options
echo nosleep-time > /sys/kernel/debug/tracing/trace_options
echo funcgraph-tail > /sys/kernel/debug/tracing/trace_options
echo 1 > /sys/kernel/debug/tracing/tracing_on

This can trace every cam_alloc and allocation time.
I think ftrace is better to debug latency.
If a buffer had allocated and had peak latency and freed,
we can check it.

But ftrace doesn't provide current status how many buffers we have and what address it is.
So I think debugging information is useful.



> Futhermore, if we really need to have such information, we need more data
> (ex, how many of pages were migrated out, how many pages were dropped
> without migrated, how many pages were written back, how many pages were
> retried with the page lock and so on).
> In this case, event trace would be better.
>
>
> > failure is fairly common but it's still important to rule out
> > a memory leak from a dma client. Seeing all the allocations is
> > also very useful for memory tuning (e.g. how big does the CMA
> > region need to be, which clients are actually allocating memory).
>
> Memory leak is really general problem and could we handle it with
> page_owner?
>
> > ftrace is certainly usable for tracing CMA allocation callers and
> > latency. ftrace is still only a fixed size buffer though so it's
> > possible for information to be lost if other logging is enabled.
>
> Sorry, I don't get with only above reasons why we need this. :(
>
> > For most of the CMA use cases, there is a very high cost if the
> > proper debugging information is not available so the more that
> > can be guaranteed the better.
> >
> > It's also worth noting that the SLUB allocator has a sysfs
> > interface for showing allocation callers when CONFIG_SLUB_DEBUG
> > is enabled.
> >
> > Thanks,
> > Laura
> >
> > --
> > Qualcomm Innovation Center, Inc.
> > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> > a Linux Foundation Collaborative Project
> >
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