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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id ADAABC43217 for ; Mon, 28 Feb 2022 15:33:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237551AbiB1Pd6 (ORCPT ); Mon, 28 Feb 2022 10:33:58 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53652 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235286AbiB1Pd4 (ORCPT ); Mon, 28 Feb 2022 10:33:56 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A219013F45 for ; Mon, 28 Feb 2022 07:33:17 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 476CD60FD8 for ; Mon, 28 Feb 2022 15:33:17 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3AF41C340EE; Mon, 28 Feb 2022 15:33:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1646062396; bh=WyaUKD0KNG92cBo3Esei4c8zkTolBVRdBS/KulblFow=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=LAY3gW5hFuQ8oUSh1itgzAjoL6/E96p44FjK1U3pvv4mRVto83GNRzWzCWYNlGJ0j El70OsW5IK4qSjOH0UHpn+ZlE91FyYwPx1RoSRFSXkB1bFyDFO+7MGAP0gJwFaK21E 60QQfyBVeyFUarU9vjnmEfoc4k9PB4B4CNmLnbkw= Date: Mon, 28 Feb 2022 07:33:15 -0800 From: Andrew Morton To: Michal Hocko 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: <20220228073315.c855e15b108089187a40d2b0@linux-foundation.org> In-Reply-To: References: <20220221111749.1928222-1-cgel.zte@gmail.com> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.