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

[Marcelo Tosatti]

On Thu, Feb 19, 2026 at 08:30:31PM +0100, Michal Hocko wrote:
> On Thu 19-02-26 12:27:23, Marcelo Tosatti 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").
> 
> Nope, I am not talking about IPIs, although those are an example of pcp
> state as well. I am sorry I do not have a link handy, I am pretty sure
> Frederic will have that. Another example, though, was vmstat flushes
> that need to be pcp. There are many other examples. 
> 
> [...]
> 
> > 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.
> 
> Why?

Because kernel code might need to use that object right away, so it
needs to be allocated right after kmalloc returns.

> > What i missing something here? (or do you have something on your mind
> > which i can't see).
> 
> I am really sorry for being really vague here. Let me try to draw
> a more abstract problem definition and let's see whether we are trying
> to solve the same problem here. Maybe not...
> 
> I believe the main usecase of the interest here is uninterrupted
> userspace execution

The main usecase of interest is uninterrupted userspace execution, yes.

It is a good thing if you can enter the kernel, say perform system 
calls, and not be interrupted as well.

> and delayed pcp work that migh disturb such workload
> after it has returned to the userspace. Right?
> That is usually hauskeeping work that for, performance reasons, doesn't
> happen in hot paths while the workload was executing in the kernel
> space.
> 
> There are more ways to deal with that. You can either change the hot
> path to not require deferred operation (tricky withtout introducing
> regressions for most workloads) or you can define a more suitable place
> to perform the housekeeping while still running in the kernel. 
> 
> Your QWP work relies on local_lock -> spin_lock transition and
> performing the pcp work remotely so you do not need to disturb that
> remote cpu. Correct?
> 
> Alternative approach is to define a moment when the housekeeping
> operation is performed on that local cpu while still running in the
> kernel space - e.g. when returning to the userspace. Delayed work is
> then not necessary and userspace is not disrupted after returning to the
> userspace.
> 
> Do I make more sense or does the above sound like a complete gibberish?

OK, sure, but can't see how you can do that with per-CPU caches for
kmalloc, for example.