From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-11.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D6100C43461 for ; Fri, 11 Sep 2020 02:52:04 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 488CD21941 for ; Fri, 11 Sep 2020 02:52:03 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 488CD21941 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 65EC78E0003; Thu, 10 Sep 2020 22:52:03 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 60F2B8E0001; Thu, 10 Sep 2020 22:52:03 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5258C8E0003; Thu, 10 Sep 2020 22:52:03 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0019.hostedemail.com [216.40.44.19]) by kanga.kvack.org (Postfix) with ESMTP id 3B9CA8E0001 for ; Thu, 10 Sep 2020 22:52:03 -0400 (EDT) Received: from smtpin21.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 030CF3624 for ; Fri, 11 Sep 2020 02:52:03 +0000 (UTC) X-FDA: 77249256126.21.yard94_2e02956270eb Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin21.hostedemail.com (Postfix) with ESMTP id CE44D180442C3 for ; Fri, 11 Sep 2020 02:52:02 +0000 (UTC) X-HE-Tag: yard94_2e02956270eb X-Filterd-Recvd-Size: 9398 Received: from out30-43.freemail.mail.aliyun.com (out30-43.freemail.mail.aliyun.com [115.124.30.43]) by imf03.hostedemail.com (Postfix) with ESMTP for ; Fri, 11 Sep 2020 02:52:01 +0000 (UTC) X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R461e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04357;MF=alex.shi@linux.alibaba.com;NM=1;PH=DS;RN=24;SR=0;TI=SMTPD_---0U8YF63K_1599792714; Received: from IT-FVFX43SYHV2H.local(mailfrom:alex.shi@linux.alibaba.com fp:SMTPD_---0U8YF63K_1599792714) by smtp.aliyun-inc.com(127.0.0.1); Fri, 11 Sep 2020 10:51:55 +0800 Subject: Re: [PATCH v18 00/32] per memcg lru_lock: reviews To: Hugh Dickins Cc: Andrew Morton , mgorman@techsingularity.net, tj@kernel.org, khlebnikov@yandex-team.ru, daniel.m.jordan@oracle.com, willy@infradead.org, hannes@cmpxchg.org, lkp@intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, shakeelb@google.com, iamjoonsoo.kim@lge.com, richard.weiyang@gmail.com, kirill@shutemov.name, alexander.duyck@gmail.com, rong.a.chen@intel.com, mhocko@suse.com, vdavydov.dev@gmail.com, shy828301@gmail.com, vbabka@suse.cz, minchan@kernel.org, cai@lca.pw References: <1598273705-69124-1-git-send-email-alex.shi@linux.alibaba.com> <20200824114204.cc796ca182db95809dd70a47@linux-foundation.org> <61a42a87-eec9-e300-f710-992756f70de6@linux.alibaba.com> From: Alex Shi Message-ID: <855ad6ee-dba4-9729-78bd-23e392905cf6@linux.alibaba.com> Date: Fri, 11 Sep 2020 10:50:56 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 X-Rspamd-Queue-Id: CE44D180442C3 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam05 Content-Transfer-Encoding: quoted-printable 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: =E5=9C=A8 2020/9/10 =E4=B8=8A=E5=8D=887:16, Hugh Dickins =E5=86=99=E9=81=93= : > On Wed, 9 Sep 2020, Alex Shi wrote: >> =E5=9C=A8 2020/9/9 =E4=B8=8A=E5=8D=887:41, Hugh Dickins =E5=86=99=E9=81= =93: >>> >>> [PATCH v18 05/32] mm/thp: remove code path which never got into >>> This is a good simplification, but I see no sign that you understand >>> why it's valid: it relies on lru_add_page_tail() being called while >>> head refcount is frozen to 0: we would not get this far if someone >>> else holds a reference to the THP - which they must hold if they have >>> isolated the page from its lru (and that's true before or after your >>> per-memcg changes - but even truer after those changes, since PageLRU >>> can then be flipped without lru_lock at any instant): please explain >>> something of this in the commit message. >> >> Is the following commit log better? >> >> split_huge_page() will never call on a page which isn't on lru lis= t, so >> this code never got a chance to run, and should not be run, to add= tail >> pages on a lru list which head page isn't there. >> >> Hugh Dickins' mentioned: >> The path should never be called since lru_add_page_tail() being ca= lled >> while head refcount is frozen to 0: we would not get this far if s= omeone >> else holds a reference to the THP - which they must hold if they h= ave >> isolated the page from its lru. >> >> Although the bug was never triggered, it'better be removed for cod= e >> correctness, and add a warn for unexpected calling. >=20 > Not much better, no. split_huge_page() can easily be called for a page > which is not on the lru list at the time,=20 Hi Hugh, Thanks for comments! There are some discussion on this point a couple of weeks ago, https://lkml.org/lkml/2020/7/9/760 Matthew Wilcox and Kirill have the following comments, > I don't understand how we get to split_huge_page() with a page that's > not on an LRU list. Both anonymous and page cache pages should be on > an LRU list. What am I missing? Right, and it's never got removed from LRU during the split. The tail pages have to be added to LRU because they now separate from the tail page. --=20 Kirill A. Shutemov > and