[PATCH 0/2] multi-threading device shutdown

[PATCH 0/2] multi-threading device shutdown

[Pavel Tatashin]

Do a faster shutdown by calling dev->*->shutdown(dev) in parallel.
device_shutdown() calls these functions for every single device but
only using one thread.

Since, nothing else is running on the machine by the device_shutdown()
s called, there is no reason not to utilize all the available CPU
resources.

Pavel Tatashin (2):
  ixgbe: release lock for the duration of ixgbe_suspend_close()
  drivers core: multi-threading device shutdown

 drivers/base/core.c                           | 238 ++++++++++++++----
 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c |   9 +-
 2 files changed, 197 insertions(+), 50 deletions(-)

-- 
2.17.0

[PATCH 1/2] ixgbe: release lock for the duration of ixgbe_suspend_close()

[Pavel Tatashin]

Currently, during device_shutdown() ixgbe holds rtnl_lock for the duration
of lengthy ixgbe_close_suspend(). On machines with multiple ixgbe cards
this lock prevents scaling if device_shutdown() function is multi-threaded.

It is not necessary to hold this lock during ixgbe_close_suspend()
as it is not held when ixgbe_close() is called also during shutdown but for
kexec case.

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
---
 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index afadba99f7b8..e7875b58854b 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
@@ -6748,8 +6748,15 @@ static int __ixgbe_shutdown(struct pci_dev *pdev, bool *enable_wake)
 	rtnl_lock();
 	netif_device_detach(netdev);
 
-	if (netif_running(netdev))
+	if (netif_running(netdev)) {
+		/* Suspend takes a long time, device_shutdown may be
+		 * parallelized this function, so drop lock for the
+		 * duration of this call.
+		 */
+		rtnl_unlock();
 		ixgbe_close_suspend(adapter);
+		rtnl_lock();
+	}
 
 	ixgbe_clear_interrupt_scheme(adapter);
 	rtnl_unlock();
-- 
2.17.0

Re: [PATCH 1/2] ixgbe: release lock for the duration of ixgbe_suspend_close()

[Alexander Duyck]

On Wed, May 2, 2018 at 8:59 PM, Pavel Tatashin
<pasha.tatashin@oracle.com> wrote:
> Currently, during device_shutdown() ixgbe holds rtnl_lock for the duration
> of lengthy ixgbe_close_suspend(). On machines with multiple ixgbe cards
> this lock prevents scaling if device_shutdown() function is multi-threaded.
>
> It is not necessary to hold this lock during ixgbe_close_suspend()
> as it is not held when ixgbe_close() is called also during shutdown but for
> kexec case.
>
> Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
> ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> index afadba99f7b8..e7875b58854b 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> @@ -6748,8 +6748,15 @@ static int __ixgbe_shutdown(struct pci_dev *pdev, bool *enable_wake)
>         rtnl_lock();
>         netif_device_detach(netdev);
>
> -       if (netif_running(netdev))
> +       if (netif_running(netdev)) {
> +               /* Suspend takes a long time, device_shutdown may be
> +                * parallelized this function, so drop lock for the
> +                * duration of this call.
> +                */
> +               rtnl_unlock();
>                 ixgbe_close_suspend(adapter);
> +               rtnl_lock();
> +       }
>
>         ixgbe_clear_interrupt_scheme(adapter);
>         rtnl_unlock();

I'm not a fan of dropping the mutex while we go through
ixgbe_close_suspend. I'm concerned it will result in us having a
number of races on shutdown.

If anything, I think we would need to find a replacement for the mutex
that can operate at the per-netdev level if you are wanting to
parallelize this.

- Alex

Re: [PATCH 1/2] ixgbe: release lock for the duration of ixgbe_suspend_close()

[Pavel Tatashin]

Hi Alex,

> I'm not a fan of dropping the mutex while we go through
> ixgbe_close_suspend. I'm concerned it will result in us having a
> number of races on shutdown.

I would agree, but ixgbe_close_suspend() is already called without this
mutex from ixgbe_close(). This path is executed also during machine
shutdown but when kexec'ed. So, it is either an existing bug or there are
no races. But, because ixgbe_close() is called from the userland, and a
little earlier than ixgbe_shutdown() I think this means there are no races.


> If anything, I think we would need to find a replacement for the mutex
> that can operate at the per-netdev level if you are wanting to
> parallelize this.

Yes, if lock is necessary, it can be replaced in this place (and added to
ixgbe_close())  with something scalable.

