Re: [PATCH v2 0/5] Introduce QPW for per-cpu operations (v2)

[Marcelo Tosatti]

Hi Frederic,

On Thu, Mar 05, 2026 at 05:55:12PM +0100, Frederic Weisbecker wrote:
> Le Mon, Mar 02, 2026 at 12:49:45PM -0300, Marcelo Tosatti a écrit :
> > The problem:
> > Some places in the kernel implement a parallel programming strategy
> > consisting on local_locks() for most of the work, and some rare remote
> > operations are scheduled on target cpu. This keeps cache bouncing low since
> > cacheline tends to be mostly local, and avoids the cost of locks in non-RT
> > kernels, even though the very few remote operations will be expensive due
> > to scheduling overhead.
> > 
> > On the other hand, for RT workloads this can represent a problem: getting
> > an important workload scheduled out to deal with remote requests is
> > sure to introduce unexpected deadline misses.
> > 
> > The idea:
> > Currently with PREEMPT_RT=y, local_locks() become per-cpu spinlocks.
> > In this case, instead of scheduling work on a remote cpu, it should
> > be safe to grab that remote cpu's per-cpu spinlock and run the required
> > work locally. That major cost, which is un/locking in every local function,
> > already happens in PREEMPT_RT.
> > 
> > Also, there is no need to worry about extra cache bouncing:
> > The cacheline invalidation already happens due to schedule_work_on().
> > 
> > This will avoid schedule_work_on(), and thus avoid scheduling-out an
> > RT workload.
> > 
> > Proposed solution:
> > A new interface called Queue PerCPU Work (QPW), which should replace
> > Work Queue in the above mentioned use case.
> > 
> > If CONFIG_QPW=n this interfaces just wraps the current
> > local_locks + WorkQueue behavior, so no expected change in runtime.
> > 
> > If CONFIG_QPW=y, and qpw kernel boot option =1, 
> > queue_percpu_work_on(cpu,...) will lock that cpu's per-cpu structure
> > and perform work on it locally. This is possible because on 
> > functions that can be used for performing remote work on remote 
> > per-cpu structures, the local_lock (which is already
> > a this_cpu spinlock()), will be replaced by a qpw_spinlock(), which
> > is able to get the per_cpu spinlock() for the cpu passed as parameter.
> 
> So let me summarize what are the possible design solutions, on top of our discussions,
> so we can compare:
> 
> 1) Never queue remotely but always queue locally and execute on userspace
>    return via task work.

How can you "queue locally" if the request is visible on a remote CPU?

That is, the event which triggers the manipulation of data structures 
which need to be performed by the owner CPU (owner of the data
structures) is triggered on a remote CPU.

This is confusing.

Can you also please give a practical example of such case ?

>    Pros:
>          - Simple and easy to maintain.
> 
>    Cons:
>          - Need a case by case handling.
> 
> 	 - Might be suitable for full userspace applications but not for
>            some HPC usecases. In the best world MPI is fully implemented in
>            userspace but that doesn't appear to be the case.
> 
> 2) Queue locally the workqueue right away

Again, the event which triggers the manipulation of data structures
by the owner CPU happens on a remote CPU. 
So how can you queue it locally ?

>    or do it remotely (if it's
>    really necessary) if the isolated CPU is in userspace, otherwise queue
>    it for execution on return to kernel. The work will be handled by preemption
>    to a worker or by a workqueue flush on return to userspace.
> 
>    Pros:
>         - The local queue handling is simple.
> 
>    Cons:
>         - The remote queue must synchronize with return to userspace and
> 	  eventually postpone to return to kernel if the target is in userspace.
> 	  Also it may need to differentiate IRQs and syscalls.
> 
>         - Therefore still involve some case by case handling eventually.
>    
>         - Flushing the global workqueues to avoid deadlocks is unadvised as shown
>           in the comment above flush_scheduled_work(). It even triggers a
>           warning. Significant efforts have been put to convert all the existing
> 	  users. It's not impossible to sell in our case because we shouldn't
> 	  hold a lock upon return to userspace. But that will restore a new
> 	  dangerous API.
> 
>         - Queueing the workqueue / flushing involves a context switch which
>           induce more noise (eg: tick restart)
> 	  
>         - As above, probably not suitable for HPC.
> 
> 3) QPW: Handle the work remotely
> 
>    Pros:
>         - Works on all cases, without any surprise.
> 
>    Cons:
>         - Introduce new locking scheme to maintain and debug.
> 
>         - Needs case by case handling.
> 
> Thoughts?

Can you please be more verbose, mindful of lesser cognitive powers ? :-) 

Note: i also dislike the added layers (and multiple cases) QPW adds.

But there is precedence with local locks...

Code would be less complex in case spinlocks were added:

01b44456a7aa7c3b24fa9db7d1714b208b8ef3d8 mm/page_alloc: replace local_lock with normal spinlock
4b23a68f953628eb4e4b7fe1294ebf93d4b8ceee mm/page_alloc: protect PCP lists with a spinlock

But people seem to reject that in the basis of performance
degradation.