Message-ID: <c1f065ec-c544-4191-85b5-e1bb01808183@siemens.com>
Date: Wed, 11 Sep 2024 07:51:31 +0200
From: Jan Kiszka <jan.kiszka@...mens.com>
To: stuart hayes <stuart.w.hayes@...il.com>, linux-kernel@...r.kernel.org,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Rafael J . Wysocki" <rafael@...nel.org>,
 Martin Belanger <Martin.Belanger@...l.com>,
 Oliver O'Halloran <oohall@...il.com>, Daniel Wagner <dwagner@...e.de>,
 Keith Busch <kbusch@...nel.org>, Lukas Wunner <lukas@...ner.de>,
 David Jeffery <djeffery@...hat.com>, Jeremy Allison <jallison@....com>,
 Jens Axboe <axboe@...com>, Christoph Hellwig <hch@....de>,
 Sagi Grimberg <sagi@...mberg.me>, linux-nvme@...ts.infradead.org
Cc: Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH v8 3/4] driver core: shut down devices asynchronously

On 11.09.24 02:14, stuart hayes wrote:
> 
> 
> On 9/8/2024 8:36 AM, Jan Kiszka wrote:
>> On 22.08.24 22:28, Stuart Hayes wrote:
>>> Add code to allow asynchronous shutdown of devices, ensuring that each
>>> device is shut down before its parents & suppliers.
>>>
>>> Only devices with drivers that have async_shutdown_enable enabled
>>> will be
>>> shut down asynchronously.
>>>
>>> This can dramatically reduce system shutdown/reboot time on systems that
>>> have multiple devices that take many seconds to shut down (like certain
>>> NVMe drives). On one system tested, the shutdown time went from 11
>>> minutes
>>> without this patch to 55 seconds with the patch.
>>>
>>> Signed-off-by: Stuart Hayes <stuart.w.hayes@...il.com>
>>> Signed-off-by: David Jeffery <djeffery@...hat.com>
>>> ---
>>>   drivers/base/base.h           |  4 +++
>>>   drivers/base/core.c           | 54 ++++++++++++++++++++++++++++++++++-
>>>   include/linux/device/driver.h |  2 ++
>>>   3 files changed, 59 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/base/base.h b/drivers/base/base.h
>>> index 0b53593372d7..aa5a2bd3f2b8 100644
>>> --- a/drivers/base/base.h
>>> +++ b/drivers/base/base.h
>>> @@ -10,6 +10,7 @@
>>>    * shared outside of the drivers/base/ directory.
>>>    *
>>>    */
>>> +#include <linux/async.h>
>>>   #include <linux/notifier.h>
>>>     /**
>>> @@ -97,6 +98,8 @@ struct driver_private {
>>>    *    the device; typically because it depends on another driver
>>> getting
>>>    *    probed first.
>>>    * @async_driver - pointer to device driver awaiting probe via
>>> async_probe
>>> + * @shutdown_after - used during device shutdown to ensure correct
>>> shutdown
>>> + *    ordering.
>>>    * @device - pointer back to the struct device that this structure is
>>>    * associated with.
>>>    * @dead - This device is currently either in the process of or has
>>> been
>>> @@ -114,6 +117,7 @@ struct device_private {
>>>       struct list_head deferred_probe;
>>>       const struct device_driver *async_driver;
>>>       char *deferred_probe_reason;
>>> +    async_cookie_t shutdown_after;
>>>       struct device *device;
>>>       u8 dead:1;
>>>   };
>>> diff --git a/drivers/base/core.c b/drivers/base/core.c
>>> index 7e50daa65ca0..dd3652ea56fe 100644
>>> --- a/drivers/base/core.c
>>> +++ b/drivers/base/core.c
>>> @@ -9,6 +9,7 @@
>>>    */
>>>     #include <linux/acpi.h>
>>> +#include <linux/async.h>
>>>   #include <linux/cpufreq.h>
>>>   #include <linux/device.h>
>>>   #include <linux/err.h>
>>> @@ -3531,6 +3532,7 @@ static int device_private_init(struct device *dev)
>>>       klist_init(&dev->p->klist_children, klist_children_get,
>>>              klist_children_put);
>>>       INIT_LIST_HEAD(&dev->p->deferred_probe);
>>> +    dev->p->shutdown_after = 0;
>>>       return 0;
>>>   }
>>>   @@ -4781,6 +4783,8 @@ int device_change_owner(struct device *dev,
>>> kuid_t kuid, kgid_t kgid)
>>>   }
>>>   EXPORT_SYMBOL_GPL(device_change_owner);
>>>   +static ASYNC_DOMAIN(sd_domain);
>>> +
>>>   static void shutdown_one_device(struct device *dev)
>>>   {
>>>       /* hold lock to avoid race with probe/release */
>>> @@ -4816,12 +4820,34 @@ static void shutdown_one_device(struct device
>>> *dev)
>>>           put_device(dev->parent);
>>>   }
>>>   +/**
>>> + * shutdown_one_device_async
>>> + * @data: the pointer to the struct device to be shutdown
>>> + * @cookie: not used
>>> + *
>>> + * Shuts down one device, after waiting for shutdown_after to complete.
>>> + * shutdown_after should be set to the cookie of the last child or
>>> consumer
>>> + * of this device to be shutdown (if any), or to the cookie of the
>>> previous
>>> + * device to be shut down for devices that don't enable asynchronous
>>> shutdown.
>>> + */
>>> +static void shutdown_one_device_async(void *data, async_cookie_t
>>> cookie)
>>> +{
>>> +    struct device *dev = data;
>>> +
>>> +    async_synchronize_cookie_domain(dev->p->shutdown_after + 1,
>>> &sd_domain);
>>> +
>>> +    shutdown_one_device(dev);