Thank you,
Pavel

Re: [PATCH 1/2] ixgbe: release lock for the duration of ixgbe_suspend_close()

[Alexander Duyck]

On Thu, May 3, 2018 at 6:30 AM, Pavel Tatashin
<pasha.tatashin@oracle.com> wrote:
> Hi Alex,

Hi Pavel,

>> I'm not a fan of dropping the mutex while we go through
>> ixgbe_close_suspend. I'm concerned it will result in us having a
>> number of races on shutdown.
>
> I would agree, but ixgbe_close_suspend() is already called without this
> mutex from ixgbe_close(). This path is executed also during machine
> shutdown but when kexec'ed. So, it is either an existing bug or there are
> no races. But, because ixgbe_close() is called from the userland, and a
> little earlier than ixgbe_shutdown() I think this means there are no races.

All callers of ixgbe_close are supposed to already be holding the RTNL
mutex. That is the whole reason why we need to hold it here as the
shutdown path doesn't have the mutex held otherwise. The mutex was
added here because shutdown was racing with the open/close calls.

>
>> If anything, I think we would need to find a replacement for the mutex
>> that can operate at the per-netdev level if you are wanting to
>> parallelize this.
>
> Yes, if lock is necessary, it can be replaced in this place (and added to
> ixgbe_close())  with something scalable.

I wouldn't suggest adding yet another lock for all this. Instead it
might make more sense for us to start looking at breaking up the
locking since most of the networking subsystem uses the rtnl_lock call
it might be time to start looking at doing something like cutting back
on the use of this in cases where all the resources needed are
specific to one device and instead have a per device lock that could
address those kind of cases.

- Alex

Re: [PATCH 1/2] ixgbe: release lock for the duration of ixgbe_suspend_close()

[Steven Sistare]

On 5/3/2018 9:46 AM, Alexander Duyck wrote:
> On Thu, May 3, 2018 at 6:30 AM, Pavel Tatashin
> <pasha.tatashin@oracle.com> wrote:
>> Hi Alex,
> 
> Hi Pavel,
> 
>>> I'm not a fan of dropping the mutex while we go through
>>> ixgbe_close_suspend. I'm concerned it will result in us having a
>>> number of races on shutdown.
>>
>> I would agree, but ixgbe_close_suspend() is already called without this
>> mutex from ixgbe_close(). This path is executed also during machine
>> shutdown but when kexec'ed. So, it is either an existing bug or there are
>> no races. But, because ixgbe_close() is called from the userland, and a
>> little earlier than ixgbe_shutdown() I think this means there are no races.
> 
> All callers of ixgbe_close are supposed to already be holding the RTNL
> mutex. That is the whole reason why we need to hold it here as the
> shutdown path doesn't have the mutex held otherwise. The mutex was
> added here because shutdown was racing with the open/close calls.
> 
>>
>>> If anything, I think we would need to find a replacement for the mutex
>>> that can operate at the per-netdev level if you are wanting to
>>> parallelize this.
>>
>> Yes, if lock is necessary, it can be replaced in this place (and added to
>> ixgbe_close())  with something scalable.
> 
> I wouldn't suggest adding yet another lock for all this. Instead it
> might make more sense for us to start looking at breaking up the
> locking since most of the networking subsystem uses the rtnl_lock call
> it might be time to start looking at doing something like cutting back
> on the use of this in cases where all the resources needed are
> specific to one device and instead have a per device lock that could
> address those kind of cases.

Hi Pavel, you might want to pull lock optimization out of this patch series.
The parallelization by itself is valuable, and optimizations for individual 
devices including locks can come later.

- Steve

Re: [PATCH 1/2] ixgbe: release lock for the duration of ixgbe_suspend_close()

[Pavel Tatashin]

> Hi Pavel, you might want to pull lock optimization out of this patch
series.
> The parallelization by itself is valuable, and optimizations for
individual
> devices including locks can come later.

Hi Steve,

Yes, I will pull this patch out of the series. Thank you for the suggestion.

Pavel

[PATCH 2/2] drivers core: multi-threading device shutdown

[Pavel Tatashin]

When system is rebooted, halted or kexeced device_shutdown() is
called.

This function shuts down every single device by calling either:
	dev->bus->shutdown(dev)
	dev->driver->shutdown(dev)

Even on a machine just with a moderate amount of devices, device_shutdown()
may take multiple seconds to complete. Because many devices require a
specific delays to perform this operation.

