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Thu, 4 Mar 2021 16:10:53 +0000 (UTC) Subject: Re: [PATCH] mm: be more verbose for alloc_contig_range faliures To: Minchan Kim , Michal Hocko Cc: Andrew Morton , linux-mm , LKML , joaodias@google.com References: <20210217163603.429062-1-minchan@kernel.org> <2f167b3c-5f0a-444a-c627-49181fc8fe0d@redhat.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: Date: Thu, 4 Mar 2021 17:10:52 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 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 2.84 on 10.5.11.23 X-Stat-Signature: tfjb98k6yhyndoubbgrz3u8rfsozgza6 X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 56C28E007A57 Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf13; 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: 1614874257-380004 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 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. -- Thanks, David / dhildenb