On 1/4/26 8:15 PM, Chen Ridong wrote:
On 2026/1/5 5:25, Waiman Long wrote:
On 1/3/26 9:48 PM, Chen Ridong wrote:
On 2026/1/2 3:15, Waiman Long wrote:
Since commit f62a5d39368e ("cgroup/cpuset: Remove remote_partition_check()
& make update_cpumasks_hier() handle remote partition"), the
compute_effective_exclusive_cpumask() helper was extended to
strip exclusive CPUs from siblings when computing effective_xcpus
(cpuset.cpus.exclusive.effective). This helper was later renamed to
compute_excpus() in commit 86bbbd1f33ab ("cpuset: Refactor exclusive
CPU mask computation logic").
This helper is supposed to be used consistently to compute
effective_xcpus. However, there is an exception within the callback
critical section in update_cpumasks_hier() when exclusive_cpus of a
valid partition root is empty. This can cause effective_xcpus value to
differ depending on where exactly it is last computed. Fix this by using
compute_excpus() in this case to give a consistent result.
Signed-off-by: Waiman Long <[email protected]>
---
kernel/cgroup/cpuset.c | 14 +++++---------
1 file changed, 5 insertions(+), 9 deletions(-)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index da2b3b51630e..37d118a9ad4d 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2168,17 +2168,13 @@ static void update_cpumasks_hier(struct cpuset *cs,
struct tmpmasks *tmp,
spin_lock_irq(&callback_lock);
cpumask_copy(cp->effective_cpus, tmp->new_cpus);
cp->partition_root_state = new_prs;
- if (!cpumask_empty(cp->exclusive_cpus) && (cp != cs))
- compute_excpus(cp, cp->effective_xcpus);
-
/*
- * Make sure effective_xcpus is properly set for a valid
- * partition root.
+ * Need to compute effective_xcpus if either exclusive_cpus
+ * is non-empty or it is a valid partition root.
*/
- if ((new_prs > 0) && cpumask_empty(cp->exclusive_cpus))
- cpumask_and(cp->effective_xcpus,
- cp->cpus_allowed, parent->effective_xcpus);
- else if (new_prs < 0)
+ if ((new_prs > 0) || !cpumask_empty(cp->exclusive_cpus))
+ compute_excpus(cp, cp->effective_xcpus);
+ if (new_prs < 0)
reset_partition_data(cp);
spin_unlock_irq(&callback_lock);
The code resets partition data only for new_prs < 0. My understanding is that a
partition is invalid
when new_prs <= 0. Shouldn't reset_partition_data() also be called when new_prs
= 0? Is there a
specific reason to skip the reset in that case?
update_cpumasks_hier() is called when changes in a cpuset or hotplug affects
other cpusets in the
hierarchy. With respect to changes in partition state, it is either from valid
to invalid or vice
versa. It will not change from a valid partition to member. The only way
new_prs = 0 is when old_prs
= 0. Even if the affected cpuset is processed again in update_cpumask_hier(),
any state change from
valid partition to member (update_prstate()), reset_partition_data() should
have been called there.
That is why we only care about when new_prs != 0.
Thank you for your patience.
The code isn't wrong here. However I can change the condition to (new_prs <= 0)
if it makes it
easier to understand.
I agree there's nothing wrong with the current logic. However, for clarity, I
suggest changing the
condition to (new_prs <= 0). This allows the function's logic to be fully
self-consistent and
focused on a single responsibility. This approach would allow us to simplify
the code to:
if (new_prs > 0)
compute_excpus(cp, cp->effective_xcpus);
else
reset_partition_data(cp);
Since reset_partition_data() already handles cases whether cp->exclusive_cpus
is empty or not, this
implementation would be more concise while correctly covering all scenarios.
effective_xcpus should be set when exclusive_cpus is not empty or when
the cpuset is a valid partition root. So just checking new_prs for
compute_excpus() is not enough.
Cheers,
Longman