I don't know what was the > bug which was never triggered. =20 So the only path to the removed part should be a bug, like sth here, https://lkml.org/lkml/2020/7/10/118 or https://lkml.org/lkml/2020/7/10/972 > Stick with whatever text you end up with > for the combination of 05/32 and 18/32, and I'll rewrite it after. I am not object to merge them into one, I just don't know how to say clear about 2 patches in commit log. As patch 18, TestClearPageLRU add the incorrect posibility of remove lru bit during split, that's the reason of code path rewrite and a WARN there. Thanks Alex >=20 >>> [PATCH v18 06/32] mm/thp: narrow lru locking >>> Why? What part does this play in the series? "narrow lru locking" can >>> also be described as "widen page cache locking":=20 >> >> Uh, the page cache locking isn't widen, it's still on the old place. >=20 > I'm not sure if you're joking there. Perhaps just a misunderstanding. >=20 > Yes, patch 06/32 does not touch the xa_lock(&mapping->i_pages) and > xa_lock(&swap_cache->i_pages) lines (odd how we've arrived at two of > those, but please do not get into cleaning it up now); but it removes > the spin_lock_irqsave(&pgdata->lru_lock, flags) which used to come > before them, and inserts a spin_lock(&pgdat->lru_lock) after them. >=20 > You call that narrowing the lru locking, okay, but I see it as also > pushing the page cache locking outwards: before this patch, page cache > lock was taken inside lru_lock; after this patch, page cache lock is > taken outside lru_lock. If you cannot see that, then I think you > should not have touched this code at all; but it's what we have > been testing, and I think we should go forward with it. >=20 >>> But I wish you could give some reason for it in the commit message! >> >> It's a head scratch task. Would you like to tell me what's detailed in= fo=20 >> should be there? Thanks! >=20 > So, you don't know why you did it either: then it will be hard to > justify. I guess I'll have to write something for it later. I'm > strongly tempted just to drop the patch, but expect it will become > useful later, for using lock_page_memcg() before getting lru_lock. >=20 I thought the xa_lock and lru_lock relationship was described clear in the commit log, and still no idea of the move_lock in the chain. Please refill them for what I overlooked. Thanks! >>> Signed-off-by: Wei Yang >>> Is that correct? Or Wei Yang suggested some part of it perhaps? >> >> Yes, we talked a lot to confirm the locking change is safe. >=20 > Okay, but the patch was written by you, and sent by you to Andrew: > that is not a case for "Signed-off-by: Someone Else". >=20 Ok. let's remove his signed-off. >>> [PATCH v18 27/32] mm/swap.c: optimizing __pagevec_lru_add lru_lock >>> Could we please drop this one for the moment? And come back to it lat= er >>> when the basic series is safely in. It's a good idea to try sorting >>> together those pages which come under the same lock (though my guess = is >>> that they naturally gather themselves together quite well already); b= ut >>> I'm not happy adding 360 bytes to the kernel stack here (and that in >>> addition to 192 bytes of horrid pseudo-vma in the shmem swapin case), >>> though that could be avoided by making it per-cpu. But I hope there's >>> a simpler way of doing it, as efficient, but also useful for the othe= r >>> pagevec operations here: perhaps scanning the pagevec for same page-> >>> mem_cgroup (and flags node bits), NULLing entries as they are done. >>> Another, easily fixed, minor defect in this patch: if I'm reading it >>> right, it reverses the order in which the pages are put on the lru? >> >> this patch could give about 10+% performance gain on my multiple memcg >> readtwice testing. fairness locking cost the performance much. >=20 > Good to know, should have been mentioned. s/fairness/Repeated/ >=20 > But what was the gain or loss on your multiple memcg readtwice > testing without this patch, compared against node-only lru_lock? > The 80% gain mentioned before, I presume. So this further > optimization can wait until the rest is solid. the gain based on the patch 26. >=20 >> >> I also tried per cpu solution but that cause much trouble of per cpu f= unc >> things, and looks no benefit except a bit struct size of stack, so if=20 >> stack size still fine. May we could use the solution and improve it be= tter. >> like, functionlize, fix the reverse issue etc. >=20 > I don't know how important the stack depth consideration is nowadays: > I still care, maybe others don't, since VMAP_STACK became an option. >=20 > Yes, please fix the reversal (if I was right on that); and I expect > you could use a singly linked list instead of the double. single linked list is more saving, but do we have to reverse walking to s= eek the head or tail for correct sequence? >=20 > But I'll look for an alternative - later, once the urgent stuff > is completed - and leave the acks on this patch to others. Ok, looking forward for your new solution! Thanks Alex