[PATCH v3 net-next 5/6] net: usb: pegasus: Move long delayed work on system_dfl_long_wq

[PATCH v3 net-next 5/6] net: usb: pegasus: Move long delayed work on system_dfl_long_wq

[Marco Crivellari]

Currently the code enqueue work items using {queue|mod}_delayed_work(),
using system_long_wq. This workqueue should be used when long works are
expected and it is a per-cpu workqueue.

The function(s) end up calling __queue_delayed_work(), which set a global
timer that could fire anywhere, enqueuing the work where the timer fired.

Unbound works could benefit from scheduler task placement, to optimize
performance and power consumption. Long work shouldn't stick to a single
CPU.

Recently, a new unbound workqueue specific for long running work has
been added:

    c116737e972e ("workqueue: Add system_dfl_long_wq for long unbound works")

Since the workqueue work doesn't rely on per-cpu variables, there is no
obvious reason that justify the use of a per-cpu workqueue. So change
system_long_wq with system_dfl_long_wq so that the work may benefit from
scheduler task placement.

Cc: Petko Manolov <petkan@nucleusys.com>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 drivers/net/usb/pegasus.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/net/usb/pegasus.c b/drivers/net/usb/pegasus.c
index 8700eeb8e22d..c1798e14b224 100644
--- a/drivers/net/usb/pegasus.c
+++ b/drivers/net/usb/pegasus.c
@@ -1126,8 +1126,9 @@ static void check_carrier(struct work_struct *work)
 	pegasus_t *pegasus = container_of(work, pegasus_t, carrier_check.work);
 	set_carrier(pegasus->net);
 	if (!(pegasus->flags & PEGASUS_UNPLUG)) {
-		queue_delayed_work(system_long_wq, &pegasus->carrier_check,
-			CARRIER_CHECK_DELAY);
+		queue_delayed_work(system_dfl_long_wq,
+				   &pegasus->carrier_check,
+				   CARRIER_CHECK_DELAY);
 	}
 }
 
@@ -1232,7 +1233,7 @@ static int pegasus_probe(struct usb_interface *intf,
 	res = register_netdev(net);
 	if (res)
 		goto out3;
-	queue_delayed_work(system_long_wq, &pegasus->carrier_check,
+	queue_delayed_work(system_dfl_long_wq, &pegasus->carrier_check,
 			   CARRIER_CHECK_DELAY);
 	dev_info(&intf->dev, "%s, %s, %pM\n", net->name,
 		 usb_dev_id[dev_index].name, net->dev_addr);
@@ -1297,7 +1298,7 @@ static int pegasus_resume(struct usb_interface *intf)
 		pegasus->intr_urb->actual_length = 0;
 		intr_callback(pegasus->intr_urb);
 	}
-	queue_delayed_work(system_long_wq, &pegasus->carrier_check,
+	queue_delayed_work(system_dfl_long_wq, &pegasus->carrier_check,
 				CARRIER_CHECK_DELAY);
 	return 0;
 }
-- 
2.54.0

Re: [PATCH v3 net-next 5/6] net: usb: pegasus: Move long delayed work on system_dfl_long_wq

[Oliver Neukum]

On 20.07.26 12:08, Marco Crivellari wrote:
Hi,

> 
> Since the workqueue work doesn't rely on per-cpu variables, there is no
> obvious reason that justify the use of a per-cpu workqueue. So change
> system_long_wq with system_dfl_long_wq so that the work may benefit from
> scheduler task placement.

these changes are problematic, although they look like a good cleanup
in first place. But the test you are using to determine whether USB
devices need their own work queue is incomplete because you are not
considering the reason they allocate their own work queues.

These drivers have their own work queues because they are part of the block layer.
USB devices can share a device with a block device (storage & UAS) and
USB devices have common, per device operations, in particular reset
and runtime power management and disconnect handling. Because these operations
can be necessary to complete block IO neither they nor anything
they depend on can use IO to allocate memory. That is they need to
perform any memory allocation with GFP_NOIO or GFP_ATOMIC.

That is also true for any operation on a work queue they need to wait
for to make progress. That means you cannot limit your check to per-cpu
variables. You also need to check for such dependencies. In particular
any usage of flush_work() on such queues can deadlock, if you use
common queues.

Please refrain from making such changes unless you have fully analyzed
the dependencies.

