Re: [PATCH] mm: be more verbose for alloc_contig_range faliures

[David Hildenbrand]

On 04.03.21 17:23, Minchan Kim wrote:
> On Thu, Mar 04, 2021 at 05:10:52PM +0100, David Hildenbrand wrote:
> > On 04.03.21 17:01, Minchan Kim wrote:
> > > On Tue, Mar 02, 2021 at 09:23:49AM -0800, Minchan Kim wrote:
> > > > On Fri, Feb 19, 2021 at 10:28:12AM +0100, Michal Hocko wrote:
> > > > > On Thu 18-02-21 08:19:50, Minchan Kim wrote:
> > > > > > On Thu, Feb 18, 2021 at 10:43:21AM +0100, David Hildenbrand wrote:
> > > > > > > On 18.02.21 10:35, Michal Hocko wrote:
> > > > > > > > On Thu 18-02-21 10:02:43, David Hildenbrand wrote:
> > > > > > > > > On 18.02.21 09:56, Michal Hocko wrote:
> > > > > > > > > > On Wed 17-02-21 08:36:03, Minchan Kim wrote:
> > > > > > > > > > > alloc_contig_range is usually used on cma area or movable zone.
> > > > > > > > > > > It's critical if the page migration fails on those areas so
> > > > > > > > > > > dump more debugging message like memory_hotplug unless user
> > > > > > > > > > > specifiy __GFP_NOWARN.
> > > > > > > > > >
> > > > > > > > > > I agree with David that this has a potential to generate a lot of output
> > > > > > > > > > and it is not really clear whether it is worth it. Page isolation code
> > > > > > > > > > already has REPORT_FAILURE mode which currently used only for the memory
> > > > > > > > > > hotplug because this was just too noisy from the CMA path - d381c54760dc
> > > > > > > > > > ("mm: only report isolation failures when offlining memory").
> > > > > > > > > >
> > > > > > > > > > Maybe migration failures are less likely to fail but still.
> > > > > > > > >
> > > > > > > > > Side note: I really dislike that uncontrolled error reporting on memory
> > > > > > > > > offlining path we have enabled as default. Yeah, it might be useful for
> > > > > > > > > ZONE_MOVABLE in some cases, but otherwise it's just noise.
> > > > > > > > >
> > > > > > > > > Just do a "sudo stress-ng --memhotplug 1" and see the log getting flooded
> > > > > > > >
> > > > > > > > Anyway we can discuss this in a separate thread but I think this is not
> > > > > > > > a representative workload.
> > > > > > >
> > > > > > > Sure, but the essence is "this is noise", and we'll have more noise on
> > > > > > > alloc_contig_range() as we see these calls more frequently. There should be
> > > > > > > an explicit way to enable such *debug* messages.
> > > > > >
> > > > > > alloc_contig_range already has gfp_mask and it respects __GFP_NOWARN.
> > > > > > Why shouldn't people use it if they don't care the failure?
> > > > > > Semantically, it makes sense to me.
> > > >
> > > > Sorry for the late response.
> > > >
> > > > > Well, alloc_contig_range doesn't really have to implement all the gfp
> > > > > flags. This is a matter of practicality. alloc_contig_range is quite
> > > > > different from the page allocator because it is to be expected that it
> > > > > can fail the request. This is avery optimistic allocation request. That
> > > > > would suggest that complaining about allocation failures is rather
> > > > > noisy.
> > > >
> > > > That was why I'd like to approach for per-call site indicator with
> > > > __GFP_NOWARN. Even though it was allocation from CMA, some of them
> > > > wouldn't be critical for the failure so those wouldn't care of
> > > > the failure. cma_alloc already has carried on "bool no_warn"
> > > > which was changed into gfp_t recently. What alloc_contig_range
> > > > should do is to take care of the request.
> > > >
> > > > > Now I do understand that some users would like to see why those
> > > > > allocations have failed. The question is whether that information is
> > > > > generally useful or it is more of a debugging aid. The amount of
> > > > > information is also an important aspect. It would be rather unfortunate
> > > > > to dump thousands of pages just because they cannot be migrated.
> > > >
> > > > Totally, agree dumping thounds of pages as debugging aid are bad.
> > > > Couldn't we simply ratelimit them like other places?
> > > >
> > > > > I do not have a strong opinion here. We can make all alloc_contig_range
> > > > > users use GFP_NOWARN by default and only skip the flag from the cma
> > > > > allocator but I am slowly leaning towards (ab)using dynamic debugging
> > > >
> > > > I agree the rest of the places are GFP_NOWARN by default except CMA
> > > > if they expect alloc_contig_range are optimistic allocation request.
> > > > However, I'd like to tweak it for CMA - accept gfp_t from cma_alloc
> > > > and take care of the __GFP_NOWARN since some sites of CMA could be
> > > > fault tolerant so no need to get the warning.
> > >
> > > Any thought to proceed?
> >
> > IMHO, add some proper debug mechanisms and don't try squeezing debug
> > messages into "WARN" semantics.
> >
> > Any alloc_contig_range() user can benefit from that.
>
> So the point is how we could add proper debug mechanism here.
> Think about call site A is not critical for the failure but
> called very frquently. Call site B is critical for the failure
> but called very rarely so turns on system wide dynamic debugging.
> You could see a lot of debug message from A even though we
> dont't want it. Even, it could hide B's debugging message
> by ratelimiting.

Do you have a real life example how this would be an issue? This sounds 
like a purely theoretical construct.

You want to debug something, so you try triggering it and capturing 
debug data. There are not that many alloc_contig_range() users such that 
this would really be an issue to isolate ...

Strictly speaking: any allocation failure on ZONE_MOVABLE or CMA is 
problematic (putting aside NORETRY logic and similar aside). So any such 
page you hit is worth investigating and, therefore, worth getting logged 
for debugging purposes.

--
Thanks,

David / dhildenb