Here is sample analysis of time it takes to call device_shutdown() on
two socket Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz machine.

device_shutdown		2.95s
 mlx4_shutdown		1.14s
 megasas_shutdown	0.24s
 ixgbe_shutdown		0.37s x 4 (four ixgbe devices on my machine).
 the rest		0.09s

In mlx4 we spent the most time, but that is because there is a 1 second
sleep:
mlx4_shutdown
 mlx4_unload_one
  mlx4_free_ownership
   msleep(1000)

With megasas we spend quoter of second, but sometimes longer (up-to 0.5s)
in this path:

    megasas_shutdown
      megasas_flush_cache
        megasas_issue_blocked_cmd
          wait_event_timeout

Finally, with ixgbe_shutdown() it takes 0.37 for each device, but that time
is spread all over the place, with bigger offenders:

    ixgbe_shutdown
      __ixgbe_shutdown
        ixgbe_close_suspend
          ixgbe_down
            ixgbe_init_hw_generic
              ixgbe_reset_hw_X540
                msleep(100);                        0.104483472
                ixgbe_get_san_mac_addr_generic      0.048414851
                ixgbe_get_wwn_prefix_generic        0.048409893
              ixgbe_start_hw_X540
                ixgbe_start_hw_generic
                  ixgbe_clear_hw_cntrs_generic      0.048581502
                  ixgbe_setup_fc_generic            0.024225800

    All the ixgbe_*generic functions end-up calling:
    ixgbe_read_eerd_X540()
      ixgbe_acquire_swfw_sync_X540
        usleep_range(5000, 6000);
      ixgbe_release_swfw_sync_X540
        usleep_range(5000, 6000);

While these are short sleeps, they end-up calling them over 24 times!
24 * 0.0055s = 0.132s. Adding-up to 0.528s for four devices.

While we should keep optimizing the individual device drivers, in some
cases this is simply a hardware property that forces a specific delay, and
we must wait.

So, the solution for this problem is to shutdown devices in parallel.
However, we must shutdown children before shutting down parents, so parent
device must wait for its children to finish.

With this patch, on the same machine devices_shutdown() takes 1.142s, and
without mlx4 one second delay only 0.38s

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
---
 drivers/base/core.c | 238 +++++++++++++++++++++++++++++++++++---------
 1 file changed, 189 insertions(+), 49 deletions(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index b610816eb887..f370369a303b 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -25,6 +25,7 @@
 #include <linux/netdevice.h>
 #include <linux/sched/signal.h>
 #include <linux/sysfs.h>
+#include <linux/kthread.h>
 
 #include "base.h"
 #include "power/power.h"
@@ -2102,6 +2103,59 @@ const char *device_get_devnode(struct device *dev,
 	return *tmp = s;
 }
 
+/**
+ * device_children_count - device children count
+ * @parent: parent struct device.
+ *
+ * Returns number of children for this device or 0 if nonde.
+ */
+static int device_children_count(struct device *parent)
+{
+	struct klist_iter i;
+	int children = 0;
+
+	if (!parent->p)
+		return 0;
+
+	klist_iter_init(&parent->p->klist_children, &i);
+	while (next_device(&i))
+		children++;
+	klist_iter_exit(&i);
+
+	return children;
+}
+
+/**
+ * device_get_child_by_index - Return child using the provide index.
+ * @parent: parent struct device.
+ * @index:  Index of the child, where 0 is the first child in the children list,
+ * and so on.
+ *
+ * Returns child or NULL if child with this index is not present.
+ */
+static struct device *
+device_get_child_by_index(struct device *parent, int index)
+{
+	struct klist_iter i;
+	struct device *dev = NULL, *d;
+	int child_index = 0;
+
+	if (!parent->p || index < 0)
+		return NULL;
+
+	klist_iter_init(&parent->p->klist_children, &i);
+	while ((d = next_device(&i)) != NULL) {
+		if (child_index == index) {
+			dev = d;
+			break;
+		}
+		child_index++;
+	}
+	klist_iter_exit(&i);
+
+	return dev;
+}
+
 /**
  * device_for_each_child - device child iterator.
  * @parent: parent struct device.
@@ -2765,71 +2819,157 @@ int device_move(struct device *dev, struct device *new_parent,
 }
 EXPORT_SYMBOL_GPL(device_move);
 
