Re: [PATCH] mm: cma: support sysfs

[John Hubbard]

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