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[115.124.30.130]) by mx.google.com with ESMTPS id o3si6230363jao.16.2019.05.26.20.00.23 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 26 May 2019 20:00:24 -0700 (PDT) Received-SPF: pass (google.com: domain of yang.shi@linux.alibaba.com designates 115.124.30.130 as permitted sender) client-ip=115.124.30.130; Authentication-Results: mx.google.com; spf=pass (google.com: domain of yang.shi@linux.alibaba.com designates 115.124.30.130 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=01201311R111e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04407;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=12;SR=0;TI=SMTPD_---0TSlZzX5_1558925998; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0TSlZzX5_1558925998) by smtp.aliyun-inc.com(127.0.0.1); Mon, 27 May 2019 10:59:58 +0800 Subject: Re: [RESEND v5 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout From: Yang Shi 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> Message-ID: Date: Mon, 27 May 2019 10:59:55 +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: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 8bit 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 10:47 AM, Yang Shi wrote: > > > 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. > > 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. Reiterating all the goto, it seems minus tail pages may be easier since there are a lot places which goto activate_locked, keep_locked and keep after THP has been added into swap cache. We can't tell if the tail pages are accounted or not unless we introduce a new flag, otherwise adding tails pages in those labels may account tail pages twice. > >> >> Best Regards, >> Huang, Ying >