linux-kernel - Re: [PATCH] mm: be more verbose for alloc_contig

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Date:   Thu, 18 Feb 2021 17:26:08 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, joaodias@...gle.com
Subject: Re: [PATCH] mm: be more verbose for alloc_contig_range faliures

On 18.02.21 17:19, 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.

I am not 100% sure it does.

> Why shouldn't people use it if they don't care the failure?

Because flooding the log with noise maybe a handful of people on this 
planet care about is absolutely useless. With the warnings in 
warn_alloc() people can at least conclude something reasonable.

> Semantically, it makes sense to me.
> 
> About the messeage flooding, shouldn't we go with ratelimiting?

At least that (see warn_alloc()). But I'd even want to see some other 
trigger to enable this explicitly on demand.

-- 
Thanks,

David / dhildenb