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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: <34110c61-9826-4cbe-8cd4-76f5e7612dbd@nvidia.com>
Date:   Thu, 4 Feb 2021 21:49:54 -0800
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On 2/4/21 9:17 PM, Minchan Kim wrote:
...
>>>> Presumably, having the source code, you can easily deduce that a bluetooth
>>>> allocation failure goes directly to a CMA allocation failure, right?
>>
>> Still wondering about this...
> 
> It would work if we have full source code and stack are not complicated for
> every usecases. Having said, having a good central place automatically
> popped up is also beneficial for not to add similar statistics for each
> call sites.
> 
> Why do we have too many item in slab sysfs instead of creating each call
> site inventing on each own?
> 

I'm not sure I understand that question fully, but I don't think we need to
invent anything unique here. So far we've discussed debugfs, sysfs, and /proc,
none of which are new mechanisms.

...

>> It's actually easier to monitor one or two simpler items than it is to monitor
>> a larger number of complicated items. And I get the impression that this is
>> sort of a top-level, production software indicator.
> 
> Let me clarify one more time.
> 
> What I'd like to get ultimately is per-CMA statistics instead of
> global vmstat for the usecase at this moment. Global vmstat
> could help the decision whether I should go deeper but it ends up
> needing per-CMA statistics. And I'd like to keep them in sysfs,
> not debugfs since it should be stable as a telemetric.
> 
> What points do you disagree in this view?


No huge disagreements, I just want to get us down to the true essential elements
of what is required--and find a good home for the data. Initial debugging always
has excesses, and those should not end up in the more carefully vetted production
code.

If I were doing this, I'd probably consider HugeTLB pages as an example to follow,
because they have a lot in common with CMA: it's another memory allocation pool, and
people also want to monitor it.

HugeTLB pages and THP pages are monitored in /proc:
	/proc/meminfo and /proc/vmstat:

# cat meminfo |grep -i huge
AnonHugePages:     88064 kB
ShmemHugePages:        0 kB
FileHugePages:         0 kB
HugePages_Total:     500
HugePages_Free:      500
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
Hugetlb:         1024000 kB

# cat vmstat | grep -i huge
nr_shmem_hugepages 0
nr_file_hugepages 0
nr_anon_transparent_hugepages 43
numa_huge_pte_updates 0

...aha, so is CMA:

# cat vmstat | grep -i cma
nr_free_cma 261718

# cat meminfo | grep -i cma
CmaTotal:        1048576 kB
CmaFree:         1046872 kB

OK, given that CMA is already in those two locations, maybe we should put
this information in one or both of those, yes?


thanks,
-- 
John Hubbard
NVIDIA