Message-ID: <CAJZ5v0jxKwtHM3vhtw44TW5=J916XvXSeP3DyNLcJDo+eTSRHw@mail.gmail.com>
Date:   Mon, 21 Aug 2023 20:49:23 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Yazen Ghannam <yazen.ghannam@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Limonciello, Mario" <mario.limonciello@....com>,
        Avadhut Naik <avadnaik@....com>,
        "Wilczynski, Michal" <michal.wilczynski@...el.com>,
        Avadhut Naik <avadhut.naik@....com>, lenb@...nel.org,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ACPI: PHAT: Add Platform Health Assessment Table support

On Mon, Aug 21, 2023 at 8:44 PM Yazen Ghannam <yazen.ghannam@....com> wrote:
>
> On 8/21/23 2:01 PM, Rafael J. Wysocki wrote:
> > On Mon, Aug 21, 2023 at 7:52 PM Rafael J. Wysocki <rafael@...nel.org> 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?
> >>
> >>> 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.
> >
> > To be more precise, I don't see why the kernel needs to be made a
> > man-in-the-middle between the firmware which is the source of the
> > information and user space that consumes it.
>
> I think that's a fair point.
>
> Is there a preferred set of tools that can be updated?

I think you need to talk to distro people about this.

> If not, would it make sense to develop a set of common kernel tools for
> this?

Yes, it would, but please see above in the first place.

> In my experience, it seems many folks use tools from their vendors or
> custom tools.

This observation matches my own experience.

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