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Date: Mon, 16 Nov 2015 13:34:23 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Bjorn Helgaas <bhelgaas@...gle.com>
Cc: Yinghai Lu <yinghai@...nel.org>, linux-pci@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] PCI: pciehp: Merge hotplug work requests
Hi folks,
any comments on this series ?
Thanks,
Guenter
On Mon, Nov 02, 2015 at 07:48:15PM -0800, Guenter Roeck wrote:
> Some oddball devices may experience a PCIe link flap after power-on.
> This may result in the following sequence of events.
>
> fpc0 kernel: pciehp 0000:02:08.0:pcie24: Card present on Slot(0)
> fpc0 kernel: pciehp 0000:02:08.0:pcie24: slot(0): Link Up event
> fpc0 kernel: pciehp 0000:02:08.0:pcie24:
> Link Up event ignored on slot(0): already powering on
> fpc0 kernel: pciehp 0000:02:08.0:pcie24: slot(0): Link Down event
> fpc0 kernel: pciehp 0000:02:08.0:pcie24:
> Link Down event queued on slot(0): currently getting powered on
> fpc0 kernel: pciehp 0000:02:08.0:pcie24: slot(0): Link Up event
> fpc0 kernel: pciehp 0000:02:08.0:pcie24:
> Link Up event queued on slot(0): currently getting powered off
>
> This causes the driver for affected devices to be instantiated, removed,
> and re-instantiated.
>
> An even worse problem is a device which resets itself continuously.
> This can result in the following endless sequence of messages.
>
> pciehp 0000:02:0a.0:pcie24: Card present on Slot(10)
> pciehp 0000:02:0a.0:pcie24: Card not present on Slot(10)
> pciehp 0000:02:0a.0:pcie24: Card present on Slot(10)
> pciehp 0000:02:0a.0:pcie24: Card not present on Slot(10)
> pciehp 0000:02:0a.0:pcie24: Card present on Slot(10)
> pciehp 0000:02:0a.0:pcie24: Card not present on Slot(10)
>
> The problem in the both cases is that all events are enqueued as hotplug
> work requests and executed in sequence, which can overwhelm the system
> and even result in "hung task" tracebacks in pciehp_power_thread().
>
> This exposes an underlying limitation of the hotplug state machine: It
> executes all received requests, even though only the most recent request
> really needs to be handled. As a result, by the time a request is handled,
> it may well be obsolete and have been superseded by many other enable /
> disable requests which have been enqueued in the meantime.
>
> To solve the problem, fold hotplug work requests into a single request.
> Store the request as well as the work data structure in 'struct slot',
> thus eliminating the need to allocate memory for each request.
> Handle a sequence of requests by setting a 'disable' flag when needed,
> indicating that a link needs to be disabled prior to re-enabling it.
>
> With this change, requests and request sequences are handled as follows.
>
> enable (single request): enable link
> disable (single request): disable link
> ... disable-enable-disable...disable: disable link
> ... disable-enable-disable...enable: disable link, then enable it
>
> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> ---
> This is a different approach to solve the problem I tried to address
> earlier with with "PCI: pciehp: Add support for delayed power-on" [1].
>
> While the earlier patch implemented an additional state in the hotplug
> state machine to solve the problem, the approach taken here is a bit
> simpler and more straightfoward. By folding multiple requests into one,
> the follow-up patch can use delayed work to implement power-on delays.
> An additional advantage is that this patch can be applied separately
> to simplify the state machine.
>
> While working on this patch, I also tried to drop multiple "disable"
> requests, and only disable a slot if it was already disabled, to reduce
> overhead. Unfortunately, this did not always work. In some instances,
> I ended up in a situation where a device could not be enabled
> because the system thought that it already existed. I don't know
> if this is a weakness in this patch or some state change I did not catch.
> This may be left for further study.
