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Wed, 8 Sep 2021 21:00:21 +0000 Received: from BY5PR10MB4196.namprd10.prod.outlook.com ([fe80::a4a2:56de:e8db:9f2b]) by BY5PR10MB4196.namprd10.prod.outlook.com ([fe80::a4a2:56de:e8db:9f2b%9]) with mapi id 15.20.4500.016; Wed, 8 Sep 2021 21:00:21 +0000 Subject: Re: [PATCH RESEND 0/8] hugetlb: add demote/split page functionality To: Hillf Danton , Vlastimil Babka Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Michal Hocko References: <20210816224953.157796-1-mike.kravetz@oracle.com> <20210816162749.22b921a61156a091f3e1d14d@linux-foundation.org> <20210816184611.07b97f4c26b83090f5d48fab@linux-foundation.org> <10d86c18-f0cf-395f-4209-17ac71b9fc03@oracle.com> <2d826470-d345-0196-1359-b79ed08dfc66@oracle.com> <02a1a50f-4e7c-4eb7-519c-35b26ec2c6af@oracle.com> <20210907085001.3773-1-hdanton@sina.com> From: Mike Kravetz Message-ID: <6c42bed7-d4dd-e5eb-5a74-24cf64bf52d3@oracle.com> Date: Wed, 8 Sep 2021 14:00:19 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 In-Reply-To: <20210907085001.3773-1-hdanton@sina.com> Content-Type: text/plain; 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dkim=pass header.d=oracle.com header.s=corp-2021-07-09 header.b=Qwca4EsP; dkim=pass header.d=oracle.com header.s=corp-2020-01-29 header.b=U7s4K33N; dkim=pass header.d=oracle.onmicrosoft.com header.s=selector2-oracle-onmicrosoft-com header.b=RjW3MtRb; spf=none (imf14.hostedemail.com: domain of mike.kravetz@oracle.com has no SPF policy when checking 205.220.165.32) smtp.mailfrom=mike.kravetz@oracle.com; dmarc=pass (policy=none) header.from=oracle.com X-HE-Tag: 1631134826-350068 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 9/7/21 1:50 AM, Hillf Danton wrote: > On Mon, 6 Sep 2021 16:40:28 +0200 Vlastimil Babka wrote: >> On 9/2/21 20:17, Mike Kravetz wrote: >>> >>> Here is some very high level information from a long stall that was >>> interrupted. This was an order 9 allocation from alloc_buddy_huge_page(). >>> >>> 55269.530564] __alloc_pages_slowpath: jiffies 47329325 tries 609673 cpu_tries 1 node 0 FAIL >>> [55269.539893] r_tries 25 c_tries 609647 reclaim 47325161 compact 607 >>> >>> Yes, in __alloc_pages_slowpath for 47329325 jiffies before being interrupted. >>> should_reclaim_retry returned true 25 times and should_compact_retry returned >>> true 609647 times. >>> Almost all time (47325161 jiffies) spent in __alloc_pages_direct_reclaim, and >>> 607 jiffies spent in __alloc_pages_direct_compact. >>> >>> Looks like both >>> reclaim retries > MAX_RECLAIM_RETRIES >>> and >>> compaction retries > MAX_COMPACT_RETRIES >>> >> Yeah AFAICS that's only possible with the scenario I suspected. I guess >> we should put a limit on compact retries (maybe some multiple of >> MAX_COMPACT_RETRIES) even if it thinks that reclaim could help, while >> clearly it doesn't (i.e. because somebody else is stealing the page like >> in your test case). > > And/or clamp reclaim retries for costly orders > > reclaim retries = MAX_RECLAIM_RETRIES - order; > > to pull down the chance for stall as low as possible. Thanks, and sorry for not replying quickly. I only get back to this as time allows. We could clamp the number of compaction and reclaim retries in __alloc_pages_slowpath as suggested. However, I noticed that a single reclaim call could take a bunch of time. As a result, I instrumented shrink_node to see what might be happening. Here is some information from a long stall. Note that I only dump stats when jiffies > 100000. [ 8136.874706] shrink_node: 507654 total jiffies, 3557110 tries [ 8136.881130] 130596341 reclaimed, 32 nr_to_reclaim [ 8136.887643] compaction_suitable results: [ 8136.893276] idx COMPACT_SKIPPED, 3557109 [ 8672.399839] shrink_node: 522076 total jiffies, 3466228 tries [ 8672.406268] 124427720 reclaimed, 32 nr_to_reclaim [ 8672.412782] compaction_suitable results: [ 8672.418421] idx COMPACT_SKIPPED, 3466227 [ 8908.099592] __alloc_pages_slowpath: jiffies 2939938 tries 17068 cpu_tries 1 node 0 success [ 8908.109120] r_tries 11 c_tries 17056 reclaim 2939865 compact 9 In this case, clamping the number of retries from should_compact_retry and should_reclaim_retry could help. Mostly because we will not be calling back into the reclaim code? Notice the long amount of time spent in shrink_node. The 'tries' in shrink_node come about from that: if (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed, sc)) goto again; compaction_suitable results is the values returned from calls to should_continue_reclaim -> compaction_suitable. Trying to think if there might be an intelligent way to quit early. -- Mike Kravetz