Messages in this thread | | | Subject | Re: [PATCH] mm: be more verbose for alloc_contig_range faliures | From | David Hildenbrand <> | Date | Thu, 4 Mar 2021 17:28:32 +0100 |
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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
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