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[192.55.52.88]) by mx.google.com with ESMTPS id w16si16311761plp.185.2019.05.26.19.58.42 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 26 May 2019 19:58:42 -0700 (PDT) Received-SPF: pass (google.com: domain of ying.huang@intel.com designates 192.55.52.88 as permitted sender) client-ip=192.55.52.88; Authentication-Results: mx.google.com; spf=pass (google.com: domain of ying.huang@intel.com designates 192.55.52.88 as permitted sender) smtp.mailfrom=ying.huang@intel.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=intel.com X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga006.fm.intel.com ([10.253.24.20]) by fmsmga101.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 26 May 2019 19:58:42 -0700 X-ExtLoop1: 1 Received: from yhuang-dev.sh.intel.com (HELO yhuang-dev) ([10.239.159.29]) by fmsmga006.fm.intel.com with ESMTP; 26 May 2019 19:58:40 -0700 From: "Huang\, Ying" To: Yang Shi Cc: , , , , , , , , , , Subject: Re: [RESEND v5 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout 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> Date: Mon, 27 May 2019 10:58:39 +0800 In-Reply-To: (Yang Shi's message of "Mon, 27 May 2019 10:47:51 +0800") Message-ID: <87imtw8uxs.fsf@yhuang-dev.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii 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: 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. Best Regards, Huang, Ying >> >> Best Regards, >> Huang, Ying