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

[Michal Hocko]

On Fri 20-02-26 11:48:00, Vlastimil Babka wrote:
> 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?)

Thanks, this is a very useful insight.
 
> 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?

This would be tricky because there is no time guarantee when isolated
workload enters the kernel again. Maybe never if all the
pre-initialization was sufficient. On the other hand if the flush
happens on the way to userspace then you only need to wait for the
isolated workload to return from a syscall (modulo task dying and
similar edge cases).
 
> 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?

That seems like a reasonable performance price to pay for very edge case
(isolated workload).
-- 
Michal Hocko
SUSE Labs