Messages in this thread | | | Date | Tue, 2 May 2023 15:56:01 -0400 | Subject | Re: [RFC PATCH 3/3] cgroup: Do not take css_set_lock in cgroup_show_path | From | Waiman Long <> |
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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 <mkoutny@suse.com> > --- > 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
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