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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id u26-20020a05600c211a00b003f42314832fsm19740769wml.18.2023.06.15.01.41.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 15 Jun 2023 01:41:30 -0700 (PDT) Message-ID: Date: Thu, 15 Jun 2023 10:41:30 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.12.0 Subject: Re: [PATCH v2] mm: compaction: skip memory hole rapidly when isolating migratable pages To: "Huang, Ying" Cc: Baolin Wang , Mel Gorman , akpm@linux-foundation.org, vbabka@suse.cz, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <770f9f61472b24b6bc89adbd71a77d9cf62bb54f.1686646361.git.baolin.wang@linux.alibaba.com> <20230614095501.m4porztaibchrgwx@techsingularity.net> <87ilbpo1d9.fsf@yhuang6-desk2.ccr.corp.intel.com> <5f340d98-4ee0-35a9-58ed-943834d68042@linux.alibaba.com> <87fs6tfaw5.fsf@yhuang6-desk2.ccr.corp.intel.com> <5b5e7dd0-d60b-ca46-215c-f59947b805fe@redhat.com> <87bkhhf7d2.fsf@yhuang6-desk2.ccr.corp.intel.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <87bkhhf7d2.fsf@yhuang6-desk2.ccr.corp.intel.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: D791220002 X-Rspam-User: X-Stat-Signature: 8xu7xfg75bpu5upiashkncfpus81u8y1 X-Rspamd-Server: rspam03 X-HE-Tag: 1686818494-16139 X-HE-Meta: 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 g/TRPcny 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000014, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 15.06.23 10:38, Huang, Ying wrote: > David Hildenbrand writes: > >> On 15.06.23 09:22, Huang, Ying wrote: >>> Baolin Wang writes: >>> >>>> On 6/15/2023 11:22 AM, Huang, Ying wrote: >>>>> Hi, Mel, >>>>> Mel Gorman writes: >>>>> >>>>>> On Tue, Jun 13, 2023 at 04:55:04PM +0800, Baolin Wang wrote: >>>>>>> On some machines, the normal zone can have a large memory hole like >>>>>>> below memory layout, and we can see the range from 0x100000000 to >>>>>>> 0x1800000000 is a hole. So when isolating some migratable pages, the >>>>>>> scanner can meet the hole and it will take more time to skip the large >>>>>>> hole. From my measurement, I can see the isolation scanner will take >>>>>>> 80us ~ 100us to skip the large hole [0x100000000 - 0x1800000000]. >>>>>>> >>>>>>> So adding a new helper to fast search next online memory section >>>>>>> to skip the large hole can help to find next suitable pageblock >>>>>>> efficiently. With this patch, I can see the large hole scanning only >>>>>>> takes < 1us. >>>>>>> >>>>>>> [ 0.000000] Zone ranges: >>>>>>> [ 0.000000] DMA [mem 0x0000000040000000-0x00000000ffffffff] >>>>>>> [ 0.000000] DMA32 empty >>>>>>> [ 0.000000] Normal [mem 0x0000000100000000-0x0000001fa7ffffff] >>>>>>> [ 0.000000] Movable zone start for each node >>>>>>> [ 0.000000] Early memory node ranges >>>>>>> [ 0.000000] node 0: [mem 0x0000000040000000-0x0000000fffffffff] >>>>>>> [ 0.000000] node 0: [mem 0x0000001800000000-0x0000001fa3c7ffff] >>>>>>> [ 0.000000] node 0: [mem 0x0000001fa3c80000-0x0000001fa3ffffff] >>>>>>> [ 0.000000] node 0: [mem 0x0000001fa4000000-0x0000001fa402ffff] >>>>>>> [ 0.000000] node 0: [mem 0x0000001fa4030000-0x0000001fa40effff] >>>>>>> [ 0.000000] node 0: [mem 0x0000001fa40f0000-0x0000001fa73cffff] >>>>>>> [ 0.000000] node 0: [mem 0x0000001fa73d0000-0x0000001fa745ffff] >>>>>>> [ 0.000000] node 0: [mem 0x0000001fa7460000-0x0000001fa746ffff] >>>>>>> [ 0.000000] node 0: [mem 0x0000001fa7470000-0x0000001fa758ffff] >>>>>>> [ 0.000000] node 0: [mem 0x0000001fa7590000-0x0000001fa7ffffff] >>>>>>> >>>>>>> Signed-off-by: Baolin Wang >>>>>> >>>>>> This may only be necessary for non-contiguous zones so a check for >>>>>> zone_contiguous could be made but I suspect the saving, if any, would be >>>>>> marginal. >>>>>> >>>>>> However, it's subtle that block_end_pfn can end up in an arbirary location >>>>>> past the end of the zone or past cc->free_pfn. As the "continue" will update >>>>>> cc->migrate_pfn, that might lead to errors in the future. It would be a >>>>>> lot safer to pass in cc->free_pfn and do two things with the value. First, >>>>>> there is no point scanning for a valid online section past cc->free_pfn so >>>>>> terminating after cc->free_pfn may save some cycles. Second, cc->migrate_pfn >>>>>> does not end up with an arbitrary value which is a more defensive approach >>>>>> to any future programming errors. >>>>> I have thought about this before. Originally, I had thought that we >>>>> were safe because cc->free_pfn should be in a online section and >>>>> block_end_pfn should reach cc->free_pfn before the end of zone. But >>>>> after checking more code and thinking about it again, I found that the >>>>> underlying sections may go offline under us during compaction. So that, >>>>> cc->free_pfn may be in a offline section or after the end of zone. So, >>>>> you are right, we need to consider the range of block_end_pfn. >>>>> But, if we thought in this way (memory online/offline at any time), >>>>> it >>>>> appears that we need to check whether the underlying section was >>>>> offlined. For example, is it safe to use "pfn_to_page()" in >>>>> "isolate_migratepages_block()"? Is it possible for the underlying >>>>> section to be offlined under us? >>>> >>>> It is possible. There is a previous discussion[1] about the race >>>> between pfn_to_online_page() and memory offline. >>>> >>>> [1] >>>> https://lore.kernel.org/lkml/87zgc6buoq.fsf@nvidia.com/T/#m642d91bcc726437e1848b295bc57ce249c7ca399 >>> Thank you very much for sharing! That answers my questions >>> directly! >> >> I remember another discussion (but can't find it) regarding why memory >> compaction can get away without pfn_to_online_page() all over the >> place. The use is limited to __reset_isolation_pfn(). > > Per my understanding, isolate_migratepages() -> pageblock_pfn_to_page() > will check whether the pageblock is online. So if the pageblock isn't > offlined afterwards, we can use pfn_to_page(). Oh, indeed, that was the magic bit, thanks! -- Cheers, David / dhildenb