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 39585C71153 for ; Thu, 24 Aug 2023 07:14:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240291AbjHXHOX (ORCPT ); Thu, 24 Aug 2023 03:14:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33534 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234019AbjHXHNw (ORCPT ); Thu, 24 Aug 2023 03:13:52 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3EFE7E4B; Thu, 24 Aug 2023 00:13:50 -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-out1.suse.de (Postfix) with ESMTPS id D13A922C73; Thu, 24 Aug 2023 07:13:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1692861228; 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=cL+xkHg3gVScj/qvNoq2aVyQCfwu6YNOsJXQogUGOQI=; b=qb8NuamNvZBFYTTN0FOFT3Ibgam1CmlB83kK5rDCRNJ33KCnGr21xK0VtE/ya4rUb/TBQF Octr+nFbz3nKd9GIf+gxbBOanAltvASnflTM5oIEWXbXNsTw55b1+9NG4YtLwiDoecBWWH XRQcqtauBjN1w1a6QfonjTxSnwFUhTo= 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 B513F138FB; Thu, 24 Aug 2023 07:13:48 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id kYj3KSwD52QZHQAAMHmgww (envelope-from ); Thu, 24 Aug 2023 07:13:48 +0000 Date: Thu, 24 Aug 2023 09:13:48 +0200 From: Michal Hocko To: Yosry Ahmed Cc: Andrew Morton , Johannes Weiner , Roman Gushchin , Shakeel Butt , Muchun Song , Ivan Babrou , Tejun Heo , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/3] mm: memcg: use non-unified stats flushing for userspace reads Message-ID: References: <20230821205458.1764662-1-yosryahmed@google.com> <20230821205458.1764662-4-yosryahmed@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 23-08-23 07:55:40, Yosry Ahmed wrote: > On Wed, Aug 23, 2023 at 12:33 AM Michal Hocko wrote: > > > > On Tue 22-08-23 08:30:05, Yosry Ahmed wrote: > > > On Tue, Aug 22, 2023 at 2:06 AM Michal Hocko wrote: > > > > > > > > On Mon 21-08-23 20:54:58, Yosry Ahmed wrote: > > [...] > > > So to answer your question, I don't think a random user can really > > > affect the system in a significant way by constantly flushing. In > > > fact, in the test script (which I am now attaching, in case you're > > > interested), there are hundreds of threads that are reading stats of > > > different cgroups every 1s, and I don't see any negative effects on > > > in-kernel flushers in this case (reclaimers). > > > > I suspect you have missed my point. > > I suspect you are right :) > > > > Maybe I am just misunderstanding > > the code but it seems to me that the lock dropping inside > > cgroup_rstat_flush_locked effectivelly allows unbounded number of > > contenders which is really dangerous when it is triggerable from the > > userspace. The number of spinners at a moment is always bound by the > > number CPUs but depending on timing many potential spinners might be > > cond_rescheded and the worst time latency to complete can be really > > high. Makes more sense? > > I think I understand better now. So basically because we might drop > the lock and resched, there can be nr_cpus spinners + other spinners > that are currently scheduled away, so these will need to wait to be > scheduled and then start spinning on the lock. This may happen for one > reader multiple times during its read, which is what can cause a high > worst case latency. > > I hope I understood you correctly this time. Did I? Yes. I would just add that this could also influence the worst case latency for a different reader - so an adversary user can stall others. Exposing a shared global lock in uncontrolable way over generally available user interface is not really a great idea IMHO. -- Michal Hocko SUSE Labs