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[115.124.30.133]) by mx.google.com with ESMTPS id i8si5609453oih.11.2019.05.26.20.15.32 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 26 May 2019 20:15:33 -0700 (PDT) Received-SPF: pass (google.com: domain of yang.shi@linux.alibaba.com designates 115.124.30.133 as permitted sender) client-ip=115.124.30.133; Authentication-Results: mx.google.com; spf=pass (google.com: domain of yang.shi@linux.alibaba.com designates 115.124.30.133 as permitted sender) smtp.mailfrom=yang.shi@linux.alibaba.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=alibaba.com X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R421e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01f04446;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=12;SR=0;TI=SMTPD_---0TSm5gg0_1558926918; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0TSm5gg0_1558926918) by smtp.aliyun-inc.com(127.0.0.1); Mon, 27 May 2019 11:15:19 +0800 Subject: Re: [RESEND v5 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout To: "Huang, Ying" Cc: hannes@cmpxchg.org, mhocko@suse.com, mgorman@techsingularity.net, kirill.shutemov@linux.intel.com, josef@toxicpanda.com, hughd@google.com, shakeelb@google.com, hdanton@sina.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <1558922275-31782-1-git-send-email-yang.shi@linux.alibaba.com> <1558922275-31782-2-git-send-email-yang.shi@linux.alibaba.com> <87muj88x3p.fsf@yhuang-dev.intel.com> <87imtw8uxs.fsf@yhuang-dev.intel.com> From: Yang Shi Message-ID: <7035abc1-6f56-cce0-6a0b-29fceeefe339@linux.alibaba.com> Date: Mon, 27 May 2019 11:15:15 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <87imtw8uxs.fsf@yhuang-dev.intel.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US 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 5/27/19 10:58 AM, Huang, Ying wrote: > Yang Shi writes: > >> On 5/27/19 10:11 AM, Huang, Ying wrote: >>> Yang Shi writes: >>> >>>> Since commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after >>>> swapped out"), THP can be swapped out in a whole. But, nr_reclaimed >>>> and some other vm counters still get inc'ed by one even though a whole >>>> THP (512 pages) gets swapped out. >>>> >>>> This doesn't make too much sense to memory reclaim. For example, direct >>>> reclaim may just need reclaim SWAP_CLUSTER_MAX pages, reclaiming one THP >>>> could fulfill it. But, if nr_reclaimed is not increased correctly, >>>> direct reclaim may just waste time to reclaim more pages, >>>> SWAP_CLUSTER_MAX * 512 pages in worst case. >>>> >>>> And, it may cause pgsteal_{kswapd|direct} is greater than >>>> pgscan_{kswapd|direct}, like the below: >>>> >>>> pgsteal_kswapd 122933 >>>> pgsteal_direct 26600225 >>>> pgscan_kswapd 174153 >>>> pgscan_direct 14678312 >>>> >>>> nr_reclaimed and nr_scanned must be fixed in parallel otherwise it would >>>> break some page reclaim logic, e.g. >>>> >>>> vmpressure: this looks at the scanned/reclaimed ratio so it won't >>>> change semantics as long as scanned & reclaimed are fixed in parallel. >>>> >>>> compaction/reclaim: compaction wants a certain number of physical pages >>>> freed up before going back to compacting. >>>> >>>> kswapd priority raising: kswapd raises priority if we scan fewer pages >>>> than the reclaim target (which itself is obviously expressed in order-0 >>>> pages). As a result, kswapd can falsely raise its aggressiveness even >>>> when it's making great progress. >>>> >>>> Other than nr_scanned and nr_reclaimed, some other counters, e.g. >>>> pgactivate, nr_skipped, nr_ref_keep and nr_unmap_fail need to be fixed >>>> too since they are user visible via cgroup, /proc/vmstat