From patchwork Tue May 2 13:38:47 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: =?utf-8?q?Michal_Koutn=C3=BD?= X-Patchwork-Id: 13228943 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 DA324C7EE26 for ; Tue, 2 May 2023 13:39:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234318AbjEBNju (ORCPT ); Tue, 2 May 2023 09:39:50 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34842 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234297AbjEBNjd (ORCPT ); Tue, 2 May 2023 09:39:33 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [IPv6:2001:67c:2178:6::1c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 06FCF6A5E; Tue, 2 May 2023 06:38:58 -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 3FC7722124; Tue, 2 May 2023 13:38:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1683034730; 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=fpw1/CS8jewUjSsdmSx7N0a/briOC9eP8uaWobg+SbE=; b=fE3Cff0u2uDDZoTwPGZOiCHhWMWW2sCmMD5CFUkuzcVoLi2vh0pm8Rwr/uTuKzp8yUeamE ny+LtbLzNp+7qONlp+LrgnBakxzmXydMKFc3U8iqdaukFUOm4Q7DmwqXXxGO0WxmdDOEX8 ka0xGI08HvsuJRFVAK3WDLeiAiUHjZA= 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 10904139D2; Tue, 2 May 2023 13:38:50 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id oDU7A2oSUWTOYQAAMHmgww (envelope-from ); Tue, 02 May 2023 13:38:50 +0000 From: =?utf-8?q?Michal_Koutn=C3=BD?= To: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org Cc: Alexander Viro , Christian Brauner , Tejun Heo , Zefan Li , Johannes Weiner , Dave Chinner , Rik van Riel , Jiri Wiesner Subject: [RFC PATCH 3/3] cgroup: Do not take css_set_lock in cgroup_show_path Date: Tue, 2 May 2023 15:38:47 +0200 Message-Id: <20230502133847.14570-4-mkoutny@suse.com> X-Mailer: git-send-email 2.40.1 In-Reply-To: <20230502133847.14570-1-mkoutny@suse.com> References: <20230502133847.14570-1-mkoutny@suse.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org /proc/$pid/mountinfo may accumulate lots of entries (causing frequent re-reads of whole file) or lots cgroupfs entries alone. The cgroupfs entries rendered with cgroup_show_path() may increase/be subject of css_set_lock contention causing further slowdown -- not only mountinfo rendering but any other css_set_lock user. We leverage the fact that mountinfo reading happens with namespace_sem taken and hierarchy roots thus cannot be freed concurrently. There are three relevant nodes for each cgroupfs entry: R ... cgroup hierarchy root M ... mount root C ... reader's cgroup NS root mountinfo is supposed to show path from C to M. Current's css_set (and linked root cgroups) are stable under namespace_sem, therefore current_cgns_cgroup_from_root() doesn't need css_set_lock. When the path is assembled in kernfs_path_from_node(), we know that: - C kernfs_node is (transitively) pinned via current->nsproxy, - M kernfs_node is pinned thanks to namespace_sem, - path C to M is pinned via child->parent references (this holds also when C and M are in distinct subtrees). Streamline mountinfo rendering a bit by relieving css_set_lock and add careful notes about that. Signed-off-by: Michal Koutný --- kernel/cgroup/cgroup.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-) diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index 32d693a797b9..e2ec6f0028be 100644 --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -1407,12 +1407,18 @@ static inline struct cgroup *__cset_cgroup_from_root(struct css_set *cset, struct cgroup *res_cgroup = NULL; if (cset == &init_css_set) { + /* callers must ensure root stability */ res_cgroup = &root->cgrp; } else if (root == &cgrp_dfl_root) { res_cgroup = cset->dfl_cgrp; } else { struct cgrp_cset_link *link; - lockdep_assert_held(&css_set_lock); + /* cset's cgroups are pinned unless they are root cgroups that + * were unmounted. We look at links to !cgrp_dfl_root + * cgroup_root, either lock ensures the list is not mutated + */ + lockdep_assert(lockdep_is_held(&css_set_lock) || + lockdep_is_held_type(&namespace_sem, -1)); list_for_each_entry(link, &cset->cgrp_links, cgrp_link) { struct cgroup *c = link->cgrp; @@ -1438,8 +1444,6 @@ current_cgns_cgroup_from_root(struct cgroup_root *root) struct cgroup *res = NULL; struct css_set *cset; - lockdep_assert_held(&css_set_lock); - /* namespace_sem ensures `root` stability on unmount */ lockdep_assert(lockdep_is_held_type(&namespace_sem, -1)); @@ -1905,10 +1909,8 @@ int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node, if (!buf) return -ENOMEM; - spin_lock_irq(&css_set_lock); ns_cgroup = current_cgns_cgroup_from_root(kf_cgroot); len = kernfs_path_from_node(kf_node, ns_cgroup->kn, buf, PATH_MAX); - spin_unlock_irq(&css_set_lock); if (len >= PATH_MAX) len = -ERANGE;