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

[Gioh Kim <gioh.kim@lge.com>]

2014-12-30 i??i?? 1:47i?? Minchan Kim i?'(e??) i?' e,?:
> 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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