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cgroup/cpuset: Find furthest isolated ancestor for invalidated isolated partition

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Commit 46b885416c ("cgroup/cpuset: Fix a missing isolated_cpus
update in update_cpumasks_hier()") searches for the closet partition
root ancestor to determine if the isolated CPUs in a to be invalidated
isolation partition should be removed from isolated_cpus or not.

As reported by sashiko, this method may not work when there is a deeply
nested isolated partition hierarchy. For instance, Consider a nested
hierarchy A0 -> A1 -> A2 -> A3 where all four are PRS_ISOLATED:

  A0: effective_xcpus = 0-4
  A1: effective_xcpus = 1-4
  A2: effective_xcpus = 2-4
  A3: effective_xcpus = 3-4

When A1 shrinks its exclusive_cpus from 1-4 to 1-2, A2 will update it
effective_xcpus to 2. A3 will find its closet partition root ancestor
A2 and decides that its effective_xcpus 3-4 should be removed from
isolated_cpus which is incorrect.

Address this corner case by adding a new isolated_cpus_release() helper
to replace the existing ancestor_partition() helper. This new helper
will search for the furthest isolated partition root ancestor to decide
if some or all of its isolated CPUs should be removed from isolated_cpus
and also perform the removal when appropriate.

The is_remote_partition() and is_local_partition() helpers are also
moved up so they can be used in earlier functions.

There are other pre-existing problems that will still prevent the
above test case to work correctly. These problems will be addressed in
subsequent patches.

Fixes: 46b885416c ("cgroup/cpuset: Fix a missing isolated_cpus update in update_cpumasks_hier()")
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Waiman Long 2026-10-25 17:44:39 -04:00 committed by Tejun Heo
parent 5f4a3f743a
commit 0bce85ff2c
1 changed files with 53 additions and 51 deletions
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104
kernel/cgroup/cpuset.c
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@ -1257,6 +1257,16 @@ static void reset_partition_data(struct cpuset *cs)
cpumask_copy(cs->effective_cpus, parent->effective_cpus);
}
static inline bool is_remote_partition(struct cpuset *cs)
{
return cs->remote_partition;
}
static inline bool is_local_partition(struct cpuset *cs)
{
return is_partition_valid(cs) && !is_remote_partition(cs);
}
/* Return true if isolated_cpus changes. */
static bool isolated_cpu_update(int new_prs, int cpu)
{
@ -1299,6 +1309,45 @@ static void isolated_cpus_update(int old_prs, int new_prs, struct cpumask *xcpus
update_housekeeping = true;
}
/*
* isolated_cpus_release - Remove the relevant CPUs in @xcpus from isolated_cpus
* @cs: the current cpuset
* @xcpus: the set of effective xcpus to be considered for removal
*
* This function is called when an isolated partition is invalidated or when
* some of its effective xcpus are to be removed from isolated_cpus due to
* changes in parent cpuset. The given @xcpus must be modifiable as the function
* will update its value to determine the right set of CPUs to be removed.
*/
static void isolated_cpus_release(struct cpuset *cs, struct cpumask *xcpus)
{
struct cpuset *ancestor = parent_cs(cs);
struct cpuset *iancestor = NULL;
/*
* Search the furthest isolated partition root ancestor available
* until top_cpuset or a remote partition root is hit.
*/
for (; ancestor; ancestor = parent_cs(ancestor)) {
if (ancestor->partition_root_state == PRS_ISOLATED)
iancestor = ancestor;
if (is_remote_partition(ancestor))
break;
}
/*
* If a furthest isolated partition acnestor is found, use it to
* determine which of the CPUs in @xcpus should be removed from
* isolated_cpus. Otherwise, all of them will be removed.
*/
if (iancestor && !cpumask_andnot(xcpus, xcpus, iancestor->effective_xcpus))
return; /* No CPU should be removed from isolated_cpus */
spin_lock_irq(&callback_lock);
isolated_cpus_update(PRS_ISOLATED, PRS_ROOT, xcpus);
spin_unlock_irq(&callback_lock);
}
/*
* partition_xcpus_add - Add new exclusive CPUs to partition
* @new_prs: new partition_root_state
@ -1571,16 +1620,6 @@ static int compute_trialcs_excpus(struct cpuset *trialcs, struct cpuset *cs)
return rm_siblings_excl_cpus(parent, cs, excpus);
}
static inline bool is_remote_partition(struct cpuset *cs)
{
return cs->remote_partition;
}
static inline bool is_local_partition(struct cpuset *cs)
{
return is_partition_valid(cs) && !is_remote_partition(cs);
}
/*
* remote_partition_enable - Enable current cpuset as a remote partition root
* @cs: the cpuset to update
@ -2155,25 +2194,6 @@ static void compute_partition_effective_cpumask(struct cpuset *cs,
rcu_read_unlock();
}
/*
* ancestor_partition - Find and return the closest valid partition root ancestor
* @cs: the current cpuset
*
* Return: The closet partition root ancestor cpuset
*/
static struct cpuset *ancestor_partition(struct cpuset *cs)
{
struct cpuset *ancestor = parent_cs(cs);
/*
* The top_cpuset is always a valid partition root which will stop
* the loop if it is reached.
*/
while (!is_partition_valid(ancestor))
ancestor = parent_cs(ancestor);
return ancestor;
}
/*
* update_cpumasks_hier - Update effective cpumasks and tasks in the subtree
* @cs: the cpuset to consider
@ -2267,7 +2287,6 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
*/
if ((cp != cs) && old_prs) {
int parent_prs = parent->partition_root_state;
struct cpumask *del_cpus = NULL;
switch (parent_prs) {
case PRS_ROOT:
@ -2281,32 +2300,15 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
* child partition roots become invalid too.
*
* For a to be invalidated isolated partition,
* its isolated CPUs may or may not need to be
* removed from isolated_cpus depending on if
* its closest partition root ancestor is a root
* or isolated partition. With a PRS_ISOLATED
* ancestor, the CPUs are checked against its
* effective xcpus to see if any of them needed
* to be removed.
* call return_isolated_cpus() to remove the
* relevant CPUs from isolated_cpus.
*/
if (is_partition_valid(cp))
new_prs = -old_prs;
if (old_prs == PRS_ISOLATED) {
struct cpuset *ancestor = ancestor_partition(cp);
if (ancestor->partition_root_state == PRS_ROOT)
del_cpus = cp->effective_xcpus;
else if (cpumask_andnot(tmp->delmask,
cp->effective_xcpus,
ancestor->effective_xcpus))
del_cpus = tmp->delmask;
if (del_cpus) {
spin_lock_irq(&callback_lock);
isolated_cpus_update(old_prs, new_prs, del_cpus);
spin_unlock_irq(&callback_lock);
}
cpumask_copy(tmp->delmask, cp->effective_xcpus);
isolated_cpus_release(cp, tmp->delmask);
}
WRITE_ONCE(cp->prs_err,
is_partition_invalid(parent)