From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 26EB8C433EF for ; Wed, 2 Mar 2022 09:46:35 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 47A948D0002; Wed, 2 Mar 2022 04:46:34 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 401BE8D0001; Wed, 2 Mar 2022 04:46:34 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 2F0D88D0002; Wed, 2 Mar 2022 04:46:34 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (relay.a.hostedemail.com [64.99.140.24]) by kanga.kvack.org (Postfix) with ESMTP id 1C1018D0001 for ; Wed, 2 Mar 2022 04:46:34 -0500 (EST) Received: from smtpin04.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay02.hostedemail.com (Postfix) with ESMTP id 1B06A24971 for ; Wed, 2 Mar 2022 09:46:33 +0000 (UTC) X-FDA: 79198966266.04.7B09A13 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by imf07.hostedemail.com (Postfix) with ESMTP id 3784E40007 for ; Wed, 2 Mar 2022 09:46:32 +0000 (UTC) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 0A27E21121; Wed, 2 Mar 2022 09:46:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1646214391; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=zU0omcjj6FNIaoNnjAuvB9rjRyhqvWJLX6/CjwTASF8=; b=QW0dRsrypn1bL/WOIg/fcK0vKQ622cJBIy91DRi1HftPB079qk0Qp3mgboAmt1E+D0+Tnm hNk7pvvJ3+hre162gXcLEsjySGt59/5GrwLPA4kkRTyr1lOVUWO2Vo9cQ0WmVOVteMnq4y As5ox4USY1QcQJoDDh11k4LmjzdsFpU= Received: from suse.cz (unknown [10.100.201.86]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 1DE3CA3B85; Wed, 2 Mar 2022 09:46:28 +0000 (UTC) Date: Wed, 2 Mar 2022 10:46:28 +0100 From: Michal Hocko To: Andrew Morton Cc: cgel.zte@gmail.com, naoya.horiguchi@nec.com, minchan@kernel.org, hannes@cmpxchg.org, rogerq@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, guo.ziliang@zte.com.cn, Zeal Robot , Ran Xiaokai , Jiang Xuexin , Yang Yang Subject: Re: [PATCH linux-next] mm: swap: get rid of deadloop in swapin readahead Message-ID: References: <20220221111749.1928222-1-cgel.zte@gmail.com> <20220228073315.c855e15b108089187a40d2b0@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220228073315.c855e15b108089187a40d2b0@linux-foundation.org> X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 3784E40007 X-Stat-Signature: jbyequf6g4zyipncmobh9cnbratz1wfx Authentication-Results: imf07.hostedemail.com; dkim=pass header.d=suse.com header.s=susede1 header.b=QW0dRsry; spf=pass (imf07.hostedemail.com: domain of mhocko@suse.com designates 195.135.220.28 as permitted sender) smtp.mailfrom=mhocko@suse.com; dmarc=pass (policy=quarantine) header.from=suse.com X-HE-Tag: 1646214392-201473 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 28-02-22 07:33:15, Andrew Morton wrote: > On Mon, 28 Feb 2022 08:57:49 +0100 Michal Hocko wrote: > > > On Mon 21-02-22 11:17:49, cgel.zte@gmail.com wrote: > > > From: Guo Ziliang > > > > > > In our testing, a deadloop task was found. Through sysrq printing, same > > > stack was found every time, as follows: > > > __swap_duplicate+0x58/0x1a0 > > > swapcache_prepare+0x24/0x30 > > > __read_swap_cache_async+0xac/0x220 > > > read_swap_cache_async+0x58/0xa0 > > > swapin_readahead+0x24c/0x628 > > > do_swap_page+0x374/0x8a0 > > > __handle_mm_fault+0x598/0xd60 > > > handle_mm_fault+0x114/0x200 > > > do_page_fault+0x148/0x4d0 > > > do_translation_fault+0xb0/0xd4 > > > do_mem_abort+0x50/0xb0 > > > > > > The reason for the deadloop is that swapcache_prepare() always returns > > > EEXIST, indicating that SWAP_HAS_CACHE has not been cleared, so that > > > it cannot jump out of the loop. We suspect that the task that clears > > > the SWAP_HAS_CACHE flag never gets a chance to run. We try to lower > > > the priority of the task stuck in a deadloop so that the task that > > > clears the SWAP_HAS_CACHE flag will run. The results show that the > > > system returns to normal after the priority is lowered. > > > > > > In our testing, multiple real-time tasks are bound to the same core, > > > and the task in the deadloop is the highest priority task of the > > > core, so the deadloop task cannot be preempted. > > > > > > Although cond_resched() is used by __read_swap_cache_async, it is an > > > empty function in the preemptive system and cannot achieve the purpose > > > of releasing the CPU. A high-priority task cannot release the CPU > > > unless preempted by a higher-priority task. But when this task > > > is already the highest priority task on this core, other tasks > > > will not be able to be scheduled. So we think we should replace > > > cond_resched() with schedule_timeout_uninterruptible(1), > > > schedule_timeout_interruptible will call set_current_state > > > first to set the task state, so the task will be removed > > > from the running queue, so as to achieve the purpose of > > > giving up the CPU and prevent it from running in kernel > > > mode for too long. > > > > I am sorry but I really do not see how this case is any different from > > any other kernel code path being hogged by a RT task. We surely > > shouldn't put sleeps into all random paths which are doing cond_resched > > at the moment. > > But this cond_resched() is different from most. This one is attempting > to yield the CPU so this task can make progress. And cond_resched() > simply isn't an appropriate way of doing this because under this fairly > common situation, it's a no-op. I might be really missing something but I really do not see how is this any different from the page allocator path which only does cond_resched as well (well, except for throttling but that might just not trigger). Or other paths which just do cond_resched while waiting for a progress somewhere else. Not that I like this situation but !PREEMPT kernel with RT priority tasks is rather limited and full of potential priblems IMHO. -- Michal Hocko SUSE Labs