>>> +}
>>> +
>>>   /**
>>>    * device_shutdown - call ->shutdown() on each device to shutdown.
>>>    */
>>>   void device_shutdown(void)
>>>   {
>>>       struct device *dev, *parent;
>>> +    async_cookie_t cookie = 0;
>>> +    struct device_link *link;
>>> +    int idx;
>>>         wait_for_device_probe();
>>>       device_block_probing();
>>> @@ -4852,11 +4878,37 @@ void device_shutdown(void)
>>>           list_del_init(&dev->kobj.entry);
>>>           spin_unlock(&devices_kset->list_lock);
>>>   -        shutdown_one_device(dev);
>>> +
>>> +        /*
>>> +         * Set cookie for devices that will be shut down synchronously
>>> +         */
>>> +        if (!dev->driver || !dev->driver->async_shutdown_enable)
>>> +            dev->p->shutdown_after = cookie;
>>> +
>>> +        get_device(dev);
>>> +        get_device(parent);
>>> +
>>> +        cookie = async_schedule_domain(shutdown_one_device_async,
>>> +                           dev, &sd_domain);
>>> +        /*
>>> +         * Ensure parent & suppliers wait for this device to shut down
>>> +         */
>>> +        if (parent) {
>>> +            parent->p->shutdown_after = cookie;
>>> +            put_device(parent);
>>> +        }
>>> +
>>> +        idx = device_links_read_lock();
>>> +        list_for_each_entry_rcu(link, &dev->links.suppliers, c_node,
>>> +                device_links_read_lock_held())
>>> +            link->supplier->p->shutdown_after = cookie;
>>
>> This will not fly if a supplier registered after its consumer. As we are
>> walking the list backward, the supplier will now wait for something that
>> is coming later during shutdown if the affected devices are still doing
>> this synchronously (as almost all at this stage). This creates a
>> circular dependency.
>>
>> Seems to explain the reboot hang that I'm seeing on an embedded target
>> with a mailbox dev waiting for a remoteproc dev while the mailbox being
>> after the remoteproc in the list (thus first on shutting down).
>>
>> This resolves the issue for me so far, but I don't think we are done yet:
>>
>> list_for_each_entry_rcu(link, &dev->links.suppliers, c_node,
>>         device_links_read_lock_held()) {
>>     if (link->supplier->driver &&
>>         link->supplier->driver->async_shutdown_enable)
>>         link->supplier->p->shutdown_after = cookie;
>> }
>>
>> I wonder if overwriting the supplier's shutdown_after unconditionally is
>> a good idea. A supplier may have multiple consumers - will we overwrite
>> in the right order then? And why do we now need this ordering when we
>> were so far shutting down suppliers while consumers were still up?
>>
> 
> The devices_kset list should already be in the right order for shutting
> stuff down--i.e., parents and suppliers should be shutdown later as the
> device_shutdown loop goes through the devices.
> 
> With async shutdown this loop still goes the devices_kset list in the same
> order it did before the patch, so my expectation was that any
> parents/suppliers
> would come later in the loop than any children/siblings, and it would
> update
> shutdown_after as it went to ensure that each device ended up with the
> cookie
> of the last child/consumer that it needed to wait for.
> 
> However, I missed that the devices_kset list isn't always reordered when a
> devlink is added and a consumer isn't dependent on the supplier (see
> device_is_dependent()).  I have a patch would address that, and add a
> sanity
> check in case any devices get in the list in the wrong order somehow:
> 
> 
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index b69b82da8837..52d64b419c01 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -4832,6 +4832,13 @@ static void shutdown_one_device_async(void *data,
> async_cookie_t cookie)
>  {
>      struct device *dev = data;
>  
> +    /*
> +     * Sanity check to prevent shutdown hang in case a parent or supplier
> +     * is in devices_kset list in the wrong order
> +     */
> +    if (dev->p->shutdown_after > cookie)
> +        dev->p->shutdown_after = cookie - 1;
> +
>      async_synchronize_cookie_domain(dev->p->shutdown_after + 1,
> &sd_domain);
>  
>      shutdown_one_device(dev);
> @@ -4898,8 +4905,11 @@ void device_shutdown(void)
>  
>          idx = device_links_read_lock();
>          list_for_each_entry_rcu(link, &dev->links.suppliers, c_node,
> -                device_links_read_lock_held())
> +                device_links_read_lock_held()) {
> +            if (device_link_flag_is_sync_state_only(link->flags))
> +                continue;
>              link->supplier->p->shutdown_after = cookie;
> +        }
>          device_links_read_unlock(idx);
>          put_device(dev);
>  
> 
> I'll submit this shortly if nobody responds with any issues with this.
> 
> Thank you!
> 