+/*
+ * device_shutdown_one - call ->shutdown() for the device passed as
+ * argument.
+ */
+static void device_shutdown_one(struct device *dev)
+{
+	/* Don't allow any more runtime suspends */
+	pm_runtime_get_noresume(dev);
+	pm_runtime_barrier(dev);
+
+	if (dev->class && dev->class->shutdown_pre) {
+		if (initcall_debug)
+			dev_info(dev, "shutdown_pre\n");
+		dev->class->shutdown_pre(dev);
+	}
+	if (dev->bus && dev->bus->shutdown) {
+		if (initcall_debug)
+			dev_info(dev, "shutdown\n");
+		dev->bus->shutdown(dev);
+	} else if (dev->driver && dev->driver->shutdown) {
+		if (initcall_debug)
+			dev_info(dev, "shutdown\n");
+		dev->driver->shutdown(dev);
+	}
+
+	/* Release device lock, and decrement the reference counter */
+	device_unlock(dev);
+	put_device(dev);
+}
+
+static DECLARE_COMPLETION(device_root_tasks_complete);
+static void device_shutdown_tree(struct device *dev);
+static atomic_t device_root_tasks;
+
+/*
+ * Passed as an argument to to device_shutdown_task().
+ * child_next_index	the next available child index.
+ * tasks_running	number of tasks still running. Each tasks decrements it
+ *			when job is finished and the last tasks signals that the
+ *			job is complete.
+ * complete		Used to signal job competition.
+ * parent		Parent device.
+ */
+struct device_shutdown_task_data {
+	atomic_t		child_next_index;
+	atomic_t		tasks_running;
+	struct completion	complete;
+	struct device		*parent;
+};
+
+static int device_shutdown_task(void *data)
+{
+	struct device_shutdown_task_data *tdata =
+		(struct device_shutdown_task_data *)data;
+	int child_idx = atomic_inc_return(&tdata->child_next_index) - 1;
+	struct device *dev = device_get_child_by_index(tdata->parent,
+						       child_idx);
+
+	if (dev)
+		device_shutdown_tree(dev);
+	if (atomic_dec_return(&tdata->tasks_running) == 0)
+		complete(&tdata->complete);
+	return 0;
+}
+
+/*
+ * Shutdown device tree with root started in dev. If dev has no children
+ * simply shutdown only this device. If dev has children recursively shutdown
+ * children first, and only then the parent. For performance reasons children
+ * are shutdown in parallel using kernel threads.
+ */
+static void device_shutdown_tree(struct device *dev)
+{
+	int children_count = device_children_count(dev);
+
+	if (children_count) {
+		struct device_shutdown_task_data tdata;
+		int i;
+
+		init_completion(&tdata.complete);
+		atomic_set(&tdata.child_next_index, 0);
+		atomic_set(&tdata.tasks_running, children_count);
+		tdata.parent = dev;
+
+		for (i = 0; i < children_count; i++) {
+			kthread_run(device_shutdown_task,
+				    &tdata, "device_shutdown.%s",
+				    dev_name(dev));
+		}
+		wait_for_completion(&tdata.complete);
+	}
+	device_shutdown_one(dev);
+}
+
+/*
+ * On shutdown each root device (the one that does not have a parent) goes
+ * through this function.
+ */
+static int
+device_shutdown_root_task(void *data)
+{
+	struct device *dev = (struct device *)data;
+
+	device_shutdown_tree(dev);
+	if (atomic_dec_return(&device_root_tasks) == 0)
+		complete(&device_root_tasks_complete);
+	return 0;
+}
+
 /**
  * device_shutdown - call ->shutdown() on each device to shutdown.
  */
 void device_shutdown(void)
 {
-	struct device *dev, *parent;
+	struct list_head *pos, *next;
+	int root_devices = 0;
+	struct device *dev;
 
 	spin_lock(&devices_kset->list_lock);
 	/*
-	 * Walk the devices list backward, shutting down each in turn.
-	 * Beware that device unplug events may also start pulling
-	 * devices offline, even as the system is shutting down.
+	 * Prepare devices for shutdown: lock, and increment references in every
+	 * devices. Remove child devices from the list, and count number of root
+	 * devices.
 	 */
-	while (!list_empty(&devices_kset->list)) {
-		dev = list_entry(devices_kset->list.prev, struct device,
-				kobj.entry);
+	list_for_each_safe(pos, next, &devices_kset->list) {
+		dev = list_entry(pos, struct device, kobj.entry);
 
