On 8/11/20 9:02 AM, Petr Mladek wrote: > On Tue 2020-08-11 14:05:12, Thomas Gleixner wrote: >> Petr Mladek <pmladek@xxxxxxxx> writes: >>> At least "crash" tool would need an update anyway. AFAIK, it checks >>> the size of struct printk_log and refuses to read it when it changes. >>> >>> It means that the hack with VMCOREINFO_FIELD_OFFSET probably is not >>> needed because we would need to update the crashdump-related tools anyway. >>> >>> Well, the timing is good. We are about to switch the printk ring >>> buffer into a lockless one. It requires updating the crashdump tools >>> as well. We could do this at the same time. The lockless ring buffer >>> already is in linux-next. It is aimed for 5.10 or 5.11. >> ... >>> It would be great to synchronize all these changes changes of the >>> printk log buffer structures. >> >> I agree that having one update is a good thing, but pretty please can we >> finally make progress with this and not create yet another dependency? > > To make it clear. I definitely do not want to block lockless printk by > this. Thanks for clarifying that. I had the same concern that tglx had. > > BTW: I am not 100% convinced that storing all three timestamps is > worth it. It increases the code complexity, metadata size. It needs > an interface with the userspace that has to stay backward compatible. > > Also it still will be racy because the timestamp is taken when the message > is printed. It might be "long" before or after the event that > it talks about. That scenario plays out today with the current timestamp. Printing debug with a better timestamp doesn't resolve that problem nor is it intended to. > > There is still the alternative to print all three timestamps regularly > for those interested. It is less user convenient but much easier > to maintain. While I agree, in general, your alternative is useful for in-person debugging it is not helpful in cases where a user only has an image of a panic where the printk time-synch message has scrolled off the screen. Consider this real debug case where I hacked the boottime stamp into printk: We had some systems with flaky HW that hit panics at 10 hours [1] of uptime. Since these systems came from different vendors with different HW the clocks were out-of-sync. I had a suspicion that it was some human-coded event causing the HW to die but wasn't sure until I did a boottime-stamped printk to prove that all the systems died after 10 hours. I could have set a stopwatch or timer to somehow catch this kind of event. I could have set up video cameras to watch the consoles, etc. There are a lot of other ways I could have debugged this situation but ultimately the fastest thing to do was code and provide kernels to the various HW companies and see if everything lined up as I thought it would. This problem happens far more often then I'd like to admit and we still see this type of problem with new HW and FW. I also recall that other kernel groups (storage, networking, etc.) were interested in the timestamps as it would make their debugging easier to have a true synchronized timestamp available. One other thing -- IIRC I had to modify the dmesg code by providing a sysfs (or proc?) interface for dmesg to identify the stamp. It's something that should be investigated with this code too. P. [1] It was 3+ years ago. I can't remember if it was 10 or 100 but you get the point. > > Best Regards, > Petr > _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