>
> [1] https://lkml.org/lkml/2015/10/12/686
>
> drivers/pci/hotplug/pciehp.h | 4 +++
> drivers/pci/hotplug/pciehp_ctrl.c | 52 ++++++++++++++++++---------------------
> drivers/pci/hotplug/pciehp_hpc.c | 1 +
> 3 files changed, 29 insertions(+), 28 deletions(-)
>
> diff --git a/drivers/pci/hotplug/pciehp.h b/drivers/pci/hotplug/pciehp.h
> index 62d6fe6c3714..364b6fa32978 100644
> --- a/drivers/pci/hotplug/pciehp.h
> +++ b/drivers/pci/hotplug/pciehp.h
> @@ -78,6 +78,9 @@ struct slot {
> struct mutex lock;
> struct mutex hotplug_lock;
> struct workqueue_struct *wq;
> + struct work_struct hotplug_work;
> + u32 hotplug_req;
> + bool disable; /* true to disable before enable */
> };
>
> struct event_info {
> @@ -130,6 +133,7 @@ void pciehp_queue_interrupt_event(struct slot *slot, u32 event_type);
> int pciehp_configure_device(struct slot *p_slot);
> int pciehp_unconfigure_device(struct slot *p_slot);
> void pciehp_queue_pushbutton_work(struct work_struct *work);
> +void pciehp_power_thread(struct work_struct *work);
> struct controller *pcie_init(struct pcie_device *dev);
> int pcie_init_notification(struct controller *ctrl);
> int pciehp_enable_slot(struct slot *p_slot);
> diff --git a/drivers/pci/hotplug/pciehp_ctrl.c b/drivers/pci/hotplug/pciehp_ctrl.c
> index 880978b6d534..ad1321e91546 100644
> --- a/drivers/pci/hotplug/pciehp_ctrl.c
> +++ b/drivers/pci/hotplug/pciehp_ctrl.c
> @@ -156,13 +156,9 @@ static int remove_board(struct slot *p_slot)
> return 0;
> }
>
> -struct power_work_info {
> - struct slot *p_slot;
> - struct work_struct work;
> - unsigned int req;
> -#define DISABLE_REQ 0
> -#define ENABLE_REQ 1
> -};
> +/* Hotplug work requests */
> +#define DISABLE_REQ 0
> +#define ENABLE_REQ 1
>
> /**
> * pciehp_power_thread - handle pushbutton events
> @@ -171,14 +167,19 @@ struct power_work_info {
> * Scheduled procedure to handle blocking stuff for the pushbuttons.
> * Handles all pending events and exits.
> */
> -static void pciehp_power_thread(struct work_struct *work)
> +void pciehp_power_thread(struct work_struct *work)
> {
> - struct power_work_info *info =
> - container_of(work, struct power_work_info, work);
> - struct slot *p_slot = info->p_slot;
> - int ret;
> + struct slot *p_slot = container_of(work, struct slot, hotplug_work);
> + int ret, req;
> + bool disable;
> +
> + mutex_lock(&p_slot->lock);
> + req = p_slot->hotplug_req;
> + disable = p_slot->disable;
> + p_slot->disable = false;
> + mutex_unlock(&p_slot->lock);
>
> - switch (info->req) {
> + switch (req) {
> case DISABLE_REQ:
> mutex_lock(&p_slot->hotplug_lock);
> pciehp_disable_slot(p_slot);
> @@ -189,6 +190,8 @@ static void pciehp_power_thread(struct work_struct *work)
> break;
> case ENABLE_REQ:
> mutex_lock(&p_slot->hotplug_lock);
> + if (disable)
> + pciehp_disable_slot(p_slot);
> ret = pciehp_enable_slot(p_slot);
> mutex_unlock(&p_slot->hotplug_lock);
> if (ret)
> @@ -200,26 +203,19 @@ static void pciehp_power_thread(struct work_struct *work)
> default:
> break;
> }
> -
> - kfree(info);
> }
>
> static void pciehp_queue_power_work(struct slot *p_slot, int req)
> {
> - struct power_work_info *info;
> -
> - p_slot->state = (req == ENABLE_REQ) ? POWERON_STATE : POWEROFF_STATE;
> -
> - info = kmalloc(sizeof(*info), GFP_KERNEL);
> - if (!info) {
> - ctrl_err(p_slot->ctrl, "no memory to queue %s request\n",
> - (req == ENABLE_REQ) ? "poweron" : "poweroff");
> - return;
> + if (req == ENABLE_REQ) {
> + p_slot->state = POWERON_STATE;
> + } else {
> + p_slot->state = POWEROFF_STATE;
> + p_slot->disable = true;
> }
> - info->p_slot = p_slot;
> - INIT_WORK(&info->work, pciehp_power_thread);
> - info->req = req;
> - queue_work(p_slot->wq, &info->work);
> + p_slot->hotplug_req = req;
> +
> + queue_work(p_slot->wq, &p_slot->hotplug_work);
> }
>
> void pciehp_queue_pushbutton_work(struct work_struct *work)
> diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
> index 5c24e938042f..e4e6fcbe1e20 100644
> --- a/drivers/pci/hotplug/pciehp_hpc.c
> +++ b/drivers/pci/hotplug/pciehp_hpc.c
> @@ -755,6 +755,7 @@ static int pcie_init_slot(struct controller *ctrl)
> mutex_init(&slot->lock);
> mutex_init(&slot->hotplug_lock);
> INIT_DELAYED_WORK(&slot->work, pciehp_queue_pushbutton_work);
> + INIT_WORK(&slot->hotplug_work, pciehp_power_thread);
> ctrl->slot = slot;
> return 0;
> abort:
> --
> 2.1.4
>
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