[PATCH-cgroup] cgroup: Move rcu_head up near the top of cgroup_root

[PATCH-cgroup] cgroup: Move rcu_head up near the top of cgroup_root

[Waiman Long]

Commit d23b5c577715 ("cgroup: Make operations on the cgroup root_list RCU
safe") adds a new rcu_head to the cgroup_root structure and kvfree_rcu()
for freeing the cgroup_root.

The use of kvfree_rcu(), however, has the limitation that the offset of
the rcu_head structure within the larger data structure cannot exceed
4096 or the compilation will fail. By putting rcu_head below the cgroup
structure, any change to the cgroup structure that makes it larger has
the risk of build failure. Commit 77070eeb8821 ("cgroup: Avoid false
cacheline sharing of read mostly rstat_cpu") happens to be the commit
that breaks it even though it is not its fault. Fix it by moving the
rcu_head structure up before the cgroup structure.

Fixes: d23b5c577715 ("cgroup: Make operations on the cgroup root_list RCU safe")
Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/cgroup-defs.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 5a97ea95b564..45359969d8cf 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -562,6 +562,10 @@ struct cgroup_root {
 	/* Unique id for this hierarchy. */
 	int hierarchy_id;
 
+	/* A list running through the active hierarchies */
+	struct list_head root_list;
+	struct rcu_head rcu;
+
 	/*
 	 * The root cgroup. The containing cgroup_root will be destroyed on its
 	 * release. cgrp->ancestors[0] will be used overflowing into the
@@ -575,10 +579,6 @@ struct cgroup_root {
 	/* Number of cgroups in the hierarchy, used only for /proc/cgroups */
 	atomic_t nr_cgrps;
 
-	/* A list running through the active hierarchies */
-	struct list_head root_list;
-	struct rcu_head rcu;
-
 	/* Hierarchy-specific flags */
 	unsigned int flags;
 
-- 
2.39.3

Re: [PATCH-cgroup] cgroup: Move rcu_head up near the top of cgroup_root

[Yosry Ahmed]

On Wed, Dec 6, 2023 at 8:38 PM Waiman Long <longman@redhat.com> wrote:
>
> Commit d23b5c577715 ("cgroup: Make operations on the cgroup root_list RCU
> safe") adds a new rcu_head to the cgroup_root structure and kvfree_rcu()
> for freeing the cgroup_root.
>
> The use of kvfree_rcu(), however, has the limitation that the offset of
> the rcu_head structure within the larger data structure cannot exceed
> 4096 or the compilation will fail. By putting rcu_head below the cgroup
> structure, any change to the cgroup structure that makes it larger has
> the risk of build failure. Commit 77070eeb8821 ("cgroup: Avoid false
> cacheline sharing of read mostly rstat_cpu") happens to be the commit
> that breaks it even though it is not its fault. Fix it by moving the
> rcu_head structure up before the cgroup structure.
>
> Fixes: d23b5c577715 ("cgroup: Make operations on the cgroup root_list RCU safe")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  include/linux/cgroup-defs.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
> index 5a97ea95b564..45359969d8cf 100644
> --- a/include/linux/cgroup-defs.h
> +++ b/include/linux/cgroup-defs.h
> @@ -562,6 +562,10 @@ struct cgroup_root {
>         /* Unique id for this hierarchy. */
>         int hierarchy_id;
>
> +       /* A list running through the active hierarchies */
> +       struct list_head root_list;
> +       struct rcu_head rcu;
> +

Perhaps the comment should mention the placement requirements, and
maybe a pointer to wherever it is specified that the offset of struct
rcu_head should not exceed 4096?

