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

Leonardo Bras
2026-03-08T18:02:23Z

On Tue, Mar 03, 2026 at 12:15:53PM +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.
>
> Ok I'm slowly considering this as a more comfortable solution than the
> flush before userspace. Despite it being perhaps a bit more complicated,
> remote handling of housekeeping work is more surprise-free against all
> the possible nohz_full usecases that we are having a hard time to envision.
>
> Reviewing this more in details now.

Awesome! Thanks!
Leo

>
> Thanks.
>
> --
> Frederic Weisbecker
> SUSE Labs