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Fri, 19 Feb 2021 10:37:15 +0000 (UTC) Received: from [10.36.113.117] (ovpn-113-117.ams2.redhat.com [10.36.113.117]) by smtp.corp.redhat.com (Postfix) with ESMTP id BBD6F5D9C2; Fri, 19 Feb 2021 10:37:13 +0000 (UTC) Subject: Re: [PATCH] mm: be more verbose for alloc_contig_range faliures To: Michal Hocko Cc: Minchan Kim , Andrew Morton , linux-mm , LKML , joaodias@google.com References: <20210217163603.429062-1-minchan@kernel.org> <2f167b3c-5f0a-444a-c627-49181fc8fe0d@redhat.com> <45f1bffe-8a0b-2969-32d4-e24a911a647d@redhat.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <58d7f10e-227c-eb05-64b5-f0a2ff05bc9c@redhat.com> Date: Fri, 19 Feb 2021 11:34:30 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 0C0A0407F8D0 X-Stat-Signature: 7xwf9ixexo65haswanpp5ywfaueh85cb Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf10; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=63.128.21.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1613731036-608711 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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