Message-ID: <CAJZ5v0jPgNsQ+=c9Du=oksP-cpdtsRBc08DY8ekX-TOv=JP-pw@mail.gmail.com>
Date:   Mon, 21 Aug 2023 20:04:16 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Limonciello, Mario" <mario.limonciello@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Avadhut Naik <avadnaik@....com>,
        "Wilczynski, Michal" <michal.wilczynski@...el.com>,
        Avadhut Naik <avadhut.naik@....com>, lenb@...nel.org,
        linux-acpi@...r.kernel.org, yazen.ghannam@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ACPI: PHAT: Add Platform Health Assessment Table support

On Mon, Aug 21, 2023 at 8:00 PM Limonciello, Mario
<mario.limonciello@....com> wrote:
>
>
>
> On 8/21/2023 12:52 PM, Rafael J. Wysocki wrote:
> > On Mon, Aug 21, 2023 at 7:35 PM Limonciello, Mario
> > <mario.limonciello@....com> wrote:
> >>
> >>
> >>
> >> On 8/21/2023 12:29 PM, Rafael J. Wysocki wrote:
> >>> On Mon, Aug 21, 2023 at 7:17 PM Limonciello, Mario
> >>> <mario.limonciello@....com> wrote:
> >>>>
> >>>> On 8/21/2023 12:12 PM, Rafael J. Wysocki wrote:
> >>>> <snip>
> >>>>>> I was just talking to some colleagues about PHAT recently as well.
> >>>>>>
> >>>>>> The use case that jumps out is "system randomly rebooted while I was
> >>>>>> doing XYZ".  You don't know what happened, but you keep using your
> >>>>>> system.  Then it happens again.
> >>>>>>
> >>>>>> If the reason for the random reboot is captured to dmesg you can cross
> >>>>>> reference your journal from the next boot after any random reboot and
> >>>>>> get the reason for it.  If a user reports this to a Gitlab issue tracker
> >>>>>> or Bugzilla it can be helpful in establishing a pattern.
> >>>>>>
> >>>>>>>> The below location may be appropriate in that case:
> >>>>>>>> /sys/firmware/acpi/
> >>>>>>>
> >>>>>>> Yes, it may. >
> >>>>>>>> We already have FPDT and BGRT being exported from there.
> >>>>>>>
> >>>>>>> In fact, all of the ACPI tables can be retrieved verbatim from
> >>>>>>> /sys/firmware/acpi/tables/ already, so why exactly do you want the
> >>>>>>> kernel to parse PHAT in particular?
> >>>>>>>
> >>>>>>
> >>>>>> It's not to say that /sys/firmware/acpi/PHAT isn't useful, but having
> >>>>>> something internal to the kernel "automatically" parsing it and saving
> >>>>>> information to a place like the kernel log that is already captured by
> >>>>>> existing userspace tools I think is "more" useful.
> >>>>>
> >>>>> What existing user space tools do you mean?  Is there anything already
> >>>>> making use of the kernel's PHAT output?
> >>>>>
> >>>>
> >>>> I was meaning things like systemd already capture the kernel long
> >>>> ringbuffer.  If you save stuff like this into the kernel log, it's going
> >>>> to be indexed and easier to grep for boots that had it.
> >>>>
> >>>>> And why can't user space simply parse PHAT by itself?
> >>>>>    > There are multiple ACPI tables that could be dumped into the kernel
> >>>>> log, but they aren't.  Guess why.
> >>>>
> >>>> Right; there's not reason it can't be done by userspace directly.
> >>>>
> >>>> Another way to approach this problem could be to modify tools that
> >>>> excavate records from a reboot to also get PHAT.  For example
> >>>> systemd-pstore will get any kernel panics from the previous boot from
> >>>> the EFI pstore and put them into /var/lib/systemd/pstore.
> >>>>
> >>>> No reason that couldn't be done automatically for PHAT too.
> >>>
> >>> I'm not sure about the connection between the PHAT dump in the kernel
> >>> log and pstore.
> >>>
> >>> The PHAT dump would be from the time before the failure, so it is
> >>> unclear to me how useful it can be for diagnosing it.  However, after
> >>> a reboot one should be able to retrieve PHAT data from the table
> >>> directly and that may include some information regarding the failure.
> >>
> >> Right so the thought is that at bootup you get the last entry from PHAT
> >> and save that into the log.
> >>
> >> Let's say you have 3 boots:
> >> X - Triggered a random reboot
> >> Y - Cleanly shut down
> >> Z - Boot after a clean shut down
> >>
> >> So on boot Y you would have in your logs the reason that boot X rebooted.
> >
> > Yes, and the same can be retrieved from the PHAT directly from user
> > space at that time, can't it?
>
> Yes it can.
>
> >
> >> On boot Z you would see something about how boot Y's reason.
> >>
> >>>
> >>> With pstore, the assumption is that there will be some information
> >>> relevant for diagnosing the failure in the kernel buffer, but I'm not
> >>> sure how the PHAT dump from before the failure can help here?
> >>
> >> Alone it's not useful.
> >> I had figured if you can put it together with other data it's useful.
> >> For example if you had some thermal data in the logs showing which
> >> component overheated or if you looked at pstore and found a NULL pointer
> >> dereference.
> >
> > IIUC, the current PHAT content can be useful.  The PHAT content from
> > boot X (before the failure) which is what will be there in pstore
> > after the random reboot, is of limited value AFAICS.
>
> Right, you would need to have the pstore logs from your bad boot and
> then the dmesg from your current (good) boot to get the info.  And
> you're right at that point you could just run a userspace tool that gets
> the info instead.

And it will get the information from the source without any (arguably
redundant) intermediate processing (which may introduce noise into
it).

> I don't think any of this is necessary in the kernel, I just am
> describing the use case.
>
> FWIW on the patch series IMO I think that the boots that don't show
> useful unexpected things (power button, cold boot, warm boot, cold
> reset) shouldn't be INFO either.  I think these should default to debug,
> and just the unexpected ones should show up.

I would still prefer user space to deal with this as it sees fit.

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