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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 123F7C432BE for ; Mon, 9 Aug 2021 09:07:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DEC5861075 for ; Mon, 9 Aug 2021 09:07:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234159AbhHIJH3 (ORCPT ); Mon, 9 Aug 2021 05:07:29 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]:41134 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234062AbhHIJH2 (ORCPT ); Mon, 9 Aug 2021 05:07:28 -0400 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 7C49B21F4E; Mon, 9 Aug 2021 09:07:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1628500027; 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=k5Y8l9w4iV5g6s1wFPPe9+DoJPU9/n1zkxnjff8sD0o=; b=BQmjr/B2oyVTVghvc5m6dVwkO+BrUfwt0cxBRJgtmey8Sgnq/GQJUs+Q5OrHBPEvBmCeCI AwoTYEPdyrMTyiEoKQElxN92XreBQd0dt83EhIojbcH4qfHTZWd8F43D6wC0Unk7mMWK1c QPDou1k2Zx0ABJ5JBhN2QKXgwJw1vOA= 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 02AD0A3B89; Mon, 9 Aug 2021 09:07:06 +0000 (UTC) Date: Mon, 9 Aug 2021 11:07:06 +0200 From: Michal Hocko To: Vlastimil Babka Cc: Thomas Gleixner , Waiman Long , Johannes Weiner , Vladimir Davydov , Andrew Morton , Roman Gushchin , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, Shakeel Butt , Muchun Song , Luis Goncalves , Sebastian Andrzej Siewior , Daniel Bristot de Oliveira , Linus Torvalds Subject: Re: [PATCH] mm/memcg: Disable task obj_stock for PREEMPT_RT Message-ID: References: <20210803175519.22298-1-longman@redhat.com> <87h7g62jxm.ffs@tglx> <93dfe0d4-c687-93f8-da75-c5d3c9bd0ac7@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 04-08-21 10:33:41, Michal Hocko wrote: > On Wed 04-08-21 09:39:23, Vlastimil Babka wrote: > > On 8/4/21 1:21 AM, Thomas Gleixner wrote: > > > /* > > > * The only protection from memory hotplug vs. drain_stock races is > > > * that we always operate on local CPU stock here with IRQ disabled > > > */ > > > - local_irq_save(flags); > > > + local_lock_irqsave(memcg_stock_lock, flags); > > > ... > > > if (use_task_obj_stock()) > > > drain_obj_stock(&stock->task_obj); > > > > > > which is incomprehensible garbage. > > > > > > The comment above the existing local_irq_save() is garbage w/o any local > > > lock conversion already today (and even before the commit which > > > introduced stock::task_obj) simply because that comment does not explain > > > the why. > > > > Michal, this seems to be your comment from commit 72f0184c8a00 ("mm, memcg: > > remove hotplug locking from try_charge"). Was "memory hotplug" a mistake, > > because the rest of the commit is about cpu hotplug, and I don't really see a > > memory hotplug connection there? > > This part of the changelog tried to explain that part IIRC > " > We can get rid of {get,put}_online_cpus, fortunately. We do not have to > be worried about races with memory hotplug because drain_local_stock, > which is called from both the WQ draining and the memory hotplug > contexts, is always operating on the local cpu stock with IRQs disabled. > " > > Now I have to admit I do not remember all the details and from a quick > look the memory hotplug doesn't seem to be draining memcg pcp stock. > Maybe this has been removed since then. The only stock draining outside > of the memcg code seems to be memcg_hotplug_cpu_dead callback. That > would indicate that I really meant the cpu hotplug here indeed. Does this look better? --- >From 5aa1c8ce0d88b8c6d59ba95c7e36ca07dc2b2161 Mon Sep 17 00:00:00 2001 From: Michal Hocko Date: Mon, 9 Aug 2021 10:59:04 +0200 Subject: [PATCH] memcg: fix up drain_local_stock comment Thomas and Vlastimil have noticed that the comment in drain_local_stock doesn't quite make sense. It talks about a synchronization with the memory hotplug but there is no actual memory hotplug involvement here. I meant to talk about cpu hotplug here. Fix that up and hopefuly make the comment more helpful by referencing the cpu hotplug callback as well. Signed-off-by: Michal Hocko --- mm/memcontrol.