On Mon, Aug 21, 2023 at 7:52 PM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote: > > On Mon, Aug 21, 2023 at 7:35 PM Limonciello, Mario > <mario.limonciello@xxxxxxx> 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@xxxxxxx> 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.
