Re: [PATCH] PCI: Avoid work_on_cpu() in async probe workers

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Rafael, Danilo (driver core question), update Alexander's email]

On Wed, Dec 31, 2025 at 03:51:05PM +0800, Jinhui Guo wrote:
> On Tue, Dec 30, 2025 at 03:52:41PM -0600, Bjorn Helgaas wrote:
> > On Tue, Dec 30, 2025 at 10:27:36PM +0800, Jinhui Guo wrote:
> > > On Mon, Dec 29, 2025 at 08:08:57AM -1000, Tejun Heo wrote:
> > > > On Sat, Dec 27, 2025 at 07:33:26PM +0800, Jinhui Guo wrote:
> > > > > To fix the issue, pci_call_probe() must not call work_on_cpu() when it is
> > > > > already running inside an unbounded asynchronous worker. Because a driver
> > > > > can be probed asynchronously either by probe_type or by the kernel command
> > > > > line, we cannot rely on PROBE_PREFER_ASYNCHRONOUS alone. Instead, we test
> > > > > the PF_WQ_WORKER flag in current->flags; if it is set, pci_call_probe() is
> > > > > executing within an unbounded workqueue worker and should skip the extra
> > > > > work_on_cpu() call.
> > > > 
> > > > Why not just use queue_work_on() on system_dfl_wq (or any other unbound
> > > > workqueue)? Those are soft-affine to cache domain but can overflow to other
> > > > CPUs?
> > > 
> > > Hi, tejun,
> > > 
> > > Thank you for your time and helpful suggestions.
> > > I had considered replacing work_on_cpu() with queue_work_on(system_dfl_wq) +
> > > flush_work(), but that would be a refactor rather than a fix for the specific
> > > problem we hit.
> > > 
> > > Let me restate the issue:
> > > 
> > > 1. With PROBE_PREFER_ASYNCHRONOUS enabled, the driver core queues work on
> > >    async_wq to speed up driver probe.
> > > 2. The PCI core then calls work_on_cpu() to tie the probe thread to the PCI
> > >    device’s NUMA node, but it always picks the same CPU for every device on
> > >    that node, forcing the PCI probes to run serially.
> > > 
> > > Therefore I test current->flags & PF_WQ_WORKER to detect that we are already
> > > inside an async_wq worker and skip the extra nested work queue.
> > > 
> > > I agree with your point—using queue_work_on(system_dfl_wq) + flush_work()
> > > would be cleaner and would let different vendors’ drivers probe in parallel
> > > instead of fighting over the same CPU. I’ve prepared and tested another patch,
> > > but I’m still unsure it’s the better approach; any further suggestions would
> > > be greatly appreciated.
> > > 
> > > Test results for that patch:
> > >   nvme 0000:01:00.0: CPU: 2, COMM: kworker/u1025:3, probe cost: 34904955 ns
> > >   nvme 0000:02:00.0: CPU: 134, COMM: kworker/u1025:1, probe cost: 34774235 ns
> > >   nvme 0000:03:00.0: CPU: 1, COMM: kworker/u1025:4, probe cost: 34573054 ns
> > > 
> > > Key changes in the patch:
> > > 
> > > 1. Keep the current->flags & PF_WQ_WORKER test to avoid nested workers.
> > > 2. Replace work_on_cpu() with queue_work_node(system_dfl_wq) + flush_work()
> > >    to enable parallel probing when PROBE_PREFER_ASYNCHRONOUS is disabled.
> > > 3. Remove all cpumask operations.
> > > 4. Drop cpu_hotplug_disable() since both cpumask manipulation and work_on_cpu()
> > >    are gone.
> > > 
> > > The patch is shown below.
> > 
> > I love this patch because it makes pci_call_probe() so much simpler.
> > 
> > I *would* like a short higher-level description of the issue that
> > doesn't assume so much workqueue background.
> > 
> > I'm not an expert, but IIUC __driver_attach() schedules async workers
> > so driver probes can run in parallel, but the problem is that the
> > workers for devices on node X are currently serialized because they
> > all bind to the same CPU on that node.
> > 
> > Naive questions: It looks like async_schedule_dev() already schedules
> > an async worker on the device node, so why does pci_call_probe() need
> > to use queue_work_node() again?
> > 
> > pci_call_probe() dates to 2005 (d42c69972b85 ("[PATCH] PCI: Run PCI
> > driver initialization on local node")), but the async_schedule_dev()
> > looks like it was only added in 2019 (c37e20eaf4b2 ("driver core:
> > Attach devices on CPU local to device node")).  Maybe the
> > pci_call_probe() node awareness is no longer necessary?
> 
> Hi, Bjorn
> 
> Thank you for your time and kind reply.
> 
> As I see it, two scenarios should be borne in mind:
> 
> 1. Driver allowed to probe asynchronously
>    The driver core schedules async workers via async_schedule_dev(),
>    so pci_call_probe() needs no extra queue_work_node().
> 
> 2. Driver not allowed to probe asynchronously
>    The driver core (__driver_attach() or __device_attach()) calls
>    pci_call_probe() directly, without any async worker from
>    async_schedule_dev(). NUMA-node awareness in pci_call_probe()
>    is therefore still required.

Good point, we need the NUMA awareness in both sync and async probe
paths.

But node affinity is orthogonal to the sync/async question, so it
seems weird to deal with affinity in two separate places.  It also
seems sub-optimal to have node affinity in the driver core async path
but not the synchronous probe path.

Maybe driver_probe_device() should do something about NUMA affinity?

Bjorn