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index eb8e87c4833f..f7be7b01395e 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -2205,8 +2205,9 @@ static void drain_local_stock(struct work_struct *dummy) unsigned long flags; /* - * The only protection from memory hotplug vs. drain_stock races is - * that we always operate on local CPU stock here with IRQ disabled + * The only protection from cpu hotplug (memcg_hotplug_cpu_dead) vs. + * drain_stock races is that we always operate on local CPU stock + * here with IRQ disabled */ local_irq_save(flags); -- 2.30.1 -- Michal Hocko SUSE Labs From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH] mm/memcg: Disable task obj_stock for PREEMPT_RT Date: Mon, 9 Aug 2021 11:07:06 +0200 Message-ID: References: <20210803175519.22298-1-longman@redhat.com> <87h7g62jxm.ffs@tglx> <93dfe0d4-c687-93f8-da75-c5d3c9bd0ac7@suse.cz> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1628500027; 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=k5Y8l9w4iV5g6s1wFPPe9+DoJPU9/n1zkxnjff8sD0o=; b=BQmjr/B2oyVTVghvc5m6dVwkO+BrUfwt0cxBRJgtmey8Sgnq/GQJUs+Q5OrHBPEvBmCeCI AwoTYEPdyrMTyiEoKQElxN92XreBQd0dt83EhIojbcH4qfHTZWd8F43D6wC0Unk7mMWK1c QPDou1k2Zx0ABJ5JBhN2QKXgwJw1vOA= Content-Disposition: inline In-Reply-To: List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Vlastimil Babka Cc: Thomas Gleixner , Waiman Long , Johannes Weiner , Vladimir Davydov , Andrew Morton , Roman Gushchin , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, Shakeel Butt , Muchun Song , Luis Goncalves , Sebastian Andrzej Siewior , Daniel Bristot de Oliveira , Linus Torvalds On Wed 04-08-21 10:33:41, Michal Hocko wrote: > On Wed 04-08-21 09:39:23, Vlastimil Babka wrote: > > On 8/4/21 1:21 AM, Thomas Gleixner wrote: > > > /* > > > * The only protection from memory hotplug vs. drain_stock races is > > > * that we always operate on local CPU stock here with IRQ disabled > > > */ > > > - local_irq_save(flags); > > > + local_lock_irqsave(memcg_stock_lock, flags); > > > ... > > > if (use_task_obj_stock()) > > > drain_obj_stock(&stock->task_obj); > > > > > > which is incomprehensible garbage. > > > > > > The comment above the existing local_irq_save() is garbage w/o any local > > > lock conversion already today (and even before the commit which > > > introduced stock::task_obj) simply because that comment does not explain > > > the why. > > > > Michal, this seems to be your comment from commit 72f0184c8a00 ("mm, memcg: > > remove hotplug locking from try_charge"). Was "memory hotplug" a mistake, > > because the rest of the commit is about cpu hotplug, and I don't really see a > > memory hotplug connection there? > > This part of the changelog tried to explain that part IIRC > " > We can get rid of {get,put}_online_cpus, fortunately. We do not have to > be worried about races with memory hotplug because drain_local_stock, > which is called from both the WQ draining and the memory hotplug > contexts, is always operating on the local cpu stock with IRQs disabled. > " > > Now I have to admit I do not remember all the details and from a quick > look the memory hotplug doesn't seem to be draining memcg pcp stock. > Maybe this has been removed since then. The only stock draining outside > of the memcg code seems to be memcg_hotplug_cpu_dead callback. That > would indicate that I really meant the cpu hotplug here indeed. Does this look better? --- >From 5aa1c8ce0d88b8c6d59ba95c7e36ca07dc2b2161 Mon Sep 17 00:00:00 2001 From: Michal Hocko Date: Mon, 9 Aug 2021 10:59:04 +0200 Subject: [PATCH] memcg: fix up drain_local_stock comment Thomas and Vlastimil have noticed that the comment in drain_local_stock doesn't quite make sense. It talks about a synchronization with the memory hotplug but there is no actual memory hotplug involvement here. I meant to talk about cpu hotplug here. Fix that up and hopefuly make the comment more helpful by referencing the cpu hotplug callback as well. Signed-off-by: Michal Hocko --- mm/memcontrol.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index eb8e87c4833f..f7be7b01395e 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -2205,8 +2205,9 @@ static void drain_local_stock(struct work_struct *dummy) unsigned long flags; /* - * The only protection from memory hotplug vs. drain_stock races is - * that we always operate on local CPU stock here with IRQ disabled + * The only protection from cpu hotplug (memcg_hotplug_cpu_dead) vs. + * drain_stock races is that we always operate on local CPU stock + * here with IRQ disabled */ local_irq_save(flags); -- 2.30.1 -- Michal Hocko SUSE Labs