On 2023-12-21 00:27, Csókás Bence wrote:
On 2023. 12. 20. 20:58, Dragan Simic wrote:
On 2023-12-20 20:24, Csókás Bence wrote:
On 2023. 12. 20. 9:29, Dragan Simic wrote:
On 2023-12-20 08:52, Csókás Bence wrote:
That's what I read as well. Is there support for U-boot to write
and
Linux to read PStores?
No and yes, but U-Boot can already read pstore. Please see
doc/usage/cmd/pstore.rst for the U-Boot part, and
Documentation/admin-guide/pstore-blk.rst for the Linux kernel part.
Irrelevant, as we only want to write out the console log to U-Boot,
and
not the other way around (that's for collecting panic logs, which is
already implemented).
Quite frankly, this is a bit confusing to me. Why should it be
irrelevant, if we want to store the U-Boot console logs into the
pstore,
and read them later in Linux? That's exactly what you asked about,
and
it was a good question.
Because we want to *write* PStore from U-Boot. The read being already
implemented is not very useful, we could only re-read an
already-written
console log (which is not helpful for us).
Well, it all depends on one's viewpoint. See, even if we had no read
support already implemented in U-Boot and wanted to just write the data,
we'd almost surely have to implement reading as well, so the written
stuff can be inspected in the U-Boot console.
Another benefit of using pstore would be no permanently wasted RAM
for
the recorded console contents. Also, having the data recorded to a
storage device also goes along with providing permanent records.
I'm positively *not gonna save boot logs to disk*, as most embedded
systems have Flash-based storage media; writing to them on every boot
would be devastating.
Writing 30-40 KB of data to an even remotely modern flash-based
storage
media on each boot is pretty much nothing. OTOH, writing 30-40 KB of
data to an SPI chip on each boot would be rather bad, but we aren't
going to do that for sure.
To put things into perspective, I'm writing this on a Pinebook Pro
with
an eMMC chip, and it has stored many GBs of data over a few years.
Even
high-endurance microSD cards are rather reliable in that regard.
Not every system has eMMC/uSD, and as you said, these arguments don't
hold for a 4 MB SPI NAND, for example, one you might find in an OpenWrt
router for example. Whereas RAM is quite cheap nowadays.
I see, but I also wonder how many such OpenWrt routers are still used
these days, and, even more importantly, how many of them are regularly
updated and can be expected to actually use this new feature?
Please, don't get me wrong, I still support having both options
available, but I'm also wondering about the target demographic.
Plus, I don't want the console subsystem to depend on any file/disk
operations/drivers.
Well, the console would still work as usual even if logging to disk
would fail for any reason, which is similar to the serial console
still
working if the graphical console fails. Moreover, if the disk fails,
the system isn't be able to boot, so any RAM-based console logs would
be
lost in that case. All this makes the RAM-based logging no more
resilient to disk failures.
Correction: if disk *reads* fail, as well as writes, then the system
will not boot. However, typical failure of Flash media is that it
becomes read-only.
That's a good point, but having a read-only root filesystem usually also
means having a non-operational system that can only have its stored data
salvaged. Unless the system is specifically crafted to survive such
scenarios, of course.
I still think that using disk-based pstore is a better option. Just
as
you don't want to wear out your flash disks with 30-40 KB of data, I
also don't want to waste 30-40 KB of RAM.
As I said, you could just unload the log after you're done processing
it. 40 KB RAM is less, than what `sshd` uses, for instance (860k on my
laptop, but it can probably be less, maybe even 10x less, so 80-90k?),
so you could, in your init, process the in-RAM log, then unload it,
then
start your other services, thereby reclaiming that RAM.
Using pstore should have that unloading already covered, and the already
existing systemd service is there to perform the archiving to the
primary filesystem, if desired so. It would all need to be tested in
detail, of course.
Also, embedded devices such
as the PinePhone would benefit additionally from using disk-based
logging, because people often can't do anything in the field if
booting
fails for some reason, so RAM-based logs would end up being lost,
preventing any post-mortem debugging later.
Post-mortem analysis could be interesting, true.
Yes, and it's often the only way to get to the bottom of the underlying
issues, which are sometimes intermittent and can't be reproduced easily
later.
Perhaps we can actually offer both options, i.e. both logging to RAM
and
to disk, so everyone can choose what they prefer. I'd also be fine
with
working on such a dual approach, but it would obviously be more
complex.
Perhaps that is the best option. But, as Sean has pointed out, U-Boot
doesn't support disk-backed PStore anyways... So then, should we
implement RAM-based PStore writing for the time being, and then maybe
expand it later?
I'd vote for implementing both options at once, so we can reduce the
overall amount of the required testing. It will surely result in more
time required for the development, but the total amount of time should
be significantly shorter.
