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[192.55.52.43]) by mx.google.com with ESMTPS id s36si16271560pld.278.2019.05.26.19.11.57 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 26 May 2019 19:11:58 -0700 (PDT) Received-SPF: pass (google.com: domain of ying.huang@intel.com designates 192.55.52.43 as permitted sender) client-ip=192.55.52.43; Authentication-Results: mx.google.com; spf=pass (google.com: domain of ying.huang@intel.com designates 192.55.52.43 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 orsmga005.jf.intel.com ([10.7.209.41]) by fmsmga105.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 26 May 2019 19:11:57 -0700 X-ExtLoop1: 1 Received: from yhuang-dev.sh.intel.com (HELO yhuang-dev) ([10.239.159.29]) by orsmga005.jf.intel.com with ESMTP; 26 May 2019 19:11:54 -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> Date: Mon, 27 May 2019 10:11:54 +0800 In-Reply-To: <1558922275-31782-2-git-send-email-yang.shi@linux.alibaba.com> (Yang Shi's message of "Mon, 27 May 2019 09:57:55 +0800") Message-ID: <87muj88x3p.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: > 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. Best Regards, Huang, Ying