Re: [PATCH v9 2/2] PCI: trace: Add a RAS tracepoint to monitor link speed changes

[Shuai Xue <xueshuai@linux.alibaba.com>]

在 2025/7/26 05:09, Bjorn Helgaas 写道:
> On Wed, Jul 23, 2025 at 11:31:08AM +0800, Shuai Xue wrote:
>> PCIe link speed degradation directly impacts system performance and
>> often indicates hardware issues such as faulty devices, physical layer
>> problems, or configuration errors.
>>
>> To this end, add a RAS tracepoint to monitor link speed changes,
>> enabling proactive health checks and diagnostic analysis.
>>
>> The output is like below:
>>
>> $ echo 1 > /sys/kernel/debug/tracing/events/pci/pci_link_event/enable
>> $ cat /sys/kernel/debug/tracing/trace_pipe
>> cat /sys/kernel/debug/tracing/trace_pipe
>>             <...>-119     [002] .....   125.776171: pci_hp_event: 0000:00:03.0 slot:30, event:CARD_PRESENT
>>
>>             <...>-119     [002] .....   125.776197: pci_hp_event: 0000:00:03.0 slot:30, event:LINK_UP
>>
>>     irq/57-pciehp-119     [002] .....   125.904335: pcie_link_event: 0000:00:03.0 type:4, reason:4, cur_bus_speed:2.5 GT/s PCIe, max_bus_speed:16.0 GT/s PCIe, width:1, flit_mode:0, status:DLLLA
>>
>>     irq/57-pciehp-119     [002] .....   125.907051: pcie_link_event: 0000:00:03.0 type:4, reason:0, cur_bus_speed:2.5 GT/s PCIe, max_bus_speed:16.0 GT/s PCIe, width:1, flit_mode:0, status:DLLLA
> 
> I guess this example would actually require both of these enables, right?
> 
>    echo 1 > /sys/kernel/debug/tracing/events/pci/pci_hp_event/enable
>    echo 1 > /sys/kernel/debug/tracing/events/pci/pci_link_event/enable

Yes, you're absolutely right. I'll correct the commit log to show both
commands.

(As a side note, echo 1 > /sys/kernel/debug/tracing/events/pci/enable
would enable both events at once.)

(echo 1 > /sys/kernel/debug/tracing/events/pci/enable will enable both
of these event.)
> 
>> Suggested-by: Bjorn Helgaas <helgaas@kernel.org>
> 
> I don't think I've suggested anything that really warrants this ;)
> 

Fair enough! I'll drop the Suggested-by tag.

>> ...
>> @@ -292,7 +292,7 @@ int pciehp_check_link_status(struct controller *ctrl)
>>   {
>>   	struct pci_dev *pdev = ctrl_dev(ctrl);
>>   	bool found;
>> -	u16 lnk_status, linksta2;
>> +	u16 lnk_status;
>>   
>>   	if (!pcie_wait_for_link(pdev, true)) {
>>   		ctrl_info(ctrl, "Slot(%s): No link\n", slot_name(ctrl));
>> @@ -319,8 +319,7 @@ int pciehp_check_link_status(struct controller *ctrl)
>>   		return -1;
>>   	}
>>   
>> -	pcie_capability_read_word(pdev, PCI_EXP_LNKSTA2, &linksta2);
>> -	__pcie_update_link_speed(ctrl->pcie->port->subordinate, lnk_status, linksta2);
>> +	pcie_update_link_speed(ctrl->pcie->port->subordinate, PCIE_HOTPLUG);
> 
> It kind of bugs me that the hot-add flow reads LNKSTA three times and
> generates both pci_hp_event LINK_UP and link_event tracepoints:
> 
>    pciehp_handle_presence_or_link_change
>      link_active = pciehp_check_link_active()
>        pcie_capability_read_word(PCI_EXP_LNKSTA)
>      if (link_active)
>        ctrl_info(ctrl, "Slot(%s): Link Up\n")
>        trace_pci_hp_event(PCI_HOTPLUG_LINK_UP)
>        pciehp_enable_slot
>          __pciehp_enable_slot
>            board_added
>              pciehp_check_link_status
>                pcie_capability_read_word(PCI_EXP_LNKSTA)
>                pcie_update_link_speed
>                  pcie_capability_read_word(PCI_EXP_LNKSTA)
>                  pcie_capability_read_word(PCI_EXP_LNKSTA2)
>                  trace_pcie_link_event(<REASON>)
> 
> Maybe there are good reasons for reading LNKSTA three times, but it
> does make me raise my eyebrows.  Not that this is a performance path,
> but it just offends my sense of propriety.
> 
> And maybe we need both a bare LINK_UP event and a link_event with all
> the details, but again it seems a little weird to me that there are
> two tracepoints when there's really only one event and we know all the
> link_event information from the very first LNKSTA read.
> 
> Bjorn

I understand your concern about the multiple LNKSTA reads and the
apparent duplication. Please see comments from Lukas.

Best Regards,
Shuai