linux-kernel - Re: [PATCH v4] PCI: hotplug: Add a generic RAS tracepoint for hotplug event

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Message-ID: <20250108175906.GA219807@bhelgaas>
Date: Wed, 8 Jan 2025 11:59:06 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Shuai Xue <xueshuai@...ux.alibaba.com>
Cc: rostedt@...dmis.org, lukas@...ner.de, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-edac@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, bhelgaas@...gle.com,
	tony.luck@...el.com, bp@...en8.de, mhiramat@...nel.org,
	mathieu.desnoyers@...icios.com, oleg@...hat.com, naveen@...nel.org,
	davem@...emloft.net, anil.s.keshavamurthy@...el.com,
	mark.rutland@....com, peterz@...radead.org
Subject: Re: [PATCH v4] PCI: hotplug: Add a generic RAS tracepoint for
 hotplug event

On Wed, Jan 08, 2025 at 05:04:25PM +0800, Shuai Xue wrote:
> 在 2025/1/8 07:19, Bjorn Helgaas 写道:
> > On Sat, Nov 23, 2024 at 07:31:08PM +0800, Shuai Xue wrote:
> > > Hotplug events are critical indicators for analyzing hardware health,
> > > particularly in AI supercomputers where surprise link downs can
> > > significantly impact system performance and reliability. The failure
> > > characterization analysis illustrates the significance of failures
> > > caused by the Infiniband link errors. Meta observes that 2% in a machine
> > > learning cluster and 6% in a vision application cluster of Infiniband
> > > failures co-occur with GPU failures, such as falling off the bus, which
> > > may indicate a correlation with PCIe.[1]
> > > 
> > > To this end, define a new TRACING_SYSTEM named pci, add a generic RAS
> > > tracepoint for hotplug event to help healthy check, and generate
> > > tracepoints for pcie hotplug event. To monitor these tracepoints in
> > > userspace, e.g. with rasdaemon, put `enum pci_hotplug_event` in uapi
> > > header.
> > > 
> > > The output like below:
> > > $ echo 1 > /sys/kernel/debug/tracing/events/pci/pci_hp_event/enable
> > > $ cat /sys/kernel/debug/tracing/trace_pipe
> > >             <...>-206     [001] .....    40.373870: pci_hp_event: 0000:00:02.0 slot:10, event:Link Down
> > > 
> > >             <...>-206     [001] .....    40.374871: pci_hp_event: 0000:00:02.0 slot:10, event:Card not present
> > > 
> > > [1]https://arxiv.org/abs/2410.21680
> > 
> > Doesn't apply on pci/main (v6.13-rc1); can you rebase it?
> 
> Sure. Do you mean Git repository at:
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/pci/pci.git
>   branch main

Yes.  The most recent -rc1 is generally a safe bet for basing patches.

> > Probably more detail than necessary about AI supercomputers,
> > Infiniband, vision applications, etc.  This is a very generic issue.
> 
> Agreed. It is generic. Are you asking for the first background paragraph to be
> deleted?

I think the important part is that hotplug and link down events are
critical indicators of hardware health.  That's enough to motivate
this patch.

> > "Falling off the bus" doesn't really mean anything to me.  I suppose
> > it's another way to describe a "link down" event that leads to UR
> > errors when trying to access the device?
> 
> Sorry for the confusion. "Falling off the bus" is a common error for
> NVIDIA GPU observed in production. The GPU driver will log a such
> message when GPU is not accessible.

Yep, I see those too, and I wish the message weren't phrased so
casually.  IIRC this is typically logged when an MMIO read returns ~0,
which happens when a UR or similar error occurs.

> > I'm guessing that monitoring these via rasdaemon requires more than
> > just adding "enum pci_hotplug_event"?  Or does rasdaemon read
> > include/uapi/linux/pci.h and automagically incorporate new events?
> > Maybe there's at least a rebuild involved?
> 
> Yes, a rebuild is needed. Rasdaemon has a basic infrastructure to manually
> register a tracepoint event handler. For example, for this new event, we can
> register to handle pci_hp_event:
> 
>     rc = add_event_handler(ras, pevent, page_size, "pci", "pci_hp_event",
> 			   ras_pci_hp_event_handler, NULL, PCI_HOTPLUG_EVENT);

I would say something like "Add enum pci_hotplug_event in
include/uapi/linux/pci.h so applications like rasdaemon can register
tracepoint event handlers for it."

Bjorn