-		/*
-		 * hold reference count of device's parent to
-		 * prevent it from being freed because parent's
-		 * lock is to be held
-		 */
-		parent = get_device(dev->parent);
 		get_device(dev);
-		/*
-		 * Make sure the device is off the kset list, in the
-		 * event that dev->*->shutdown() doesn't remove it.
-		 */
-		list_del_init(&dev->kobj.entry);
-		spin_unlock(&devices_kset->list_lock);
-
-		/* hold lock to avoid race with probe/release */
-		if (parent)
-			device_lock(parent);
 		device_lock(dev);
 
-		/* Don't allow any more runtime suspends */
-		pm_runtime_get_noresume(dev);
-		pm_runtime_barrier(dev);
-
-		if (dev->class && dev->class->shutdown_pre) {
-			if (initcall_debug)
-				dev_info(dev, "shutdown_pre\n");
-			dev->class->shutdown_pre(dev);
-		}
-		if (dev->bus && dev->bus->shutdown) {
-			if (initcall_debug)
-				dev_info(dev, "shutdown\n");
-			dev->bus->shutdown(dev);
-		} else if (dev->driver && dev->driver->shutdown) {
-			if (initcall_debug)
-				dev_info(dev, "shutdown\n");
-			dev->driver->shutdown(dev);
-		}
-
-		device_unlock(dev);
-		if (parent)
-			device_unlock(parent);
-
-		put_device(dev);
-		put_device(parent);
-
+		if (!dev->parent)
+			root_devices++;
+		else
+			list_del_init(&dev->kobj.entry);
+	}
+	atomic_set(&device_root_tasks, root_devices);
+	/*
+	 * Shutdown the root devices in parallel. The children are going to be
+	 * shutdown first.
+	 */
+	list_for_each_safe(pos, next, &devices_kset->list) {
+		dev = list_entry(pos, struct device, kobj.entry);
+		list_del_init(&dev->kobj.entry);
+		spin_unlock(&devices_kset->list_lock);
+		kthread_run(device_shutdown_root_task,
+			    dev, "device_root_shutdown.%s",
+			    dev_name(dev));
 		spin_lock(&devices_kset->list_lock);
 	}
 	spin_unlock(&devices_kset->list_lock);
+	wait_for_completion(&device_root_tasks_complete);
 }
 
 /*
-- 
2.17.0

Re: [PATCH 2/2] drivers core: multi-threading device shutdown

[Tobin C. Harding]

This code was a pleasure to read, super clean.

