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[115.124.30.133]) by mx.google.com with ESMTPS id f62si18039579plb.339.2019.05.20.02.43.24 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 20 May 2019 02:43:24 -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=01201311R151e4;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_---0TSCntOc_1558345401; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0TSCntOc_1558345401) by smtp.aliyun-inc.com(127.0.0.1); Mon, 20 May 2019 17:43:22 +0800 Subject: Re: [v2 PATCH] mm: vmscan: correct nr_reclaimed for THP To: Johannes Weiner Cc: Michal Hocko , Yang Shi , Huang Ying , Mel Gorman , kirill.shutemov@linux.intel.com, Hugh Dickins , Shakeel Butt , william.kucharski@oracle.com, Andrew Morton , Linux MM , Linux Kernel Mailing List References: <1557505420-21809-1-git-send-email-yang.shi@linux.alibaba.com> <20190513080929.GC24036@dhcp22.suse.cz> <20190513214503.GB25356@dhcp22.suse.cz> <20190514062039.GB20868@dhcp22.suse.cz> <509de066-17bb-e3cf-d492-1daf1cb11494@linux.alibaba.com> <20190516151012.GA20038@cmpxchg.org> From: Yang Shi Message-ID: Date: Mon, 20 May 2019 17:43:20 +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: <20190516151012.GA20038@cmpxchg.org> Content-Type: text/plain; charset=utf-8; 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 5/16/19 11:10 PM, Johannes Weiner wrote: > On Tue, May 14, 2019 at 01:44:35PM -0700, Yang Shi wrote: >> On 5/13/19 11:20 PM, Michal Hocko wrote: >>> On Mon 13-05-19 21:36:59, Yang Shi wrote: >>>> On Mon, May 13, 2019 at 2:45 PM Michal Hocko wrote: >>>>> On Mon 13-05-19 14:09:59, Yang Shi wrote: >>>>> [...] >>>>>> I think we can just account 512 base pages for nr_scanned for >>>>>> isolate_lru_pages() to make the counters sane since PGSCAN_KSWAPD/DIRECT >>>>>> just use it. >>>>>> >>>>>> And, sc->nr_scanned should be accounted as 512 base pages too otherwise we >>>>>> may have nr_scanned < nr_to_reclaim all the time to result in false-negative >>>>>> for priority raise and something else wrong (e.g. wrong vmpressure). >>>>> Be careful. nr_scanned is used as a pressure indicator to slab shrinking >>>>> AFAIR. Maybe this is ok but it really begs for much more explaining >>>> I don't know why my company mailbox didn't receive this email, so I >>>> replied with my personal email. >>>> >>>> It is not used to double slab pressure any more since commit >>>> 9092c71bb724 ("mm: use sc->priority for slab shrink targets"). It uses >>>> sc->priority to determine the pressure for slab shrinking now. >>>> >>>> So, I think we can just remove that "double slab pressure" code. It is >>>> not used actually and looks confusing now. Actually, the "double slab >>>> pressure" does something opposite. The extra inc to sc->nr_scanned >>>> just prevents from raising sc->priority. >>> I have to get in sync with the recent changes. I am aware there were >>> some patches floating around but I didn't get to review them. I was >>> trying to point out that nr_scanned used to have a side effect to be >>> careful about. If it doesn't have anymore then this is getting much more >>> easier of course. Please document everything in the changelog. >> Thanks for reminding. Yes, I remembered nr_scanned would double slab >> pressure. But, when I inspected into the code yesterday, it turns out it is >> not true anymore. I will run some test to make sure it doesn't introduce >> regression. > Yeah, sc->nr_scanned is used for three things right now: > > 1. vmpressure - this looks at the scanned/reclaimed ratio so it won't > change semantics as long as scanned & reclaimed are fixed in parallel > > 2. compaction/reclaim - this is broken. Compaction wants a certain > number of physical pages freed up before going back to compacting. > Without Yang Shi's fix, we can overreclaim by a factor of 512. > > 3. kswapd priority raising - this is broken. 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. > > Both sc->nr_scanned & sc->nr_reclaimed should be fixed. Yes, v3 patch (sit in my local repo now) did fix both. > >> BTW, I noticed the counter of memory reclaim is not correct with THP swap on >> vanilla kernel, please see the below: >> >> pgsteal_kswapd 21435 >> pgsteal_direct 26573329 >> pgscan_kswapd 3514 >> pgscan_direct 14417775 >> >> pgsteal is always greater than pgscan, my patch could fix the problem. > Ouch, how is that possible with the current code? > > I think it happens when isolate_lru_pages() counts 1 nr_scanned for a > THP, then shrink_page_list() splits the THP and we reclaim tail pages > one by one. This goes all the way back to the initial THP patch! I think so. It does make sense. But, the weird thing is I just see this with synchronous swap device (some THPs got swapped out in a whole, some got split), but I've never seen this with rotate swap device (all THPs got split). I haven't figured out why. > > isolate_lru_pages() needs to be fixed. Its return value, nr_taken, is > correct, but its *nr_scanned parameter is wrong, which causes issues: > > 1. The trace point, as Yang Shi pointed out, will underreport the > number of pages scanned, as it reports it along with nr_to_scan (base > pages) and nr_taken (base pages) > > 2. vmstat and memory.stat count 'struct page' operations rather than > base pages, which makes zero sense to neither user nor kernel > developers (I routinely multiply these counters by 4096 to get a sense > of work performed). > > All of isolate_lru_pages()'s accounting should be in base pages, which > includes nr_scanned and PGSCAN_SKIPPED. > > That should also simplify the code; e.g.: > > for (total_scan = 0; > scan < nr_to_scan && nr_taken < nr_to_scan && !list_empty(src); > total_scan++) { > > scan < nr_to_scan && nr_taken >= nr_to_scan is a weird condition that > does not make sense in page reclaim imo. Reclaim cares about physical > memory - freeing one THP is as much progress for reclaim as freeing > 512 order-0 pages. Yes, I do agree. The v3 patch did this. > > IMO *all* '++' in vmscan.c are suspicious and should be reviewed: > nr_scanned, nr_reclaimed, nr_dirty, nr_unqueued_dirty, nr_congested, > nr_immediate, nr_writeback, nr_ref_keep, nr_unmap_fail, pgactivate, > total_scan & scan, nr_skipped. Some of them should be fine but I'm not sure the side effect. IMHO, let's fix the most obvious problem first. > > Yang Shi, it would be nice if you could convert all of these to base > page accounting in one patch, as it's a single logical fix for the > initial introduction of THP that had huge pages show up on the LRUs.\ Yes, sure. > > [ check_move_unevictable_pages() seems weird. It gets a pagevec from > find_get_entries(), which, if I understand the THP page cache code > correctly, might contain the same compound page over and over. It'll > be !unevictable after the first iteration, so will only run once. So > it produces incorrect numbers now, but it is probably best to ignore > it until we figure out THP cache. Maybe add an XXX comment. ]