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[213.151.95.130]) by smtp.gmail.com with ESMTPSA id o7sm2657604wmh.11.2020.02.18.00.59.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 18 Feb 2020 00:59:51 -0800 (PST) Date: Tue, 18 Feb 2020 09:59:51 +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: <20200218085951.GE21113@dhcp22.suse.cz> References: <20200217092459.GG31531@dhcp22.suse.cz> <20200217132443.GM31531@dhcp22.suse.cz> <20200217140430.GO31531@dhcp22.suse.cz> <20200217143529.GQ31531@dhcp22.suse.cz> <20200217151417.GS31531@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Tue 18-02-20 10:09:06, Yafang Shao wrote: > On Mon, Feb 17, 2020 at 11:14 PM Michal Hocko wrote: > > > > On Mon 17-02-20 22:40:22, Yafang Shao wrote: > > > On Mon, Feb 17, 2020 at 10:35 PM Michal Hocko wrote: > > > > > > > > On Mon 17-02-20 22:28:38, Yafang Shao wrote: > > > > > On Mon, Feb 17, 2020 at 10:04 PM Michal Hocko wrote: > > > > > > > > > > > > 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? > > > > > > > > > > Because for the new reclaimer the memory.{emin, elow} should be 0. > > > > > The old value may be not 0, but it was thought as 0 in the if > > > > > statement (if (memcg == root)). > > > > > > > > Why should it be 0 when the A.min is still 512MB? > > > > > > Because A's hard limit is reached and A is the root of memcg relcaimer. > > > > Confused. But your examples suggests that memory.max > memory.min so > > having an effective emin 0 or not doesn't make any difference. > > > > Why is it having an effective emin 0 if memory.max > memory.min ? > Note that effective emin is only set in function > mem_cgroup_protected(), so if we don't set it explicitly to 0 then it > can't be 0. > > Besides mem_cgroup_protected(), the effective emin also take effect in > the function mem_cgroup_protection(), but in this function it only use > the existed memory.emin rather than verifying memory.max > memory.min. > > So the real issue is in mem_cgroup_protection(), because the value it > is using may be an old value. I am sorry but I still do not follow. You keep focusing on talking about the code while I am really interested in the user visible semantic that you want to achieve. I am sorry to be dense here but believe me I am trying. Your example doesn't help much because the effective protection doesn't play any role in the limit reclaim there AFAICS. I would even argue that emin == min is the proper thing in your example. So I can only recommend you to rethink your usecase and try to describe it in a higher level way. Thanks! -- Michal Hocko SUSE Labs