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Subject: Re: [PATCH] mm: be more verbose for alloc_contig_range faliures
To:     Michal Hocko <mhocko@suse.com>
Cc:     Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, joaodias@google.com
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <58d7f10e-227c-eb05-64b5-f0a2ff05bc9c@redhat.com>
Date:   Fri, 19 Feb 2021 11:34:30 +0100
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On 19.02.21 11:02, Michal Hocko wrote:
> On Fri 19-02-21 10:30:12, David Hildenbrand wrote:
>> On 19.02.21 10:28, 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.
>>>
>>> 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.
>>>
>>> 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.
>>>
>>> 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
>>> infrastructure for this.
>>
>> Just so I understand what you are referring to - trace points?
> 
> Documentation/admin-guide/dynamic-debug-howto.rst
> but I have to confess I have 0 experience with this.

Me too, but it does sound like a good fit.

-- 
Thanks,

David / dhildenb