cgroup/cpuset: Fix remote root partition creation problem

Since commit 181c8e091aae ("cgroup/cpuset: Introduce remote partition"),
a remote partition can be created underneath a non-partition root cpuset
as long as its exclusive_cpus are set to distribute exclusive CPUs down
to its children. The generate_sched_domains() function, however, doesn't
take into account this new behavior and hence will fail to create the
sched domain needed for a remote root (non-isolated) partition.

There are two issues related to remote partition support. First of
all, generate_sched_domains() has a fast path that is activated if
root_load_balance is true and top_cpuset.nr_subparts is non-zero. The
later condition isn't quite correct for remote partitions as nr_subparts
just shows the number of local child partitions underneath it. There
can be no local child partition under top_cpuset even if there are
remote partitions further down the hierarchy. Fix that by checking
for subpartitions_cpus which contains exclusive CPUs allocated to both
local and remote partitions.

Secondly, the valid partition check for subtree skipping in the csa[]
generation loop isn't enough as remote partition does not need to
have a partition root parent. Fix this problem by breaking csa[] array
generation loop of generate_sched_domains() into v1 and v2 specific parts
and checking a cpuset's exclusive_cpus before skipping its subtree in
the v2 case.

Also simplify generate_sched_domains() for cgroup v2 as only
non-isolating partition roots should be included in building the cpuset
array and none of the v1 scheduling attributes other than a different
way to create an isolated partition are supported.

Fixes: 181c8e091aae ("cgroup/cpuset: Introduce remote partition")
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index f9b97f65e204b4fd31eda76fcfca82910abae8a2..fb71d710a60310c867e270018872f6e232f0626e 100644 (file)
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -169,7 +169,7 @@ struct cpuset {
        /* for custom sched domain */
        int relax_domain_level;
 
-       /* number of valid sub-partitions */
+       /* number of valid local child partitions */
        int nr_subparts;
 
        /* partition root state */
@@ -957,13 +957,14 @@ static int generate_sched_domains(cpumask_var_t **domains,
        int nslot;              /* next empty doms[] struct cpumask slot */
        struct cgroup_subsys_state *pos_css;
        bool root_load_balance = is_sched_load_balance(&top_cpuset);
+       bool cgrpv2 = cgroup_subsys_on_dfl(cpuset_cgrp_subsys);
 
        doms = NULL;
        dattr = NULL;
        csa = NULL;
 
        /* Special case for the 99% of systems with one, full, sched domain */
-       if (root_load_balance && !top_cpuset.nr_subparts) {
+       if (root_load_balance && cpumask_empty(subpartitions_cpus)) {
 single_root_domain:
                ndoms = 1;
                doms = alloc_sched_domains(ndoms);
@@ -992,16 +993,18 @@ single_root_domain:
        cpuset_for_each_descendant_pre(cp, pos_css, &top_cpuset) {
                if (cp == &top_cpuset)
                        continue;
+
+               if (cgrpv2)
+                       goto v2;
+
                /*
+                * v1:
                 * Continue traversing beyond @cp iff @cp has some CPUs and
                 * isn't load balancing.  The former is obvious.  The
                 * latter: All child cpusets contain a subset of the
                 * parent's cpus, so just skip them, and then we call
                 * update_domain_attr_tree() to calc relax_domain_level of
                 * the corresponding sched domain.
-                *
-                * If root is load-balancing, we can skip @cp if it
-                * is a subset of the root's effective_cpus.
                 */
                if (!cpumask_empty(cp->cpus_allowed) &&
                    !(is_sched_load_balance(cp) &&
@@ -1009,16 +1012,28 @@ single_root_domain:
                                         housekeeping_cpumask(HK_TYPE_DOMAIN))))
                        continue;
 
-               if (root_load_balance &&
-                   cpumask_subset(cp->cpus_allowed, top_cpuset.effective_cpus))
-                       continue;
-
                if (is_sched_load_balance(cp) &&
                    !cpumask_empty(cp->effective_cpus))
                        csa[csn++] = cp;
 
-               /* skip @cp's subtree if not a partition root */
-               if (!is_partition_valid(cp))
+               /* skip @cp's subtree */
+               pos_css = css_rightmost_descendant(pos_css);
+               continue;
+
+v2:
+               /*
+                * Only valid partition roots that are not isolated and with
+                * non-empty effective_cpus will be saved into csn[].
+                */
+               if ((cp->partition_root_state == PRS_ROOT) &&
+                   !cpumask_empty(cp->effective_cpus))
+                       csa[csn++] = cp;
+
+               /*
+                * Skip @cp's subtree if not a partition root and has no
+                * exclusive CPUs to be granted to child cpusets.
+                */
+               if (!is_partition_valid(cp) && cpumask_empty(cp->exclusive_cpus))
                        pos_css = css_rightmost_descendant(pos_css);
        }
        rcu_read_unlock();
@@ -1072,6 +1087,20 @@ restart:
        dattr = kmalloc_array(ndoms, sizeof(struct sched_domain_attr),
                              GFP_KERNEL);
 
+       /*
+        * Cgroup v2 doesn't support domain attributes, just set all of them
+        * to SD_ATTR_INIT. Also non-isolating partition root CPUs are a
+        * subset of HK_TYPE_DOMAIN housekeeping CPUs.
+        */
+       if (cgrpv2) {
+               for (i = 0; i < ndoms; i++) {
+                       cpumask_copy(doms[i], csa[i]->effective_cpus);
+                       if (dattr)
+                               dattr[i] = SD_ATTR_INIT;
+               }
+               goto done;
+       }
+
        for (nslot = 0, i = 0; i < csn; i++) {
                struct cpuset *a = csa[i];
                struct cpumask *dp;
@@ -1231,7 +1260,7 @@ static void rebuild_sched_domains_locked(void)
         * root should be only a subset of the active CPUs.  Since a CPU in any
         * partition root could be offlined, all must be checked.
         */
-       if (top_cpuset.nr_subparts) {
+       if (!cpumask_empty(subpartitions_cpus)) {
                rcu_read_lock();
                cpuset_for_each_descendant_pre(cs, pos_css, &top_cpuset) {
                        if (!is_partition_valid(cs)) {
@@ -4575,7 +4604,7 @@ static void cpuset_handle_hotplug(void)
         * In the rare case that hotplug removes all the cpus in
         * subpartitions_cpus, we assumed that cpus are updated.
         */
-       if (!cpus_updated && top_cpuset.nr_subparts)
+       if (!cpus_updated && !cpumask_empty(subpartitions_cpus))
                cpus_updated = true;
 
        /* For v1, synchronize cpus_allowed to cpu_active_mask */