Date:   Tue, 25 Apr 2023 08:26:14 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Chen-Yu Tsai <wenst@...omium.org>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
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        linux-kernel@...r.kernel.org, linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH] hardlockup: detect hard lockups using secondary (buddy) cpus

Hi,

On Mon, Apr 24, 2023 at 9:58 PM Chen-Yu Tsai <wenst@...omium.org> wrote:
>
> On Mon, Apr 24, 2023 at 11:42 PM Doug Anderson <dianders@...omium.org> wrote:
> >
> > Hi,
> >
> > On Mon, Apr 24, 2023 at 5:54 AM Daniel Thompson
> > <daniel.thompson@...aro.org> wrote:
> > >
> > > On Fri, Apr 21, 2023 at 03:53:30PM -0700, Douglas Anderson wrote:
> > > > From: Colin Cross <ccross@...roid.com>
> > > >
> > > > Implement a hardlockup detector that can be enabled on SMP systems
> > > > that don't have an arch provided one or one implemented atop perf by
> > > > using interrupts on other cpus. Each cpu will use its softlockup
> > > > hrtimer to check that the next cpu is processing hrtimer interrupts by
> > > > verifying that a counter is increasing.
> > > >
> > > > NOTE: unlike the other hard lockup detectors, the buddy one can't
> > > > easily provide a backtrace on the CPU that locked up. It relies on
> > > > some other mechanism in the system to get information about the locked
> > > > up CPUs. This could be support for NMI backtraces like [1], it could
> > > > be a mechanism for printing the PC of locked CPUs like [2], or it
> > > > could be something else.
> > > >
> > > > This style of hardlockup detector originated in some downstream
> > > > Android trees and has been rebased on / carried in ChromeOS trees for
> > > > quite a long time for use on arm and arm64 boards. Historically on
> > > > these boards we've leveraged mechanism [2] to get information about
> > > > hung CPUs, but we could move to [1].
> > >
> > > On the Arm platforms is this code able to leverage the existing
> > > infrastructure to extract status from stuck CPUs:
> > > https://docs.kernel.org/trace/coresight/coresight-cpu-debug.html
> >
> > Yup! I wasn't explicit about this, but that's where you end up if you
> > follow the whole bug tracker item that was linked as [2].
> > Specifically, we used to have downstream patches in the ChromeOS that
> > just reached into the coresight range from a SoC specific driver and
> > printed out the CPU_DBGPCSR. When Brian was uprevving rk3399
> > Chromebooks he found that the equivalent functionality had made it
> > upstream in a generic way through the coresight framework. Brian
> > confirmed it was working on rk3399 and made all of the device tree
> > changes needed to get it all hooked up, so (at least for that SoC) it
> > should work on that SoC.
> >
> > [2] https://issuetracker.google.com/172213129
>
> IIRC with the coresight CPU debug driver enabled and the proper DT nodes
> added, the panic handler does dump out information from the hardware.
> I don't think it's wired up for hung tasks though.

Yes, that's correct. The coresight CPU debug driver doesn't work for
hung tasks because it can't get a real stack crawl. All it can get is
the PC of the last branch that the CPU took. This is why combining
${SUBJECT} patch with the ability to get stack traces via pseudo-NMI
is superior. That being said, even with just the coresight CPU debug
driver ${SUBJECT} patch is still helpful because (assuming
"hardlockup_panic" is set) we'll do a panic which will then trigger
the coresight CPU debug driver. :-)

-Doug

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