On Mon, Aug 21, 2023 at 8:44 PM Yazen Ghannam <yazen.ghannam@xxxxxxx> 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@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. > > 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.
