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Subject: Re: [PATCH] mm: cma: support sysfs
To:     Minchan Kim <minchan@kernel.org>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        <gregkh@linuxfoundation.org>, <surenb@google.com>,
        <joaodias@google.com>, LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>
References: <20210203155001.4121868-1-minchan@kernel.org>
 <7e7c01a7-27fe-00a3-f67f-8bcf9ef3eae9@nvidia.com>
 <YBxT9XFE6QAQ4T9N@google.com>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <cda5547b-0c78-756b-bd0c-f3e534d04bff@nvidia.com>
Date:   Thu, 4 Feb 2021 15:14:56 -0800
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On 2/4/21 12:07 PM, Minchan Kim wrote:
> On Thu, Feb 04, 2021 at 12:50:58AM -0800, John Hubbard wrote:
>> On 2/3/21 7:50 AM, Minchan Kim wrote:
>>> Since CMA is getting used more widely, it's more important to
>>> keep monitoring CMA statistics for system health since it's
>>> directly related to user experience.
>>>
>>> This patch introduces sysfs for the CMA and exposes stats below
>>> to keep monitor for telemetric in the system.
>>>
>>>    * the number of CMA allocation attempts
>>>    * the number of CMA allocation failures
>>>    * the number of CMA page allocation attempts
>>>    * the number of CMA page allocation failures
>>
>> The desire to report CMA data is understandable, but there are a few
>> odd things here:
>>
>> 1) First of all, this has significant overlap with /sys/kernel/debug/cma
>> items. I suspect that all of these items could instead go into
> 
> At this moment, I don't see any overlap with item from cma_debugfs.
> Could you specify what item you are mentioning?

Just the fact that there would be two systems under /sys, both of which are
doing very very similar things: providing information that is intended to
help diagnose CMA.

> 
>> /sys/kernel/debug/cma, right?
> 
> Anyway, thing is I need an stable interface for that and need to use
> it in Android production build, too(Unfortunately, Android deprecated
> the debugfs
> https://source.android.com/setup/start/android-11-release#debugfs
> )

That's the closest hint to a "why this is needed" that we've seen yet.
But it's only a hint.

> 
> What should be in debugfs and in sysfs? What's the criteria?

Well, it's a gray area. "Debugging support" goes into debugfs, and
"production-level monitoring and control" goes into sysfs, roughly
speaking. And here you have items that could be classified as either.

> 
> Some statistic could be considered about debugging aid or telemetric
> depening on view point and usecase. And here, I want to use it for
> telemetric, get an stable interface and use it in production build
> of Android. In this chance, I'd like to get concrete guideline
> what should be in sysfs and debugfs so that pointing out this thread
> whenever folks dump their stat into sysfs to avoid waste of time
> for others in future. :)
> 
>>
>> 2) The overall CMA allocation attempts/failures (first two items above) seem
>> an odd pair of things to track. Maybe that is what was easy to track, but I'd
>> vote for just omitting them.
> 
> Then, how to know how often CMA API failed?

Why would you even need to know that, *in addition* to knowing specific
page allocation numbers that failed? Again, there is no real-world motivation
cited yet, just "this is good data". Need more stories and support here.


thanks,
-- 
John Hubbard
NVIDIA

> There are various size allocation request for a CMA so only page
> allocation stat are not enough to know it.
> 
>>>
>>> Signed-off-by: Minchan Kim <minchan@kernel.org>
>>> ---
>>>    Documentation/ABI/testing/sysfs-kernel-mm-cma |  39 +++++
>>>    include/linux/cma.h                           |   1 +
>>>    mm/Makefile                                   |   1 +
>>>    mm/cma.c                                      |   6 +-
>>>    mm/cma.h                                      |  20 +++
>>>    mm/cma_sysfs.c                                | 143 ++++++++++++++++++
>>>    6 files changed, 209 insertions(+), 1 deletion(-)
>>>    create mode 100644 Documentation/ABI/testing/sysfs-kernel-mm-cma
>>>    create mode 100644 mm/cma_sysfs.c
>>>
>>> diff --git a/Documentation/ABI/testing/sysfs-kernel-mm-cma b/Documentation/ABI/testing/sysfs-kernel-mm-cma
>>> new file mode 100644
>>> index 000000000000..2a43c0aacc39
>>> --- /dev/null
>>> +++ b/Documentation/ABI/testing/sysfs-kernel-mm-cma
>>> @@ -0,0 +1,39 @@
>>> +What:		/sys/kernel/mm/cma/
>>> +Date:		Feb 2021
>>> +Contact:	Minchan Kim <minchan@kernel.org>
>>> +Description:
>>> +		/sys/kernel/mm/cma/ contains a number of subdirectories by
>>> +		cma-heap name. The subdirectory contains a number of files
>>> +		to represent cma allocation statistics.
>>
>> Somewhere, maybe here, there should be a mention of the closely related
>> /sys/kernel/debug/cma files.
>>
>>> +
>>> +		There are number of files under
>>> +				/sys/kernel/mm/cma/<cma-heap-name> directory
>>> +
>>> +			- cma_alloc_attempt
>>> +			- cma_alloc_fail
>>
>> Are these really useful? They a summary of the alloc_pages items, really.
>>
>>> +			- alloc_pages_attempt
>>> +			- alloc_pages_fail
>>
>> This should also have "cma" in the name, really: cma_alloc_pages_*.
> 
> No problem.
> 
>>
>>> +
>>> +What:		/sys/kernel/mm/cma/<cma-heap-name>/cma_alloc_attempt
>>> +Date:		Feb 2021
>>> +Contact:	Minchan Kim <minchan@kernel.org>
>>> +Description:
>>> +		the number of cma_alloc API attempted
>>> +
>>> +What:		/sys/kernel/mm/cma/<cma-heap-name>/cma_alloc_fail
>>> +Date:		Feb 2021
>>> +Contact:	Minchan Kim <minchan@kernel.org>
>>> +Description:
>>> +		the number of CMA_alloc API failed
>>> +
>>> +What:		/sys/kernel/mm/cma/<cma-heap-name>/alloc_pages_attempt
>>> +Date:		Feb 2021
>>> +Contact:	Minchan Kim <minchan@kernel.org>
>>> +Description:
>>> +		the number of pages CMA API tried to allocate
>>> +
>>> +What:		/sys/kernel/mm/cma/<cma-heap-name>/alloc_pages_fail
>>> +Date:		Feb 2021
>>> +Contact:	Minchan Kim <minchan@kernel.org>
>>> +Description:
>>> +		the number of pages CMA API failed to allocate
>>> diff --git a/include/linux/cma.h b/include/linux/cma.h
>>> index 217999c8a762..71a28a5bb54e 100644
>>> --- a/include/linux/cma.h
>>> +++ b/include/linux/cma.h
>>> @@ -49,4 +49,5 @@ extern struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
>>>    extern bool cma_release(struct cma *cma, const struct page *pages, unsigned int count);
>>>    extern int cma_for_each_area(int (*it)(struct cma *cma, void *data), void *data);
>>> +
>>
>> A single additional blank line seems to be the only change to this file. :)
> 
> Oops.
>