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Subject: Re: [PATCH] mm: cma: support sysfs
To:     Minchan Kim <minchan@kernel.org>
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From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <87d7ec1f-d892-0491-a2de-3d0feecca647@nvidia.com>
Date:   Thu, 4 Feb 2021 16:24:20 -0800
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On 2/4/21 4:12 PM, Minchan Kim wrote:
...
>>> 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.
> 
> Let me give an example.
> 
> Let' assume we use memory buffer allocation via CMA for bluetooth
> enable of  device.
> If user clicks the bluetooth button in the phone but fail to allocate
> the memory from CMA, user will still see bluetooth button gray.
> User would think his touch was not enough powerful so he try clicking
> again and fortunately CMA allocation was successful this time and
> they will see bluetooh button enabled and could listen the music.
> 
> Here, product team needs to monitor how often CMA alloc failed so
> if the failure ratio is steadily increased than the bar,
> it means engineers need to go investigation.
> 
> Make sense?
> 

Yes, except that it raises more questions:

1) Isn't this just standard allocation failure? Don't you already have a way
to track that?

Presumably, having the source code, you can easily deduce that a bluetooth
allocation failure goes directly to a CMA allocation failure, right?

Anyway, even though the above is still a little murky, I expect you're right
that it's good to have *some* indication, somewhere about CMA behavior...

Thinking about this some more, I wonder if this is really /proc/vmstat sort
of data that we're talking about. It seems to fit right in there, yes?


thanks,
-- 
John Hubbard
NVIDIA