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

Frederic Weisbecker
2026-03-03T11:16:00Z

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.

Thanks.

--
Frederic Weisbecker
SUSE Labs