linux-kernel - Re: [PATCH] mm: cma: support sysfs

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Date:   Fri, 5 Feb 2021 13:57:03 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Minchan Kim <minchan@...nel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        <gregkh@...uxfoundation.org>, <surenb@...gle.com>,
        <joaodias@...gle.com>, LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: cma: support sysfs

On 2/5/21 1:28 PM, Minchan Kim wrote:
> On Fri, Feb 05, 2021 at 12:25:52PM -0800, John Hubbard wrote:
>> On 2/5/21 8:15 AM, Minchan Kim wrote:
>> ...
>> 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
> 

aha, finally. I had no idea that sort of use case was happening.

This would be good to put in the patch commit description.

> 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.
> 

Yes it does. Based on the (rather surprising) use of cma-area-per-device,
it seems clear that you will need this, so I'll drop my objections to
putting it in sysfs.

I still think the "number of allocation failures" needs refining, probably
via a range-based thing, as we've discussed. But the number of pages
failed per cma looks OK now.



thanks,
-- 
John Hubbard
NVIDIA