On Wed, May 02, 2018 at 11:59:31PM -0400, Pavel Tatashin wrote:
> When system is rebooted, halted or kexeced device_shutdown() is
> called.
> 
> This function shuts down every single device by calling either:
> 	dev->bus->shutdown(dev)
> 	dev->driver->shutdown(dev)
> 
> Even on a machine just with a moderate amount of devices, device_shutdown()
> may take multiple seconds to complete. Because many devices require a
> specific delays to perform this operation.
> 
> Here is sample analysis of time it takes to call device_shutdown() on
> two socket Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz machine.
> 
> device_shutdown		2.95s
>  mlx4_shutdown		1.14s
>  megasas_shutdown	0.24s
>  ixgbe_shutdown		0.37s x 4 (four ixgbe devices on my machine).
>  the rest		0.09s
> 
> In mlx4 we spent the most time, but that is because there is a 1 second
> sleep:
> mlx4_shutdown
>  mlx4_unload_one
>   mlx4_free_ownership
>    msleep(1000)
> 
> With megasas we spend quoter of second, but sometimes longer (up-to 0.5s)
> in this path:
> 
>     megasas_shutdown
>       megasas_flush_cache
>         megasas_issue_blocked_cmd
>           wait_event_timeout
> 
> Finally, with ixgbe_shutdown() it takes 0.37 for each device, but that time
> is spread all over the place, with bigger offenders:
> 
>     ixgbe_shutdown
>       __ixgbe_shutdown
>         ixgbe_close_suspend
>           ixgbe_down
>             ixgbe_init_hw_generic
>               ixgbe_reset_hw_X540
>                 msleep(100);                        0.104483472
>                 ixgbe_get_san_mac_addr_generic      0.048414851
>                 ixgbe_get_wwn_prefix_generic        0.048409893
>               ixgbe_start_hw_X540
>                 ixgbe_start_hw_generic
>                   ixgbe_clear_hw_cntrs_generic      0.048581502
>                   ixgbe_setup_fc_generic            0.024225800
> 
>     All the ixgbe_*generic functions end-up calling:
>     ixgbe_read_eerd_X540()
>       ixgbe_acquire_swfw_sync_X540
>         usleep_range(5000, 6000);
>       ixgbe_release_swfw_sync_X540
>         usleep_range(5000, 6000);
> 
> While these are short sleeps, they end-up calling them over 24 times!
> 24 * 0.0055s = 0.132s. Adding-up to 0.528s for four devices.
> 
> While we should keep optimizing the individual device drivers, in some
> cases this is simply a hardware property that forces a specific delay, and
> we must wait.
> 
> So, the solution for this problem is to shutdown devices in parallel.
> However, we must shutdown children before shutting down parents, so parent
> device must wait for its children to finish.
> 
> With this patch, on the same machine devices_shutdown() takes 1.142s, and
> without mlx4 one second delay only 0.38s
> 
> Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
> ---
>  drivers/base/core.c | 238 +++++++++++++++++++++++++++++++++++---------
>  1 file changed, 189 insertions(+), 49 deletions(-)
> 
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index b610816eb887..f370369a303b 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -25,6 +25,7 @@
>  #include <linux/netdevice.h>
>  #include <linux/sched/signal.h>
>  #include <linux/sysfs.h>
> +#include <linux/kthread.h>
>  
>  #include "base.h"
>  #include "power/power.h"
> @@ -2102,6 +2103,59 @@ const char *device_get_devnode(struct device *dev,
>  	return *tmp = s;
>  }
>  
> +/**
> + * device_children_count - device children count
> + * @parent: parent struct device.
> + *
> + * Returns number of children for this device or 0 if nonde.
> + */
> +static int device_children_count(struct device *parent)
> +{
> +	struct klist_iter i;
> +	int children = 0;
> +
> +	if (!parent->p)
> +		return 0;
> +
> +	klist_iter_init(&parent->p->klist_children, &i);
> +	while (next_device(&i))
> +		children++;
> +	klist_iter_exit(&i);
> +
> +	return children;
> +}
> +
> +/**
> + * device_get_child_by_index - Return child using the provide index.
> + * @parent: parent struct device.
> + * @index:  Index of the child, where 0 is the first child in the children list,
> + * and so on.
> + *
> + * Returns child or NULL if child with this index is not present.
> + */
> +static struct device *
> +device_get_child_by_index(struct device *parent, int index)
> +{
> +	struct klist_iter i;
> +	struct device *dev = NULL, *d;
> +	int child_index = 0;
> +
> +	if (!parent->p || index < 0)
> +		return NULL;
> +
> +	klist_iter_init(&parent->p->klist_children, &i);
> +	while ((d = next_device(&i)) != NULL) {

perhaps:
	while ((d = next_device(&i))) {

> +		if (child_index == index) {
> +			dev = d;
> +			break;
> +		}
> +		child_index++;
> +	}
> +	klist_iter_exit(&i);
> +
> +	return dev;
> +}
> +
>  /**
>   * device_for_each_child - device child iterator.
>   * @parent: parent struct device.
> @@ -2765,71 +2819,157 @@ int device_move(struct device *dev, struct device *new_parent,
>  }
>  EXPORT_SYMBOL_GPL(device_move);
>  
> +/*
> + * device_shutdown_one - call ->shutdown() for the device passed as
> + * argument.
> + */
> +static void device_shutdown_one(struct device *dev)
> +{
> +	/* Don't allow any more runtime suspends */
> +	pm_runtime_get_noresume(dev);
> +	pm_runtime_barrier(dev);
> +
> +	if (dev->class && dev->class->shutdown_pre) {
> +		if (initcall_debug)
> +			dev_info(dev, "shutdown_pre\n");
> +		dev->class->shutdown_pre(dev);
> +	}
> +	if (dev->bus && dev->bus->shutdown) {
> +		if (initcall_debug)
> +			dev_info(dev, "shutdown\n");
> +		dev->bus->shutdown(dev);
> +	} else if (dev->driver && dev->driver->shutdown) {
> +		if (initcall_debug)
> +			dev_info(dev, "shutdown\n");
> +		dev->driver->shutdown(dev);
> +	}
> +
> +	/* Release device lock, and decrement the reference counter */
> +	device_unlock(dev);
> +	put_device(dev);
> +}
> +
> +static DECLARE_COMPLETION(device_root_tasks_complete);
> +static void device_shutdown_tree(struct device *dev);
> +static atomic_t device_root_tasks;
> +
> +/*
> + * Passed as an argument to to device_shutdown_task().
> + * child_next_index	the next available child index.
> + * tasks_running	number of tasks still running. Each tasks decrements it
> + *			when job is finished and the last tasks signals that the
> + *			job is complete.
> + * complete		Used to signal job competition.
> + * parent		Parent device.
> + */
> +struct device_shutdown_task_data {
> +	atomic_t		child_next_index;
> +	atomic_t		tasks_running;
> +	struct completion	complete;
> +	struct device		*parent;
> +};
> +
> +static int device_shutdown_task(void *data)
> +{
> +	struct device_shutdown_task_data *tdata =
> +		(struct device_shutdown_task_data *)data;

