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[213.151.95.130]) by smtp.gmail.com with ESMTPSA id x17sm1123676wrt.74.2020.02.17.06.04.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 17 Feb 2020 06:04:31 -0800 (PST) Date: Mon, 17 Feb 2020 15:04:30 +0100 From: Michal Hocko To: Yafang Shao Cc: Roman Gushchin , Johannes Weiner , Vladimir Davydov , Andrew Morton , Chris Down , Linux MM , stable@vger.kernel.org Subject: Re: [PATCH resend] mm, memcg: reset memcg's memory.{min, low} for reclaiming itself Message-ID: <20200217140430.GO31531@dhcp22.suse.cz> References: <20200216145249.6900-1-laoar.shao@gmail.com> <20200217092459.GG31531@dhcp22.suse.cz> <20200217132443.GM31531@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 Mon 17-02-20 21:51:23, Yafang Shao wrote: > On Mon, Feb 17, 2020 at 9:24 PM Michal Hocko wrote: > > > > On Mon 17-02-20 21:08:12, Yafang Shao wrote: > > > On Mon, Feb 17, 2020 at 5:25 PM Michal Hocko wrote: > > > > > > > > On Sun 16-02-20 09:52:49, Yafang Shao wrote: > > > > > memory.{emin, elow} are set in mem_cgroup_protected(), and the values of > > > > > them won't be changed until next recalculation in this function. After > > > > > either or both of them are set, the next reclaimer to relcaim this memcg > > > > > may be a different reclaimer, e.g. this memcg is also the root memcg of > > > > > the new reclaimer, and then in mem_cgroup_protection() in get_scan_count() > > > > > the old values of them will be used to calculate scan count, that is not > > > > > proper. We should reset them to zero in this case. > > > > > > > > > > Here's an example of this issue. > > > > > > > > > > root_mem_cgroup > > > > > / > > > > > A memory.max=1024M memory.min=512M memory.current=800M > > > > > > > > > > Once kswapd is waked up, it will try to scan all MEMCGs, including > > > > > this A, and it will assign memory.emin of A with 512M. > > > > > After that, A may reach its hard limit(memory.max), and then it will > > > > > do memcg reclaim. Because A is the root of this reclaimer, so it will > > > > > not calculate its memory.emin. So the memory.emin is the old value > > > > > 512M, and then this old value will be used in > > > > > mem_cgroup_protection() in get_scan_count() to get the scan count. > > > > > That is not proper. > > > > > > > > Please document user visible effects of this patch. What does it mean > > > > that this is not proper behavior? > > > > > > In the memcg reclaim, if the target memcg is the root of the reclaimer, > > > the reclaimer should scan this memcg's all page cache pages in the LRU, > > > but now as the old memcg.{emin, elow} value are still there, it will get > > > a wrong protection value, > > > and the reclaimer can't reclaim the page cache pages protected by this > > > wrong protection. > > > > Could you be more specific please. Your example above says that emin is > > not going to be recalculated and stays at 512M even for a potential max > > limit reclaim. The min limit is still 512M so why is this value wrong? > > > > Because the relcaimers are changed or the root the relcaimer is changed. > > Kswapd begins to relcaim memcg-A. > kswapd > | > calculate the {emin, elow} for memcg-A > | > stores {emin, elow} in memory.{emin, elow} of memcg-A > | > This memory.{emin, elow} will protect the page cache pages in memcg-A > (See get_scan_count->mem_cgroup_protection) > | > exit > (And it won't relcaim memcg-A for a long time) > > > Then memcg relcaimer is woke up (reached the hard limit of memcg-A), > and the root of this new reclaimer is memcg-A. > > This memcg relcaimer begins to reclaim memcg-A. > memcg relcaimer > | > As the root of the relcaimer is memcg-A, it won't calculate emin, elow > for memcg-A. > (See if (memcg == root) in mem_cgroup_protected()) > | > The old memory.{emin, elow} will protect the page cache pages in memcg-A > (SO WE SHOULD CLEAR THE OLD VALUE) I am sorry but I still do not follow. Could you focus on _why_ the old value is no longer valid? Btw. have you seen the latest patch from Johannes touching this area [1]? Is it possible that the issue you are referring to is related with the one he has fixed? [1] http://lkml.kernel.org/r/20191219200718.15696-2-hannes@cmpxchg.org -- Michal Hocko SUSE Labs