Re: [PATCH-next 6/6] cgroup/cpuset: Prevent recursive call of remote_cpus_update() via update_cpumasks_hier()

[Guopeng Zhang <guopeng.zhang@linux.dev>]

在 2026/10/22 11:16, Waiman Long 写道:
> Guopeng reports that changing the cpumask of a remote partition root
> may set the effective_cpus of the top_cpuset incorrectly. It is
> due to the following call sequence:
> 
>   update_cpumask()
>    -> partition_cpus_change()
>     -> remote_cpus_update()
>      -> update_sibling_cpumasks()
>       -> update_cpumasks_hier()
>        -> remote_cpus_update()
> 
> The second remote_cpus_update() call passes in the cs->effective_xcpus
> instead of the trialcs->effective_xcpus of the first remote_cpus_update()
> call.  As a result, the second call can revert what has been done in
> the first call leading to incorrect effective_cpus of the top_cpuset
> as well as subpartitions_cpus.
> 
> Fix that by adding a new remote_partition_updating flag into the cpuset
> structure to indicate that a remote partition is being updated and
> update_cpumasks_hier() should skip the whole subtree as update_cpumask()
> or update_exclusive_cpumask() which calls partition_cpus_change()
> will eventually calls update_cpumasks_hier() for that subtree anyway.
> 
> This new flag is currently set only in remote_cpus_update() to prevent
> recursive call of remote_cpus_update() on the same cpuset.
> 
> A test case is added to test_cpuset_prs.sh to verify this change.
> 
> Reported-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
> Link: https://lore.kernel.org/lkml/20261010075947.333549-2-guopeng.zhang@linux.dev
> Fixes: f62a5d39368e ("cgroup/cpuset: Remove remote_partition_check() & make update_cpumasks_hier() handle remote partition")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/cgroup/cpuset-internal.h                   |  5 +++++
>  kernel/cgroup/cpuset.c                            | 13 +++++++++----
>  tools/testing/selftests/cgroup/test_cpuset_prs.sh |  1 +
>  3 files changed, 15 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
> index 2e44dc8634dc..1ba94ffaff9d 100644
> --- a/kernel/cgroup/cpuset-internal.h
> +++ b/kernel/cgroup/cpuset-internal.h
> @@ -161,6 +161,11 @@ struct cpuset {
>  	 */
>  	bool remote_partition;
>  
> +	/*
> +	 * Set to true when a remote partition is undergoing update.
> +	 */
> +	bool remote_partition_updating;
> +
>  	/*
>  	 * number of SCHED_DEADLINE tasks attached to this cpuset, so that we
>  	 * know when to rebuild associated root domain bandwidth information.
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 6e1bef12f86d..952b31246d62 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1736,6 +1736,7 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
>  			goto invalidate;
>  	}
>  
> +	cs->remote_partition_updating = true;
>  	spin_lock_irq(&callback_lock);
>  	if (adding)
>  		partition_xcpus_add(prs, NULL, tmp->addmask);
> @@ -1757,6 +1758,7 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
>  	 */
>  	cpuset_update_tasks_cpumask(&top_cpuset, tmp->new_cpus);
>  	update_sibling_cpumasks(&top_cpuset, NULL, tmp);
> +	cs->remote_partition_updating = false;
>  	return;
>  
>  invalidate:
> @@ -2208,16 +2210,19 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
>  		old_prs = new_prs = cp->partition_root_state;
>  
>  		/*
> -		 * For child remote partition root (!= cs), we need to call
> -		 * remote_cpus_update() if effective_xcpus will be changed.
> -		 * Otherwise, we can skip the whole subtree.
> +		 * For a child remote partition with remote_partition_updating
> +		 * set, we can skip the whole subtree as the this function will
> +		 * be called eventually. We can also skip the subtree if the
> +		 * effective_xcpus isn't going to be changed. Otherwise, we
> +		 * will have to call remote_cpus_update().
>  		 *
>  		 * remote_cpus_update() will reuse tmp->new_cpus only after
>  		 * its value is being processed.
>  		 */
>  		if (remote && (cp != cs)) {
>  			compute_excpus(cp, tmp->new_cpus);
> -			if (cpumask_equal(cp->effective_xcpus, tmp->new_cpus)) {
> +			if (cp->remote_partition_updating ||
> +			    cpumask_equal(cp->effective_xcpus, tmp->new_cpus)) {
>  				pos_css = css_rightmost_descendant(pos_css);
>  				continue;
>  			}
> diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> index 5bc144836bdf..19c50819065c 100755
> --- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> +++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> @@ -347,6 +347,7 @@ TEST_MATRIX=(
>  	"   C0-3    C1-3  C2-3    C4    X2-3   X2-3  X2-3:P2 P2:C4-5 0 A3:2-3|B1:4-5 A3:P2|B1:P2 2-5"
>  	"  C4:X0-3  X1-3  X2-3     .      .      P2     .      .     0 A1:4|A2:1-3|A3:1-3 A2:P2 1-3"
>  	"  C4:X0-3  X1-3  X2-3     .      .      .      P2     .     0 A1:4|A2:4|A3:2-3 A3:P2 2-3"
> +	"   CX1-6  C1-4:P1  .    C4-5     .    C1-3     .      .     0 A1:4-6|A2:1-3|B1:4-5 A2:P1"
>  
>  	# Nested remote/local partition tests
>  	"   C0-3    C1-3  C2-3   C4-5   X2-3  X2-3:P1   P2     P1    0 A1:0-1|A2:|A3:2-3|B1:4-5 \

Reviewed-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Tested-by: Guopeng Zhang <zhangguopeng@kylinos.cn>

Thanks,
Guopeng