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Tue, 2 May 2023 19:56:02 +0000 (UTC) Received: from [10.22.10.239] (unknown [10.22.10.239]) by smtp.corp.redhat.com (Postfix) with ESMTP id CEBEB2166B26; Tue, 2 May 2023 19:56:01 +0000 (UTC) Message-ID: <9e149289-c1c0-3297-145d-ad3a890056ac@redhat.com> Date: Tue, 2 May 2023 15:56:01 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1 Subject: Re: [RFC PATCH 3/3] cgroup: Do not take css_set_lock in cgroup_show_path Content-Language: en-US To: =?UTF-8?Q?Michal_Koutn=c3=bd?= , 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 References: <20230502133847.14570-1-mkoutny@suse.com> <20230502133847.14570-4-mkoutny@suse.com> From: Waiman Long In-Reply-To: <20230502133847.14570-4-mkoutny@suse.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.6 X-Spam-Status: No, score=-3.7 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,NICE_REPLY_A, RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE, T_SCC_BODY_TEXT_LINE autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/2/23 09:38, Michal Koutný wrote: > /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)); Again lockdep_is_held(&namespace_sem) is good enough. > > 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; Cheers, Longman