From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f43.google.com (mail-oi0-f43.google.com [209.85.218.43]) by kanga.kvack.org (Postfix) with ESMTP id 9E85F828DF for ; Thu, 14 Jan 2016 17:21:30 -0500 (EST) Received: by mail-oi0-f43.google.com with SMTP id w75so93230881oie.0 for ; Thu, 14 Jan 2016 14:21:30 -0800 (PST) Received: from www262.sakura.ne.jp (www262.sakura.ne.jp. [2001:e42:101:1:202:181:97:72]) by mx.google.com with ESMTPS id xu2si9422906oec.67.2016.01.14.14.21.29 for (version=TLS1 cipher=AES128-SHA bits=128/128); Thu, 14 Jan 2016 14:21:29 -0800 (PST) Subject: Re: [PATCH v2] mm,oom: Exclude TIF_MEMDIE processes from candidates. From: Tetsuo Handa References: <201601131952.HAJ18298.OQLtSOFOFFMVJH@I-love.SAKURA.ne.jp> <201601141926.JHG56933.OFFHOFOLQMtJSV@I-love.SAKURA.ne.jp> In-Reply-To: Message-Id: <201601150721.HJC60832.LMOtQHFFFSJOOV@I-love.SAKURA.ne.jp> Date: Fri, 15 Jan 2016 07:21:17 +0900 Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: owner-linux-mm@kvack.org List-ID: To: rientjes@google.com Cc: mhocko@kernel.org, hannes@cmpxchg.org, akpm@linux-foundation.org, mgorman@suse.de, torvalds@linux-foundation.org, oleg@redhat.com, hughd@google.com, andrea@kernel.org, riel@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org David Rientjes wrote: > On Thu, 14 Jan 2016, Tetsuo Handa wrote: > > > > > > > @@ -171,7 +195,7 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg, > > > > > > if (oom_unkillable_task(p, memcg, nodemask)) > > > > > > return 0; > > > > > > > > > > > > - p = find_lock_task_mm(p); > > > > > > + p = find_lock_non_victim_task_mm(p); > > > > > > if (!p) > > > > > > return 0; > > > > > > > > > > > > > > > > I understand how this may make your test case pass, but I simply don't > > > > > understand how this could possibly be the correct thing to do. This would > > > > > cause oom_badness() to return 0 for any process where a thread has > > > > > TIF_MEMDIE set. If the oom killer is called from the page allocator, > > > > > kills a thread, and it is recalled before that thread may exit, then this > > > > > will panic the system if there are no other eligible processes to kill. > > > > > > > > > Why? oom_badness() is called after oom_scan_process_thread() returned OOM_SCAN_OK. > > > > oom_scan_process_thread() returns OOM_SCAN_ABORT if a thread has TIF_MEMDIE set. > > > > > > > > > > oom_scan_process_thread() checks for TIF_MEMDIE on p, not on p's threads. > > > If one of p's threads has TIF_MEMDIE set and p does not, we actually want > > > to set TIF_MEMDIE for p. That's the current behavior since it will lead > > > to p->mm memory freeing. Your patch is excluding such processes entirely > > > and selecting another process to kill unnecessarily. > > > > > > > I think p's threads are checked by oom_scan_process_thread() for TIF_MEMDIE > > even if p does not have TIF_MEMDIE. What am I misunderstanding about what > > for_each_process_thread(g, p) is doing? > > > > #define for_each_process_thread(p, t) for_each_process(p) for_each_thread(p, t) > > > > select_bad_process() { > > for_each_process_thread(g, p) { > > oom_scan_process_thread(oc, p, totalpages)); > > oom_badness(p); > > } > > } > > > > Yes, select_bad_process() iterates over threads, that is obvious. The > point is that today it can select a thread independent of whether any of > its other threads have TIF_MEMDIE set, which is the desired behavior per > the above. With your change, that is no longer possible because we > disregard _all_ threads if one of them has TIF_MEMDIE set. > I still cannot understand. Today select_bad_process() can select a thread independent of whether any of its other threads have TIF_MEMDIE set. But select_bad_process() after all ignores that thread selected by oom_badness() logic and aborts the iterate loop as soon as oom_scan_process_thread() finds a TIF_MEMDIE thread from all threads. Changing oom_badness() logic to skip processes with TIF_MEMDIE threads does not change the task select_bad_process() logic will finally return. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org