	Regards
		Oliver

Re: [PATCH v3 net-next 5/6] net: usb: pegasus: Move long delayed work on system_dfl_long_wq

[Sebastian Andrzej Siewior]

On 2026-07-22 10:29:50 [+0200], Oliver Neukum wrote:
> On 20.07.26 12:08, Marco Crivellari wrote:
> Hi,
Hi Oliver,

> > Since the workqueue work doesn't rely on per-cpu variables, there is no
> > obvious reason that justify the use of a per-cpu workqueue. So change
> > system_long_wq with system_dfl_long_wq so that the work may benefit from
> > scheduler task placement.
> 
> these changes are problematic, although they look like a good cleanup
> in first place. But the test you are using to determine whether USB
> devices need their own work queue is incomplete because you are not
> considering the reason they allocate their own work queues.

This driver does not using its own workqueue.

> These drivers have their own work queues because they are part of the block layer.
> USB devices can share a device with a block device (storage & UAS) and
> USB devices have common, per device operations, in particular reset
> and runtime power management and disconnect handling. Because these operations
> can be necessary to complete block IO neither they nor anything
> they depend on can use IO to allocate memory. That is they need to
> perform any memory allocation with GFP_NOIO or GFP_ATOMIC.

This is a networking driver. It has nothing to do with storage and UAS.
It uses USB, yes.

> That is also true for any operation on a work queue they need to wait
> for to make progress. That means you cannot limit your check to per-cpu
> variables. You also need to check for such dependencies. In particular
> any usage of flush_work() on such queues can deadlock, if you use
> common queues.

It has nothing to do with per-CPU variables. In fact its
queue_delayed_work(,, CARRIER_CHECK_DELAY) usage already ensures that it
can be executed on a random and not on the submitting CPU.

> Please refrain from making such changes unless you have fully analyzed
> the dependencies.

Did you refer to the wrong patch? As far as this patch goes I can not
reason why you assume Marco did not fully analyze the dependencies. I
don't see anything wrong with this patch.

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

> 	Regards
> 		Oliver
> 

Sebastian

Re: [PATCH v3 net-next 5/6] net: usb: pegasus: Move long delayed work on system_dfl_long_wq

[Alan Stern]

On Tue, Aug 25, 2026 at 05:18:12PM +0200, Sebastian Andrzej Siewior wrote:
> On 2026-07-22 10:29:50 [+0200], Oliver Neukum wrote:
> > On 20.07.26 12:08, Marco Crivellari wrote:
> > Hi,
> Hi Oliver,
> 
> > > Since the workqueue work doesn't rely on per-cpu variables, there is no
> > > obvious reason that justify the use of a per-cpu workqueue. So change
> > > system_long_wq with system_dfl_long_wq so that the work may benefit from
> > > scheduler task placement.
> > 
> > these changes are problematic, although they look like a good cleanup
> > in first place. But the test you are using to determine whether USB
> > devices need their own work queue is incomplete because you are not
> > considering the reason they allocate their own work queues.
> 
> This driver does not using its own workqueue.
> 
> > These drivers have their own work queues because they are part of the block layer.
> > USB devices can share a device with a block device (storage & UAS) and
> > USB devices have common, per device operations, in particular reset
> > and runtime power management and disconnect handling. Because these operations
> > can be necessary to complete block IO neither they nor anything
> > they depend on can use IO to allocate memory. That is they need to
> > perform any memory allocation with GFP_NOIO or GFP_ATOMIC.
> 
> This is a networking driver. It has nothing to do with storage and UAS.
> It uses USB, yes.

Ah, but a composite USB device can have both a networking interface and 
a mass-storage interface.

Suppose you have such a device, and suppose the disk attached to its 
mass-storage interface contains a swap partition.  Now suppose the 
device is being reset, and as part of the preparation for that reset the 
networking driver needs to flush its workqueue.  This means waiting 
until the work routines that are already running have completed.

Since it's a general-purpose workqueue, you don't know what those work 
routines are going to do.  One of them might try to allocate memory 
using GFP_KERNEL.  Suppose that in order to satisfy the memory request, 
the kernel decides it needs to write some pages to the swap partition on 
the USB mass-storage interface.  But the mass-storage driver is stuck; 
it can't do anything until the device reset finishes.  Deadlock.

That's why USB drivers have to use their own workqueues.

