linux-kernel - Re: [PATCH v2] PCI/MSI: Avoid torn updates to MSI pairs

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Date:   Thu, 23 Jan 2020 14:59:25 -0800
From:   Evan Green <evgreen@...omium.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Rajat Jain <rajatja@...gle.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] PCI/MSI: Avoid torn updates to MSI pairs

On Thu, Jan 23, 2020 at 12:59 PM Evan Green <evgreen@...omium.org> wrote:
>
> On Thu, Jan 23, 2020 at 10:17 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > Evan,
> >
> > Thomas Gleixner <tglx@...utronix.de> writes:
> > > This is not yet debugged fully and as this is happening on MSI-X I'm not
> > > really convinced yet that your 'torn write' theory holds.
> >
> > can you please apply the debug patch below and run your test. When the
> > failure happens, stop the tracer and collect the trace.
> >
> > Another question. Did you ever try to change the affinity of that
> > interrupt without hotplug rapidly while the device makes traffic? If
> > not, it would be interesting whether this leads to a failure as well.
>
> Thanks for the patch. Looks pretty familiar :)
> I ran into issues where trace_printks on offlined cores seem to
> disappear. I even made sure the cores were back online when I
> collected the trace. So your logs might not be useful. Known issue
> with the tracer?
>
> I figured I'd share my own debug chicken scratch, in case you could
> glean anything from it. The LOG entries print out timestamps (divide
> by 1000000) that you can match up back to earlier in the log (ie so
> the last XHCI MSI change occurred at 74.032501, the last interrupt
> came in at 74.032405). Forgive the mess.
>
> I also tried changing the affinity rapidly without CPU hotplug, but
> didn't see the issue, at least not in the few minutes I waited
> (normally repros easily within 1 minute). An interesting datapoint.

One additional datapoint. The intel guys suggested enabling
CONFIG_IRQ_REMAP, which does seem to eliminate the issue for me. I'm
still hoping there's a smaller fix so I don't have to add all that in.
-Evan