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slub: Update frozen slabs documentations in the source 

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

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@bytedance.com>
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: Christoph Lameter (Ampere) <cl@linux.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
This commit is contained in:
Chengming Zhou 2023-11-02 03:23:30 +00:00 committed by Vlastimil Babka
parent 21316fdc79
commit 31bda717d7
1 changed files with 18 additions and 4 deletions
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22
mm/slub.c
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@ -76,13 +76,28 @@
*
* 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 are also exempt from list management,
* by clearing the PG_workingset flag when moving out of the node
* partial list. Please see __slab_free() for more details.
*
* To sum up, the current scheme is:
* - node partial slab: PG_Workingset && !frozen
* - cpu partial slab: !PG_Workingset && !frozen
* - cpu slab: !PG_Workingset && frozen
* - full slab: !PG_Workingset && !frozen
*
* list_lock
*
* The list_lock protects the partial and full list on each node and
@ -2617,8 +2632,7 @@ 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.