Re: [RFC PATCH v4 9/9] slub: Update frozen slabs documentations in the source

[Vlastimil Babka]

On 10/31/23 15:07, chengming.zhou@xxxxxxxxx wrote:
> From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> 
> The current updated scheme (which this series implemented) is:
>  - node partial slabs: PG_Workingset && !frozen
>  - cpu partial slabs: !PG_Workingset && !frozen
>  - cpu slabs: !PG_Workingset && frozen
>  - full slabs: !PG_Workingset && !frozen

It could be useful to put this also to the initial comment description.
Towards the end of the comment, there's a block explaining
"slab->frozen". It could be extended to cover all 4 combination (but not
all of them need such long explanation).

> 
> The most important change is that "frozen" bit is not set for the
> cpu partial slabs anymore, __slab_free() will grab node list_lock
> then check by !PG_Workingset that it's not on a node partial list.
> 
> And the "frozen" bit is still kept for the cpu slabs for performance,
> since we don't need to grab node list_lock to check whether the
> PG_Workingset is set or not if the "frozen" bit is set in __slab_free().
> 
> Update related documentations and comments in the source.
> 
> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> ---
>  mm/slub.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index bb7368047103..89d3f7a18a73 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -76,13 +76,22 @@
>   *
>   *   Frozen slabs
>   *
> - *   If a slab is frozen then it is exempt from list management. It is not
> - *   on any list except per cpu partial list. The processor that froze the
> + *   If a slab is frozen then it is exempt from list management. It is
> + *   the cpu slab which is actively allocated from by the processor that
> + *   froze it and it is not on any list. The processor that froze the
>   *   slab is the one who can perform list operations on the slab. Other
>   *   processors may put objects onto the freelist but the processor that
>   *   froze the slab is the only one that can retrieve the objects from the
>   *   slab's freelist.
>   *
> + *   CPU partial slabs
> + *
> + *   The partially empty slabs cached on the CPU partial list are used
> + *   for performance reasons, which speeds up the allocation process.
> + *   These slabs are not frozen, but also exempt from list management,

					^ are also

(otherwise somebody could read it as "also are not")

> + *   by clearing the PG_workingset flag when moving out of the node
> + *   partial list. Please see __slab_free() for more details.
> + *
>   *   list_lock
>   *
>   *   The list_lock protects the partial and full list on each node and
> @@ -2620,8 +2629,7 @@ static void put_partials_cpu(struct kmem_cache *s,
>  }
>  
>  /*
> - * Put a slab that was just frozen (in __slab_free|get_partial_node) into a
> - * partial slab slot if available.
> + * Put a slab into a partial slab slot if available.
>   *
>   * If we did not find a slot then simply move all the partials to the
>   * per node partial list.