Alan Stern

Re: [PATCH v3 net-next 5/6] net: usb: pegasus: Move long delayed work on system_dfl_long_wq

[Sebastian Andrzej Siewior]

On 2026-08-27 12:01:02 [-0400], Alan Stern wrote:
> > > These drivers have their own work queues because they are part of the block layer.
> > > USB devices can share a device with a block device (storage & UAS) and
> > > USB devices have common, per device operations, in particular reset
> > > and runtime power management and disconnect handling. Because these operations
> > > can be necessary to complete block IO neither they nor anything
> > > they depend on can use IO to allocate memory. That is they need to
> > > perform any memory allocation with GFP_NOIO or GFP_ATOMIC.
> > 
> > This is a networking driver. It has nothing to do with storage and UAS.
> > It uses USB, yes.
> 
> Ah, but a composite USB device can have both a networking interface and 
> a mass-storage interface.

okay.

> Suppose you have such a device, and suppose the disk attached to its 
> mass-storage interface contains a swap partition.  Now suppose the 
> device is being reset, and as part of the preparation for that reset the 
> networking driver needs to flush its workqueue.  This means waiting 
> until the work routines that are already running have completed.

The individual functions are usually independent. But if the USB core
would reset the whole device it would reset each function.

> Since it's a general-purpose workqueue, you don't know what those work 
> routines are going to do.  One of them might try to allocate memory 
> using GFP_KERNEL.  Suppose that in order to satisfy the memory request, 
> the kernel decides it needs to write some pages to the swap partition on 
> the USB mass-storage interface.  But the mass-storage driver is stuck; 
> it can't do anything until the device reset finishes.  Deadlock.

So you are saying, the USB-storage device is in reset and we wait until
the networking part finishes its workqueue flush. That flush is stuck
behind behind a memory allocation which waits on the storage device. So
any URB passed to usb_submit_urb() just waits for the reset to complete?

> That's why USB drivers have to use their own workqueues.

While this does make sense I don't see how this is related to this
patch. The pegasus driver uses `system_long_wq'. This is a system wide
workqueue_struct and is not limited to USB or this driver.
This workqueue is per-CPU meaning if you enqueue the work item on CPU3
it will be executed on CPU3. However pegasus uses a delayed work item
and the timer can fire on any CPU so even if it is enqueued on CPU3 it
could be executed on CPU1.

Therefore the suggested change system_long_wq -> system_dfl_long_wq
should not make a difference here: it is a different workqueue and it is
unbound (instead of per-CPU) but given the usage it is unchanged but
more obvious. Also its usage recommendations (use this for long running
items) is the same.

The plan is remove system_long_wq from the tree.

> Alan Stern

Sebastian

Re: [PATCH v3 net-next 5/6] net: usb: pegasus: Move long delayed work on system_dfl_long_wq

[Alan Stern]

On Fri, Aug 28, 2026 at 11:43:18AM +0200, Sebastian Andrzej Siewior wrote:
> On 2026-08-27 12:01:02 [-0400], Alan Stern wrote:
> > > > These drivers have their own work queues because they are part of the block layer.
> > > > USB devices can share a device with a block device (storage & UAS) and
> > > > USB devices have common, per device operations, in particular reset
> > > > and runtime power management and disconnect handling. Because these operations
> > > > can be necessary to complete block IO neither they nor anything
> > > > they depend on can use IO to allocate memory. That is they need to
> > > > perform any memory allocation with GFP_NOIO or GFP_ATOMIC.
> > > 
> > > This is a networking driver. It has nothing to do with storage and UAS.
> > > It uses USB, yes.
> > 
> > Ah, but a composite USB device can have both a networking interface and 
> > a mass-storage interface.
> 
> okay.
> 
> > Suppose you have such a device, and suppose the disk attached to its 
> > mass-storage interface contains a swap partition.  Now suppose the 
> > device is being reset, and as part of the preparation for that reset the 
> > networking driver needs to flush its workqueue.  This means waiting 
> > until the work routines that are already running have completed.
> 
> The individual functions are usually independent. But if the USB core
> would reset the whole device it would reset each function.
> 
> > Since it's a general-purpose workqueue, you don't know what those work 
> > routines are going to do.  One of them might try to allocate memory 
> > using GFP_KERNEL.  Suppose that in order to satisfy the memory request, 
> > the kernel decides it needs to write some pages to the swap partition on 
> > the USB mass-storage interface.  But the mass-storage driver is stuck; 
> > it can't do anything until the device reset finishes.  Deadlock.
> 
> So you are saying, the USB-storage device is in reset and we wait until
> the networking part finishes its workqueue flush. That flush is stuck
> behind behind a memory allocation which waits on the storage device. So
> any URB passed to usb_submit_urb() just waits for the reset to complete?
> 
> > That's why USB drivers have to use their own workqueues.
> 
> While this does make sense I don't see how this is related to this
> patch. The pegasus driver uses `system_long_wq'. This is a system wide
> workqueue_struct and is not limited to USB or this driver.
> This workqueue is per-CPU meaning if you enqueue the work item on CPU3
> it will be executed on CPU3. However pegasus uses a delayed work item
> and the timer can fire on any CPU so even if it is enqueued on CPU3 it
> could be executed on CPU1.

