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s=arc-20220608; d=hostedemail.com; t=1734920594; a=rsa-sha256; cv=none; b=XVMI50zOqhcl1OPMzD+qB7wJX3j4NxQ+myEz6ghajBiaybe5+Q+nxlEa6Q7pJuUTnZfWF0 2HtEYyb375fPeQqJ+8nsn5JaGKoimu+V2U3hG2uVVaDwnW4fwEmgWU7KpMh0ZatbnQv5c5 DWyiAOJjnSaRM6uJCsCjbkFMj/pPLxc= ARC-Authentication-Results: i=1; imf15.hostedemail.com; dkim=none; spf=pass (imf15.hostedemail.com: domain of chenridong@huaweicloud.com designates 45.249.212.51 as permitted sender) smtp.mailfrom=chenridong@huaweicloud.com; dmarc=none Received: from mail.maildlp.com (unknown [172.19.163.216]) by dggsgout11.his.huawei.com (SkyGuard) with ESMTP id 4YGhdV1JPyz4f3kvP for ; Mon, 23 Dec 2024 10:23:10 +0800 (CST) Received: from mail02.huawei.com (unknown [10.116.40.112]) by mail.maildlp.com (Postfix) with ESMTP id DD2951A0196 for ; Mon, 23 Dec 2024 10:23:30 +0800 (CST) Received: from [10.67.109.79] (unknown [10.67.109.79]) by APP1 (Coremail) with SMTP id cCh0CgB3a62hyWhniXD6FA--.18923S2; Mon, 23 Dec 2024 10:23:30 +0800 (CST) Message-ID: <6319a3df-b19c-4e35-a46c-c4a5ea003a6b@huaweicloud.com> Date: Mon, 23 Dec 2024 10:23:29 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] memcg: fix soft lockup in the OOM process To: Michal Hocko , Andrew Morton Cc: hannes@cmpxchg.org, yosryahmed@google.com, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, davidf@vimeo.com, vbabka@suse.cz, handai.szj@taobao.com, rientjes@google.com, kamezawa.hiroyu@jp.fujitsu.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, chenridong@huawei.com, wangweiyang2@huawei.com References: <20241220103123.3677988-1-chenridong@huaweicloud.com> <20241220144734.05d62ef983fa92e96e29470d@linux-foundation.org> Content-Language: en-US From: Chen Ridong In-Reply-To: Content-Type: text/plain; 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As a result, this lead to soft lockup >>> in the OOM process. >>> >>> To fix this issue, call 'cond_resched' in the 'mem_cgroup_scan_tasks' >>> function per 1000 iterations. For global OOM, call >>> 'touch_softlockup_watchdog' per 1000 iterations to avoid this issue. >>> >>> ... >>> >>> --- a/include/linux/oom.h >>> +++ b/include/linux/oom.h >>> @@ -14,6 +14,13 @@ struct notifier_block; >>> struct mem_cgroup; >>> struct task_struct; >>> >>> +/* When it traverses for long time, to prevent softlockup, call >>> + * cond_resched/touch_softlockup_watchdog very 1000 iterations. >>> + * The 1000 value is not exactly right, it's used to mitigate the overhead >>> + * of cond_resched/touch_softlockup_watchdog. >>> + */ >>> +#define SOFTLOCKUP_PREVENTION_LIMIT 1000 >> >> If this is to have potentially kernel-wide scope, its name should >> identify which subsystem it belongs to. Maybe OOM_KILL_RESCHED or >> something. >> >> But I'm not sure that this really needs to exist. Are the two usage >> sites particularly related? > > Yes, I do not think this needs to pretend to be a more generic mechanism > to prevent soft lockups. The number of iterations highly depends on the > operation itself. > Thanks， I will update. >> >>> enum oom_constraint { >>> CONSTRAINT_NONE, >>> CONSTRAINT_CPUSET, >>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >>> index 5c373d275e7a..f4c12d6e7b37 100644 >>> --- a/mm/memcontrol.c >>> +++ b/mm/memcontrol.c >>> @@ -1161,6 +1161,7 @@ void mem_cgroup_scan_tasks(struct mem_cgroup *memcg, >>> { >>> struct mem_cgroup *iter; >>> int ret = 0; >>> + int i = 0; >>> >>> BUG_ON(mem_cgroup_is_root(memcg)); >>> >>> @@ -1169,8 +1170,11 @@ void mem_cgroup_scan_tasks(struct mem_cgroup *memcg, >>> struct task_struct *task; >>> >>> css_task_iter_start(&iter->css, CSS_TASK_ITER_PROCS, &it); >>> - while (!ret && (task = css_task_iter_next(&it))) >>> + while (!ret && (task = css_task_iter_next(&it))) { >>> ret = fn(task, arg); >>> + if (++i % SOFTLOCKUP_PREVENTION_LIMIT) >> >> And a modulus operation is somewhat expensive. > > This is a cold path used during OOM. While we can make it more optimal I > doubt it matters in practice so we should aim at readbility. I do not > mind either way, I just wanted to note that this is not performance > sensitive. > I think '(++i & 1023)' is much better, I will update. Thank you all gays. Best regards Ridong >> >> Perhaps a simple >> >> /* Avoid potential softlockup warning */ >> if ((++i & 1023) == 0) >> >> at both sites will suffice. Opinions might vary... >> >