Re: [PATCH-cgroup] cgroup: Move rcu_head up near the top of cgroup_root

[Waiman Long]

On 12/6/23 23:51, Yosry Ahmed wrote:
> On Wed, Dec 6, 2023 at 8:38 PM Waiman Long <longman@redhat.com> wrote:
>> Commit d23b5c577715 ("cgroup: Make operations on the cgroup root_list RCU
>> safe") adds a new rcu_head to the cgroup_root structure and kvfree_rcu()
>> for freeing the cgroup_root.
>>
>> The use of kvfree_rcu(), however, has the limitation that the offset of
>> the rcu_head structure within the larger data structure cannot exceed
>> 4096 or the compilation will fail. By putting rcu_head below the cgroup
>> structure, any change to the cgroup structure that makes it larger has
>> the risk of build failure. Commit 77070eeb8821 ("cgroup: Avoid false
>> cacheline sharing of read mostly rstat_cpu") happens to be the commit
>> that breaks it even though it is not its fault. Fix it by moving the
>> rcu_head structure up before the cgroup structure.
>>
>> Fixes: d23b5c577715 ("cgroup: Make operations on the cgroup root_list RCU safe")
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   include/linux/cgroup-defs.h | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
>> index 5a97ea95b564..45359969d8cf 100644
>> --- a/include/linux/cgroup-defs.h
>> +++ b/include/linux/cgroup-defs.h
>> @@ -562,6 +562,10 @@ struct cgroup_root {
>>          /* Unique id for this hierarchy. */
>>          int hierarchy_id;
>>
>> +       /* A list running through the active hierarchies */
>> +       struct list_head root_list;
>> +       struct rcu_head rcu;
>> +
> Perhaps the comment should mention the placement requirements, and
> maybe a pointer to wherever it is specified that the offset of struct
> rcu_head should not exceed 4096?

Fair. I will update the patch description to give more details about 
that. It is new to me too before I got a compilation failure report in 
linux-next.

Cheers,
Longman

>

Re: [PATCH-cgroup] cgroup: Move rcu_head up near the top of cgroup_root

[Yafang Shao]

On Thu, Dec 7, 2023 at 12:38 PM Waiman Long <longman@redhat.com> wrote:
>
> Commit d23b5c577715 ("cgroup: Make operations on the cgroup root_list RCU
> safe") adds a new rcu_head to the cgroup_root structure and kvfree_rcu()
> for freeing the cgroup_root.
>
> The use of kvfree_rcu(), however, has the limitation that the offset of
> the rcu_head structure within the larger data structure cannot exceed
> 4096 or the compilation will fail. By putting rcu_head below the cgroup
> structure, any change to the cgroup structure that makes it larger has
> the risk of build failure. Commit 77070eeb8821 ("cgroup: Avoid false
> cacheline sharing of read mostly rstat_cpu") happens to be the commit
> that breaks it even though it is not its fault. Fix it by moving the
> rcu_head structure up before the cgroup structure.
>
> Fixes: d23b5c577715 ("cgroup: Make operations on the cgroup root_list RCU safe")
> Signed-off-by: Waiman Long <longman@redhat.com>

Acked-by: Yafang Shao <laoar.shao@gmail.com>

> ---
>  include/linux/cgroup-defs.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
> index 5a97ea95b564..45359969d8cf 100644
> --- a/include/linux/cgroup-defs.h
> +++ b/include/linux/cgroup-defs.h
> @@ -562,6 +562,10 @@ struct cgroup_root {
>         /* Unique id for this hierarchy. */
>         int hierarchy_id;
>
> +       /* A list running through the active hierarchies */
> +       struct list_head root_list;
> +       struct rcu_head rcu;
> +
>         /*
>          * The root cgroup. The containing cgroup_root will be destroyed on its
>          * release. cgrp->ancestors[0] will be used overflowing into the
> @@ -575,10 +579,6 @@ struct cgroup_root {
>         /* Number of cgroups in the hierarchy, used only for /proc/cgroups */
>         atomic_t nr_cgrps;
>
> -       /* A list running through the active hierarchies */
> -       struct list_head root_list;
> -       struct rcu_head rcu;
> -
>         /* Hierarchy-specific flags */
>         unsigned int flags;
>
> --
> 2.39.3
>


-- 
Regards
Yafang