Re: [PATCH-next v3 01/11] cgroup/cpuset: Find furthest isolated ancestor for invalidated isolated partition

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

在 2026/10/26 05:44, Waiman Long 写道:
> Commit 46b885416c27 ("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: 46b885416c27 ("cgroup/cpuset: Fix a missing isolated_cpus update in update_cpumasks_hier()")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/cgroup/cpuset.c | 104 +++++++++++++++++++++--------------------
>  1 file changed, 53 insertions(+), 51 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 585a982249ad..bbd44782d4bf 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -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)

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

Thanks,
Guopeng