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 X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7CE53C33CA9 for ; Mon, 13 Jan 2020 00:54:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 581612187F for ; Mon, 13 Jan 2020 00:54:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387575AbgAMAyG (ORCPT ); Sun, 12 Jan 2020 19:54:06 -0500 Received: from out4436.biz.mail.alibaba.com ([47.88.44.36]:23108 "EHLO out4436.biz.mail.alibaba.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387502AbgAMAyF (ORCPT ); Sun, 12 Jan 2020 19:54:05 -0500 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R151e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04423;MF=shile.zhang@linux.alibaba.com;NM=1;PH=DS;RN=5;SR=0;TI=SMTPD_---0TnVzo8d_1578876843; Received: from ali-6c96cfdd1403.local(mailfrom:shile.zhang@linux.alibaba.com fp:SMTPD_---0TnVzo8d_1578876843) by smtp.aliyun-inc.com(127.0.0.1); Mon, 13 Jan 2020 08:54:03 +0800 Subject: Re: [PATCH RESEND] mm: fix tick_sched timer blocked by pgdat_resize_lock To: Kirill Tkhai , Andrew Morton , Pavel Tatashin Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20200110082510.172517-2-shile.zhang@linux.alibaba.com> <20200110093053.34777-1-shile.zhang@linux.alibaba.com> <1ee6088c-9e72-8824-3a9a-fc099d196faf@virtuozzo.com> From: Shile Zhang Message-ID: Date: Mon, 13 Jan 2020 08:54:03 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:60.0) Gecko/20100101 Thunderbird/60.9.1 MIME-Version: 1.0 In-Reply-To: <1ee6088c-9e72-8824-3a9a-fc099d196faf@virtuozzo.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020/1/10 19:42, Kirill Tkhai wrote: > On 10.01.2020 12:30, Shile Zhang wrote: >> When 'CONFIG_DEFERRED_STRUCT_PAGE_INIT' is set, 'pgdat_resize_lock' >> will be called inside 'pgdatinit' kthread to initialise the deferred >> pages with local interrupts disabled. Which is introduced by >> commit 3a2d7fa8a3d5 ("mm: disable interrupts while initializing deferred >> pages"). >> >> But 'pgdatinit' kthread is possible be pined on the boot CPU (CPU#0 by >> default), especially in small system with NRCPUS <= 2. In this case, the >> interrupts are disabled on boot CPU during memory initialising, which >> caused the tick_sched timer be blocked, leading to wall clock stuck. >> >> Fixes: commit 3a2d7fa8a3d5 ("mm: disable interrupts while initializing >> deferred pages") >> >> Signed-off-by: Shile Zhang >> --- >> include/linux/memory_hotplug.h | 16 ++++++++++++++-- >> 1 file changed, 14 insertions(+), 2 deletions(-) >> >> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h >> index ba0dca6aac6e..be69a6dc4fee 100644 >> --- a/include/linux/memory_hotplug.h >> +++ b/include/linux/memory_hotplug.h >> @@ -6,6 +6,8 @@ >> #include >> #include >> #include >> +#include >> +#include >> >> struct page; >> struct zone; >> @@ -282,12 +284,22 @@ static inline bool movable_node_is_enabled(void) >> static inline >> void pgdat_resize_lock(struct pglist_data *pgdat, unsigned long *flags) >> { >> - spin_lock_irqsave(&pgdat->node_size_lock, *flags); >> + /* >> + * Disable local interrupts on boot CPU will stop the tick_sched >> + * timer, which will block jiffies(wall clock) update. >> + */ >> + if (current->cpu != get_boot_cpu_id()) >> + spin_lock_irqsave(&pgdat->node_size_lock, *flags); >> + else >> + spin_lock(&pgdat->node_size_lock); >> } >> static inline >> void pgdat_resize_unlock(struct pglist_data *pgdat, unsigned long *flags) >> { >> - spin_unlock_irqrestore(&pgdat->node_size_lock, *flags); >> + if (current->cpu != get_boot_cpu_id()) >> + spin_unlock_irqrestore(&pgdat->node_size_lock, *flags); >> + else >> + spin_unlock(&pgdat->node_size_lock); >> } >> static inline >> void pgdat_resize_init(struct pglist_data *pgdat) > 1)Linux kernel is *preemptible*. Kernel with CONFIG_PREEMPT_RT option even may preempt > *kernel* code in the middle of function. When you are executing a code containing > pgdat_resize_lock() and pgdat_resize_unlock(), the process may migrate to another cpu > between them. > > bool cpu another cpu > ---------------------------------- > pgdat_resize_lock() > spin_lock() > --> migrate to another cpu > pgdat_resize_unlock() > spin_unlock_irqrestore() > > (Yes, in case of CONFIG_PREEMPT_RT, process is preemptible even after spin_lock() call). > > This looks like a bad helpers, and we should not introduce such the design. Hi Kirill, Thanks for your comments! Sorry for I'm not very clear about this lock/unlock, but I encountered this issue with "CONFIG_PREEMPT is not set". > 2)I think there is no the problem this patch solves. Do we really this statistics? > Can't we simple remove print message from deferred_init_memmap() and solve this? Sorry for I've not put this issue very clearly. It's *not* just one statistics log with wrong time calculate, but the wall clock is stuck. So the 'systemd-analyze' command also give a wrong time as I mentioned in the cover letter. I don't think is OK just remove the log, it cannot solve the wall clock latency. > Also, you may try to check that sched_clock() gives better results with interrupts > disabled (on x86 it uses rdtsc, when it's possible. But it also may fallback to > jiffies-based clock in some hardware cases, and they also won't go with interrupts > disabled). > > Kirill