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 F3F77C761AF for ; Thu, 30 Mar 2023 07:06:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230346AbjC3HGw (ORCPT ); Thu, 30 Mar 2023 03:06:52 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:32810 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230091AbjC3HGt (ORCPT ); Thu, 30 Mar 2023 03:06:49 -0400 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D08D365B5; Thu, 30 Mar 2023 00:06:48 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id 654981FE87; Thu, 30 Mar 2023 07:06:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1680160007; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=QLfYeLt+TXwV+pwToLzrHHrJTaYboOCW9UaiwVxS7lA=; b=mAI7MPiHSl8rJ93izo+zY41ldY7Q5UIHq3cnqnGtElFi7az9dtYt0NrZYGnhHulCp4xuiB tCKde3AIvlhN6fTukiLKYF3ViOCSzqBHTGWpE8aQCHMCXW6+8GcWX7Keeil3ozJ3OOGTHK IGmmNg/5vANvb4KhA3aiQh5XlXtsXR4= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 4169A1348E; Thu, 30 Mar 2023 07:06:47 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id yVwIDQc1JWR+bAAAMHmgww (envelope-from ); Thu, 30 Mar 2023 07:06:47 +0000 Date: Thu, 30 Mar 2023 09:06:46 +0200 From: Michal Hocko To: Shakeel Butt Cc: Yosry Ahmed , Tejun Heo , Josef Bacik , Jens Axboe , Zefan Li , Johannes Weiner , Roman Gushchin , Muchun Song , Andrew Morton , Michal =?iso-8859-1?Q?Koutn=FD?= , Vasily Averin , cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, bpf@vger.kernel.org Subject: Re: [PATCH v2 4/9] cgroup: rstat: add WARN_ON_ONCE() if flushing outside task context Message-ID: References: <20230328221644.803272-1-yosryahmed@google.com> <20230328221644.803272-5-yosryahmed@google.com> <20230329192059.2nlme5ubshzdbpg6@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20230329192059.2nlme5ubshzdbpg6@google.com> Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Wed 29-03-23 19:20:59, Shakeel Butt wrote: > On Wed, Mar 29, 2023 at 11:41:39AM -0700, Yosry Ahmed wrote: > > On Wed, Mar 29, 2023 at 4:22 AM Michal Hocko wrote: > > > > > > On Tue 28-03-23 22:16:39, Yosry Ahmed wrote: > > > > rstat flushing is too expensive to perform in irq context. > > > > The previous patch removed the only context that may invoke an rstat > > > > flush from irq context, add a WARN_ON_ONCE() to detect future > > > > violations, or those that we are not aware of. > > > > > > > > Ideally, we wouldn't flush with irqs disabled either, but we have one > > > > context today that does so in mem_cgroup_usage(). Forbid callers from > > > > irq context for now, and hopefully we can also forbid callers with irqs > > > > disabled in the future when we can get rid of this callsite. > > > > > > I am sorry to be late to the discussion. I wanted to follow up on > > > Johannes reply in the previous version but you are too fast ;) > > > > > > I do agree that this looks rather arbitrary. You do not explain how the > > > warning actually helps. Is the intention to be really verbose to the > > > kernel log when somebody uses this interface from the IRQ context and > > > get bug reports? What about configurations with panic on warn? Do we > > > really want to crash their systems for something like that? > > > > Thanks for taking a look, Michal! > > > > The ultimate goal is not to flush in irq context or with irqs > > disabled, as in some cases it causes irqs to be disabled for a long > > time, as flushing is an expensive operation. The previous patch in the > > series should have removed the only context that flushes in irq > > context, and the purpose of the WARN_ON_ONCE() is to catch future uses > > or uses that we might have missed. > > > > There is still one code path that flushes with irqs disabled (also > > mem_cgroup_usage()), and we cannot remove this just yet; we need to > > deprecate usage threshold events for root to do that. So we cannot > > enforce not flushing with irqs disabled yet. > > > > So basically the patch is trying to enforce what we have now, not > > flushing in irq context, and hopefully at some point we will also be > > able to enforce not flushing with irqs disabled. > > > > If WARN_ON_ONCE() is the wrong tool for this, please let me know. > > > > If I understand Michal's concern, the question is should be start with > pr_warn_once() instead of WARN_ON_ONCE() and I think yes we should start > with pr_warn_once(). Yes, I do not really like the WARN_ON here. It is an overkill. pr_warn would much less intrusive but potentially incomplete because you won't know who that offender is. So if you really care about those then you would need to call dump_stack as well. So the real question is. Do we really care so deeply? After all somebody might be calling this from within a spin lock or irq disabled section resulting in a similar situation without noticing. -- Michal Hocko SUSE Labs