or trace >>>> points, otherwise they would be underreported. >>>> >>>> When isolating pages from LRUs, nr_taken has been accounted in base >>>> page, but nr_scanned and nr_skipped are still accounted in THP. It >>>> doesn't make too much sense too since this may cause trace point >>>> underreport the numbers as well. >>>> >>>> So accounting those counters in base page instead of accounting THP as >>>> one page. >>>> >>>> nr_dirty, nr_unqueued_dirty, nr_congested and nr_writeback are used by >>>> file cache, so they are not impacted by THP swap. >>>> >>>> This change may result in lower steal/scan ratio in some cases since >>>> THP may get split during page reclaim, then a part of tail pages get >>>> reclaimed instead of the whole 512 pages, but nr_scanned is accounted >>>> by 512, particularly for direct reclaim. But, this should be not a >>>> significant issue. >>>> >>>> Cc: "Huang, Ying" >>>> Cc: Johannes Weiner >>>> Cc: Michal Hocko >>>> Cc: Mel Gorman >>>> Cc: "Kirill A . Shutemov" >>>> Cc: Hugh Dickins >>>> Cc: Shakeel Butt >>>> Signed-off-by: Yang Shi >>>> --- >>>> v5: Fixed sc->nr_scanned double accounting per Huang Ying >>>> Added some comments to address the concern about premature OOM per Hillf Danton >>>> v4: Fixed the comments from Johannes and Huang Ying >>>> v3: Removed Shakeel's Reviewed-by since the patch has been changed significantly >>>> Switched back to use compound_order per Matthew >>>> Fixed more counters per Johannes >>>> v2: Added Shakeel's Reviewed-by >>>> Use hpage_nr_pages instead of compound_order per Huang Ying and William Kucharski >>>> >>>> mm/vmscan.c | 42 +++++++++++++++++++++++++++++++----------- >>>> 1 file changed, 31 insertions(+), 11 deletions(-) >>>> >>>> diff --git a/mm/vmscan.c b/mm/vmscan.c >>>> index b65bc50..f4f4d57 100644 >>>> --- a/mm/vmscan.c >>>> +++ b/mm/vmscan.c >>>> @@ -1118,6 +1118,7 @@ static unsigned long shrink_page_list(struct list_head *page_list, >>>> int may_enter_fs; >>>> enum page_references references = PAGEREF_RECLAIM_CLEAN; >>>> bool dirty, writeback; >>>> + unsigned int nr_pages; >>>> cond_resched(); >>>> @@ -1129,6 +1130,13 @@ static unsigned long >>>> shrink_page_list(struct list_head *page_list, >>>> VM_BUG_ON_PAGE(PageActive(page), page); >>>> + nr_pages = 1 << compound_order(page); >>>> + >>>> + /* >>>> + * Accounted one page for THP for now. If THP gets swapped >>>> + * out in a whole, will account all tail pages later to >>>> + * avoid accounting tail pages twice. >>>> + */ >>>> sc->nr_scanned++; >>>> if (unlikely(!page_evictable(page))) >>>> @@ -1250,7 +1258,7 @@ static unsigned long shrink_page_list(struct list_head *page_list, >>>> case PAGEREF_ACTIVATE: >>>> goto activate_locked; >>>> case PAGEREF_KEEP: >>>> - stat->nr_ref_keep++; >>>> + stat->nr_ref_keep += nr_pages; >>>> goto keep_locked; >>>> case PAGEREF_RECLAIM: >>>> case PAGEREF_RECLAIM_CLEAN: >>> If the "Accessed bit" of a THP is set in the page table that maps it, it >>> will go PAGEREF_ACTIVATE path here. And the sc->nr_scanned should >>> increase 512 instead of 1. Otherwise sc->nr_activate may be larger than >>> sc->nr_scanned. >> Yes, it looks so. It seems the easiest way is to add "nr_pages - 1" in >> activate_locked label if the page is still a THP. > Add keep_locked label. > >> If we add all tail pages at the very beginning, then we have to minus >> tail pages when THP gets split, there are a few places do this. > I think we can do that in one place too. Just before try_to_unmap() via > checking nr_pages and page order. And we need to update nr_pages if > the THP is split anyway. Yes, I agree. > > Best Regards, > Huang, Ying > >>> Best Regards, >>> Huang, Ying