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

[Minchan Kim <minchan@kernel.org>]

Hey, Gioh

On Wed, Dec 31, 2014 at 09:58:04AM +0900, Gioh Kim wrote:
> 
> 
> 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

I didn't know such detail. Thanks for the tip, Gioh.

> 
> 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.

Agree.

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

I didn't say debug information is useless.
If we need to know snapshot of cma at the moment,
describe why we need it and send a patch to implement the idea
rather than dumping lots of information is always better.

> 
> 
> 
> >
> >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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> >
> 
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-- 
Kind regards,
Minchan Kim

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