Re: [PATCH 0/4] Introduce QPW for per-cpu operations

[Vlastimil Babka]

On 2/19/26 16:27, Marcelo Tosatti wrote:
> On Mon, Feb 16, 2026 at 12:00:55PM +0100, Michal Hocko wrote:
> 
> Michal,
> 
> Again, i don't see how moving operations to happen at return to 
> kernel would help (assuming you are talking about 
> "context_tracking,x86: Defer some IPIs until a user->kernel transition").
> 
> The IPIs in the patchset above can be deferred until user->kernel
> transition because they are TLB flushes, for addresses which do not
> exist on the address space mapping in userspace.
> 
> What are the per-CPU objects in SLUB ?
> 
> struct slab_sheaf {
>         union {
>                 struct rcu_head rcu_head;
>                 struct list_head barn_list;
>                 /* only used for prefilled sheafs */
>                 struct {
>                         unsigned int capacity;
>                         bool pfmemalloc;
>                 };
>         };
>         struct kmem_cache *cache;
>         unsigned int size;
>         int node; /* only used for rcu_sheaf */
>         void *objects[];
> };
> 
> struct slub_percpu_sheaves {
>         local_trylock_t lock;
>         struct slab_sheaf *main; /* never NULL when unlocked */
>         struct slab_sheaf *spare; /* empty or full, may be NULL */
>         struct slab_sheaf *rcu_free; /* for batching kfree_rcu() */
> };
> 
> Examples of local CPU operation that manipulates the data structures:
> 1) kmalloc, allocates an object from local per CPU list.
> 2) kfree, returns an object to local per CPU list.
> 
> Examples of an operation that would perform changes on the per-CPU lists 
> remotely:
> kmem_cache_shrink (cache shutdown), kmem_cache_shrink.
> 
> You can't delay either kmalloc (removal of object from per-CPU freelist), 
> or kfree (return of object from per-CPU freelist), or kmem_cache_shrink 
> or kmem_cache_shrink to return to userspace.
> 
> What i missing something here? (or do you have something on your mind
> which i can't see).

Let's try and analyze when we need to do the flushing in SLUB

- memory offline - would anyone do that with isolcpus? if yes, they probably
deserve the disruption

- cache shrinking (mainly from sysfs handler) - not necessary for
correctness, can probably skip cpu if needed, also kinda shooting your own
foot on isolcpu systems

- kmem_cache is being destroyed (__kmem_cache_shutdown()) - this is
important for correctness. destroying caches should be rare, but can't rule
it out

- kvfree_rcu_barrier() - a very tricky one; currently has only a debugging
caller, but that can change

(BTW, see the note in flush_rcu_sheaves_on_cache() and how it relies on the
flush actually happening on the cpu. Won't QPW violate that?)

How would this work with houskeeping on return to userspace approach?

- Would we just walk the list of all caches to flush them? could be
expensive. Would we somehow note only those that need it? That would make
the fast paths do something extra?

- If some other CPU executed kmem_cache_destroy(), it would have to wait for
the isolated cpu returning to userspace. Do we have the means for
synchronizing on that? Would that risk a deadlock? We used to have a
deferred finishing of the destroy for other reasons but were glad to get rid
of it when it was possible, now it might be necessary to revive it?

How would this work with QPW?

- probably fast paths more expensive due to spin lock vs local_trylock_t

- flush_rcu_sheaves_on_cache() needs to be solved safely (see above)

What if we avoid percpu sheaves completely on isolated cpus and instead
allocate/free using the slowpaths?

- It could probably be achieved without affecting fastpaths, as we already
handle bootstrap without sheaves, so it's implemented in a way to not affect
fastpaths.

- Would it slow the isolcpu workloads down too much when they do a syscall?
  - compared to "houskeeping on return to userspace" flushing, maybe not?
Because in that case the syscall starts with sheaves flushed from previous
return, it has to do something expensive to get the initial sheaf, then
maybe will use only on or few objects, then on return has to flush
everything. Likely the slowpath might be faster, unless it allocates/frees
many objects from the same cache.
  - compared to QPW - it would be slower as QPW would mostly retain sheaves
populated, the need for flushes should be very rare

So if we can assume that workloads on isolated cpus make syscalls only
rarely, and when they do they can tolerate them being slower, I think the
"avoid sheaves on isolated cpus" would be the best way here.