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Sender: Minchan Kim <minchan.kim@gmail.com>
Date:   Thu, 4 Feb 2021 17:44:30 -0800
From:   Minchan Kim <minchan@kernel.org>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [PATCH] mm: cma: support sysfs
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On Thu, Feb 04, 2021 at 04:24:20PM -0800, John Hubbard wrote:
> 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?

Thing is CMA instance are multiple, cma-A, cma-B, cma-C and each of CMA
heap has own specific scenario. /proc/vmstat could be bloated a lot
while CMA instance will be increased.