Re: [PATCH] mm: cma: support sysfs

[Pintu Agarwal <pintu.ping@gmail.com>]

On Sat, 6 Feb 2021 at 04:17, Minchan Kim <minchan@kernel.org> wrote:
>
> On Fri, Feb 05, 2021 at 01:58:06PM -0800, John Hubbard wrote:
> > On 2/5/21 1:52 PM, Suren Baghdasaryan wrote:
> > > > > > 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
> > >
> > > The interface above seems to me the most useful actually, if by
> > > [alloc_range] you mean the different allocation orders. This would
> > > cover Minchan's per-CMA failure tracking and would also allow us to
> > > understand what kind of allocations are failing and therefore if the
> > > problem is caused by pinning/fragmentation or by over-utilization.
> > >
> >
> > I agree. That seems about right, now that we've established that
> > cma areas are a must-have.
>
> Okay, now we agreed the dirction right now so let me do that in next
> version. If you don't see it's reasonable, let me know.
>
> * I will drop the number of CMA *page* allocation attemtps/failures to
> make simple start
> * I will keep CMA allocation attemtps/failures
> * They will be under /sys/kernel/mm/cma/cma-XX/success,fail
> * It will turn on CONFIG_CMA && CONFIG_SYSFS
>
> Orthognal work(diffrent patchset)
>
> * adding global CMA alloc/fail into vmstat
> * adding alloc_range success/failure under CONFIG_CMA_ALLOC_TRACKING
>   whatever configuration or just by default if everyone agree
>

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

This CMA info was added by me way back in 2014.
At that time I even thought about adding this cma alloc/fail counter in vmstat.
That time I also had an internal patch about it but later dropped
(never released to mainline).
If required I can re-work again on this part.

And I have few more points to add here.
1) In the past I have faced this cma allocation failure (which could
be common in small embedded devices).
Earlier it was even difficult to figure if cma failure actually happened.
Thus I added this simple patch:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.11-rc6&id=5984af1082f3b115082178ed88c47033d43b924d

2) IMO just reporting CMA alloc/fail may not be enough (at times). The
main point is for which client/dev is this failure happening ?
Sometimes just tuning the size or fixing the alignment can resolve the
failure if we know the client. For global CMA it will be just NULL
(dev).

3) IMO having 2 options SYSFS and DEBUGFS may confuse the
developer/user (personal experience). So are we going to remove the
DEBUGFS or merge it ?

4) Sometimes CMA (or DMA) allocation failure can happen even very
early in boot time itself. At that time these are anyways not useful.
Some system may not proceed if CMA/DMA allocation is failing (again
personal experience).
==> Anyways this is altogether is different case...

5) The default max CMA areas are defined to be 7 but user can
configure it to any number, may be 20 or 50 (???)

6) Thus I would like to propose another thing here.
Just like we have /proc/vmallocinfo, /proc/slabinfo, etc., we can also
have: /proc/cmainfo
Here in /proc/cmaminfo we can capute more detailed information:
Global CMA Data: Alloc/Free
Client specific data: name, size, success, fail, flags, align, etc
(just a random example).
==> This can dynamically grow as large as possible
==> It can also be enabled/disabled based on CMA config itself (no
additional config required)

Any feedback on point (6) specifically ?


Thanks,
Pintu