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Sender: Minchan Kim <minchan.kim@gmail.com>
Date:   Fri, 5 Feb 2021 13:28:01 -0800
From:   Minchan Kim <minchan@kernel.org>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
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        LKML <linux-kernel@vger.kernel.org>,
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Subject: Re: [PATCH] mm: cma: support sysfs
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On Fri, Feb 05, 2021 at 12:25:52PM -0800, John Hubbard wrote:
> On 2/5/21 8:15 AM, Minchan Kim wrote:
> ...
> > > Yes, approximately. I was wondering if this would suffice at least as a baseline:
> > > 
> > > cma_alloc_success   125
> > > cma_alloc_failure   25
> > 
> > IMO, regardless of the my patch, it would be good to have such statistics
> > in that CMA was born to replace carved out memory with dynamic allocation
> > ideally for memory efficiency ideally so failure should regard critical
> > so admin could notice it how the system is hurt.
> 
> Right. So CMA failures are useful for the admin to see, understood.
> 
> > 
> > Anyway, it's not enough for me and orthgonal with my goal.
> > 
> 
> OK. But...what *is* your goal, and why is this useless (that's what
> orthogonal really means here) for your goal?

As I mentioned, the goal is to monitor the failure from each of CMA
since they have each own purpose.

Let's have an example.

System has 5 CMA area and each CMA is associated with each
user scenario. They have exclusive CMA area to avoid
fragmentation problem.

CMA-1 depends on bluetooh
CMA-2 depends on WIFI
CMA-3 depends on sensor-A
CMA-4 depends on sensor-B
CMA-5 depends on sensor-C

With this, we could catch which module was affected but with global failure,
I couldn't find who was affected.

> 
> Also, would you be willing to try out something simple first,
> such as providing indication that cma is active and it's overall success
> rate, like this:
> 
> /proc/vmstat:
> 
> cma_alloc_success   125
> cma_alloc_failure   25
> 
> ...or is the only way to provide the more detailed items, complete with
> per-CMA details, in a non-debugfs location?
> 
> 
> > > 
> > > ...and then, to see if more is needed, some questions:
> > > 
> > > a)  Do you know of an upper bound on how many cma areas there can be
> > > (I think Matthew also asked that)?
> > 
> > There is no upper bound since it's configurable.
> > 
> 
> OK, thanks,so that pretty much rules out putting per-cma details into
> anything other than a directory or something like it.
> 
> > > 
> > > b) Is tracking the cma area really as valuable as other possibilities? We can put
> > > "a few" to "several" items here, so really want to get your very favorite bits of
> > > information in. If, for example, there can be *lots* of cma areas, then maybe tracking
> > 
> > At this moment, allocation/failure for each CMA area since they have
> > particular own usecase, which makes me easy to keep which module will
> > be affected. I think it is very useful per-CMA statistics as minimum
> > code change so I want to enable it by default under CONFIG_CMA && CONFIG_SYSFS.
> > 
> > > by a range of allocation sizes is better...
> > 
> > I takes your suggestion something like this.
> > 
> > [alloc_range] could be order or range by interval
> > 
> > /sys/kernel/mm/cma/cma-A/[alloc_range]/success
> > /sys/kernel/mm/cma/cma-A/[alloc_range]/fail
> > ..
> > ..
> > /sys/kernel/mm/cma/cma-Z/[alloc_range]/success
> > /sys/kernel/mm/cma/cma-Z/[alloc_range]/fail
> 
> Actually, I meant, "ranges instead of cma areas", like this:
> 
> /<path-to-cma-data/[alloc_range_1]/success
> /<path-to-cma-data/[alloc_range_1]/fail
> /<path-to-cma-data/[alloc_range_2]/success
> /<path-to-cma-data/[alloc_range_2]/fail
> ...
> /<path-to-cma-data/[alloc_range_max]/success
> /<path-to-cma-data/[alloc_range_max]/fail
> 
> The idea is that knowing the allocation sizes that succeeded
> and failed is maybe even more interesting and useful than
> knowing the cma area that contains them.

Understand your point but it would make hard to find who was
affected by the failure. That's why I suggested to have your
suggestion under additional config since per-cma metric with
simple sucess/failure are enough.

> 
> > 
> > I agree it would be also useful but I'd like to enable it under
> > CONFIG_CMA_SYSFS_ALLOC_RANGE as separate patchset.
> > 
> 
> I will stop harassing you very soon, just want to bottom out on
> understanding the real goals first. :)
> 

I hope my example makes the goal more clear for you.