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[37.188.253.35]) by smtp.gmail.com with ESMTPSA id i10sm70877339wrn.53.2020.03.12.13.16.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 12 Mar 2020 13:16:25 -0700 (PDT) Date: Thu, 12 Mar 2020 21:16:24 +0100 From: Michal Hocko To: David Rientjes Cc: Andrew Morton , Vlastimil Babka , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [patch] mm, oom: prevent soft lockup on memcg oom for UP systems Message-ID: <20200312201624.GD23944@dhcp22.suse.cz> References: <20200310221019.GE8447@dhcp22.suse.cz> <20200311082736.GA23944@dhcp22.suse.cz> <20200312083241.GT23944@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 Thu 12-03-20 11:20:33, David Rientjes wrote: > On Thu, 12 Mar 2020, Michal Hocko wrote: > > > > I think the changelog clearly states that we need to guarantee that a > > > reclaimer will yield the processor back to allow a victim to exit. This > > > is where we make the guarantee. If it helps for the specific reason it > > > triggered in my testing, we could add: > > > > > > "For example, mem_cgroup_protected() can prohibit reclaim and thus any > > > yielding in page reclaim would not address the issue." > > > > I would suggest something like the following: > > " > > The reclaim path (including the OOM) relies on explicit scheduling > > points to hand over execution to tasks which could help with the reclaim > > process. > > Are there other examples where yielding in the reclaim path would "help > with the reclaim process" other than oom victims? This sentence seems > vague. In the context of UP and !PREEMPT this also includes IO flushers, filesystems rely on workers and there are things I am very likely not aware of. If you think this is vaague then feel free to reformulate. All I really do care about is what the next paragraph is explaining. > > Currently it is mostly shrink_page_list which yields CPU for > > each reclaimed page. This might be insuficient though in some > > configurations. E.g. when a memcg OOM path is triggered in a hierarchy > > which doesn't have any reclaimable memory because of memory reclaim > > protection (MEMCG_PROT_MIN) then there is possible to trigger a soft > > lockup during an out of memory situation on non preemptible kernels > > > > > > Fix this by adding a cond_resched up in the reclaim path and make sure > > there is a yield point regardless of reclaimability of the target > > hierarchy. > > " > > -- Michal Hocko SUSE Labs