This sounds widely reasonable to me, and a quick check confirmed that it
apparently resolves the issue I was seeing.

I'm still wondering, though, if overwriting the parent's shutdown_after
and only checking later on in shutdown_one_device_async is sufficient or
if it wouldn't be safer to have a check when we write. The fact that
there could be multiple children for a parent is worrying me.

Jan

> 
>> Same overwrite question applies to setting shutdown_after in parents.
>> Don't we rather need a list for shutdown_after, at least once everything
>> is async?
>>
>> This needs to be thought through once more, I guess.
>>
>> Jan
>>
>>> +        device_links_read_unlock(idx);
>>> +        put_device(dev);
>>>             spin_lock(&devices_kset->list_lock);
>>>       }
>>>       spin_unlock(&devices_kset->list_lock);
>>> +    async_synchronize_full_domain(&sd_domain);
>>>   }
>>>     /*
>>> diff --git a/include/linux/device/driver.h
>>> b/include/linux/device/driver.h
>>> index 1fc8b68786de..2b6127faaa25 100644
>>> --- a/include/linux/device/driver.h
>>> +++ b/include/linux/device/driver.h
>>> @@ -56,6 +56,7 @@ enum probe_type {
>>>    * @mod_name:    Used for built-in modules.
>>>    * @suppress_bind_attrs: Disables bind/unbind via sysfs.
>>>    * @probe_type:    Type of the probe (synchronous or asynchronous)
>>> to use.
>>> + * @async_shutdown_enable: Enables devices to be shutdown
>>> asynchronously.
>>>    * @of_match_table: The open firmware table.
>>>    * @acpi_match_table: The ACPI match table.
>>>    * @probe:    Called to query the existence of a specific device,
>>> @@ -102,6 +103,7 @@ struct device_driver {
>>>         bool suppress_bind_attrs;    /* disables bind/unbind via
>>> sysfs */
>>>       enum probe_type probe_type;
>>> +    bool async_shutdown_enable;
>>>         const struct of_device_id    *of_match_table;
>>>       const struct acpi_device_id    *acpi_match_table;
>>

-- 
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