It doesn't matter what CPU the work item runs on.  Here's the deadlock 
sequence, in brief:

	USB device reset cannot proceed until network interface's
	->pre_reset() method returns.

	The ->pre_reset() method cannot return until its call to
	flush_workqueue() returns.

	flush_workqueue() cannot return until the already executing
	work item finishes.

	The work item cannot finish until its kmalloc() call returns.

	kmalloc() won't return until the kernel can free up memory by 
	writing some pages to the swap partition.

	The write to the swap partition cannot take place until the
	usb_storage/uas driver carries it out.

	usb_storage/uas cannot do anything until the USB device reset
	is finished.

> Therefore the suggested change system_long_wq -> system_dfl_long_wq
> should not make a difference here: it is a different workqueue and it is
> unbound (instead of per-CPU) but given the usage it is unchanged but
> more obvious. Also its usage recommendations (use this for long running
> items) is the same.
> 
> The plan is remove system_long_wq from the tree.

The point Oliver was making is that the driver shouldn't be using a 
general-purpose workqueue at all.  Switching from one general-purpose 
workqueue to another ignores this point; it's not the right thing to do.

Alan Stern

[PATCH v3 net-next 6/6] net: usb: r8152: Move long delayed work on system_dfl_long_wq

[Marco Crivellari]

Currently the code enqueue work items using {queue|mod}_delayed_work(),
using system_long_wq. This workqueue should be used when long works are
expected and it is a per-cpu workqueue.

The function(s) end up calling __queue_delayed_work(), which set a global
timer that could fire anywhere, enqueuing the work where the timer fired.

Unbound works could benefit from scheduler task placement, to optimize
performance and power consumption. Long work shouldn't stick to a single
CPU.

Recently, a new unbound workqueue specific for long running work has
been added:

    c116737e972e ("workqueue: Add system_dfl_long_wq for long unbound works")

Since the workqueue work doesn't rely on per-cpu variables, there is no
obvious reason that justify the use of a per-cpu workqueue. So change
system_long_wq with system_dfl_long_wq so that the work may benefit from
scheduler task placement.

Cc: Ethan Nelson-Moore <enelsonmoore@gmail.com>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 drivers/net/usb/r8152.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/net/usb/r8152.c b/drivers/net/usb/r8152.c
index f61686433031..f6af66f294db 100644
--- a/drivers/net/usb/r8152.c
+++ b/drivers/net/usb/r8152.c
@@ -7072,7 +7072,8 @@ static void rtl_hw_phy_work_func_t(struct work_struct *work)
 
 		/* Delay execution in case request_firmware() is not ready yet.
 		 */
-		queue_delayed_work(system_long_wq, &tp->hw_phy_work, HZ * 10);
+		queue_delayed_work(system_dfl_long_wq, &tp->hw_phy_work,
+				   HZ * 10);
 		goto ignore_once;
 	}
 
@@ -8840,7 +8841,7 @@ static int rtl8152_reset_resume(struct usb_interface *intf)
 	clear_bit(SELECTIVE_SUSPEND, &tp->flags);
 	rtl_reset_ocp_base(tp);
 	tp->rtl_ops.init(tp);
-	queue_delayed_work(system_long_wq, &tp->hw_phy_work, 0);
+	queue_delayed_work(system_dfl_long_wq, &tp->hw_phy_work, 0);
 	set_ethernet_addr(tp, true);
 	return rtl8152_resume(intf);
 }
@@ -10295,7 +10296,7 @@ static int rtl8152_probe_once(struct usb_interface *intf,
 	/* Retry in case request_firmware() is not ready yet. */
 	tp->rtl_fw.retry = true;
 #endif
-	queue_delayed_work(system_long_wq, &tp->hw_phy_work, 0);
+	queue_delayed_work(system_dfl_long_wq, &tp->hw_phy_work, 0);
 	set_ethernet_addr(tp, false);
 
 	usb_set_intfdata(intf, tp);
-- 
2.54.0