perhaps:
	struct device_shutdown_task_data *tdata = data;

> +	int child_idx = atomic_inc_return(&tdata->child_next_index) - 1;
> +	struct device *dev = device_get_child_by_index(tdata->parent,
> +						       child_idx);

perhaps:
	struct device *dev = device_get_child_by_index(tdata->parent, child_idx);

This is over the 80 character limit but only by one character :)

> +
> +	if (dev)
> +		device_shutdown_tree(dev);
> +	if (atomic_dec_return(&tdata->tasks_running) == 0)
> +		complete(&tdata->complete);
> +	return 0;
> +}
> +
> +/*
> + * Shutdown device tree with root started in dev. If dev has no children
> + * simply shutdown only this device. If dev has children recursively shutdown
> + * children first, and only then the parent. For performance reasons children
> + * are shutdown in parallel using kernel threads.
> + */
> +static void device_shutdown_tree(struct device *dev)
> +{
> +	int children_count = device_children_count(dev);
> +
> +	if (children_count) {
> +		struct device_shutdown_task_data tdata;
> +		int i;
> +
> +		init_completion(&tdata.complete);
> +		atomic_set(&tdata.child_next_index, 0);
> +		atomic_set(&tdata.tasks_running, children_count);
> +		tdata.parent = dev;
> +
> +		for (i = 0; i < children_count; i++) {
> +			kthread_run(device_shutdown_task,
> +				    &tdata, "device_shutdown.%s",
> +				    dev_name(dev));
> +		}
> +		wait_for_completion(&tdata.complete);
> +	}
> +	device_shutdown_one(dev);
> +}
> +
> +/*
> + * On shutdown each root device (the one that does not have a parent) goes
> + * through this function.
> + */
> +static int
> +device_shutdown_root_task(void *data)
> +{
> +	struct device *dev = (struct device *)data;
> +
> +	device_shutdown_tree(dev);
> +	if (atomic_dec_return(&device_root_tasks) == 0)
> +		complete(&device_root_tasks_complete);
> +	return 0;
> +}
> +
>  /**
>   * device_shutdown - call ->shutdown() on each device to shutdown.
>   */
>  void device_shutdown(void)
>  {
> -	struct device *dev, *parent;
> +	struct list_head *pos, *next;
> +	int root_devices = 0;
> +	struct device *dev;
>  
>  	spin_lock(&devices_kset->list_lock);
>  	/*
> -	 * Walk the devices list backward, shutting down each in turn.
> -	 * Beware that device unplug events may also start pulling
> -	 * devices offline, even as the system is shutting down.
> +	 * Prepare devices for shutdown: lock, and increment references in every
> +	 * devices. Remove child devices from the list, and count number of root

         * Prepare devices for shutdown: lock, and increment reference in each
         * device. Remove child devices from the list, and count number of root


Hope this helps,
Tobin.

Re: [PATCH 2/2] drivers core: multi-threading device shutdown

[Pavel Tatashin]

On Thu, May 3, 2018 at 1:54 AM Tobin C. Harding <tobin@apporbit.com> wrote:

> This code was a pleasure to read, super clean.


Hi Tobin,

Thank you very much for your review, I will address all of your comments in
the next revision.

BTW, I found  a lock ordering issue in my work that  that I will need to
fix:

In device_shutdown() device_lock() must be taken before
devices_kset->list_lock. Instead I will use device_trylock(), and if that
fails, will drop devices_kset->list_lock and acquiring the device lock
outside, and check that the device is still in the list after taking the
list lock again.

Pavel