Re: [PATCH 01/40] lib/string_helpers: Drop space in string_get_size's output

Vlastimil Babka
2023-05-03T09:28:53Z

On 5/3/23 00:50, Dave Chinner wrote:
> On Tue, May 02, 2023 at 07:42:59AM -0400, James Bottomley wrote:
>> On Mon, 2023-05-01 at 23:17 -0400, Kent Overstreet wrote:
>> > On Mon, May 01, 2023 at 10:22:18PM -0400, James Bottomley wrote:
>> > > It is not used just for debug.  It's used all over the kernel for
>> > > printing out device sizes.  The output mostly goes to the kernel
>> > > print buffer, so it's anyone's guess as to what, if any, tools are
>> > > parsing it, but the concern about breaking log parsers seems to be
>> > > a valid one.
>> >
>> > Ok, there is sd_print_capacity() - but who in their right mind would
>> > be trying to scrape device sizes, in human readable units,
>>
>> If you bother to google "kernel log parser", you'll discover it's quite
>> an active area which supports a load of company business models.
>
> That doesn't mean log messages are unchangable ABI. Indeed, we had
> the whole "printk_index_emit()" addition recently to create
> an external index of printk message formats for such applications to
> use. [*]
>
>> > from log messages when it's available in sysfs/procfs (actually, is
>> > it in sysfs? if not, that's an oversight) in more reasonable units?
>>
>> It's not in sysfs, no. As aren't a lot of things, which is why log
>> parsing for system monitoring is big business.
>
> And that big business is why printk_index_emit() exists to allow
> them to easily determine how log messages change format and come and
> go across different kernel versions.
>
>> > Correct me if I'm wrong, but I've yet to hear about kernel log
>> > messages being consider a stable interface, and this seems a bit out
>> > there.
>>
>> It might not be listed as stable, but when it's known there's a large
>> ecosystem out there consuming it we shouldn't break it just because you
>> feel like it.
>
> But we've solved this problem already, yes?
>
> If the userspace applications are not using the kernel printk format
> index to detect such changes between kernel version, then they
> should be. This makes trivial issues like whether we have a space or
> not between units is completely irrelevant because the entry in the
> printk format index for the log output we emit will match whatever
> is output by the kernel....

If I understand that correctly from the commit changelog, this would have
indeed helped, but if the change was reflected in format string. But with
string_get_size() it's always an %s and the change of the helper's or a
switch to another variant of the helper that would omit the space, wouldn't
be reflected in the format string at all? I guess that would be an argument
for Andy's suggestion for adding a new %pt / %pT which would then be
reflected in the format string. And also more concise to use than using the
helper, fwiw.

> Cheers,
>
> Dave.
>
> [*]
> commit 337015573718b161891a3473d25f59273f2e626b
> Author: Chris Down <chris@chrisdown.name>
> Date: Tue Jun 15 17:52:53 2021 +0100
>
> printk: Userspace format indexing support
>
> We have a number of systems industry-wide that have a subset of their
> functionality that works as follows:
>
> 1. Receive a message from local kmsg, serial console, or netconsole;
> 2. Apply a set of rules to classify the message;
> 3. Do something based on this classification (like scheduling a
> remediation for the machine), rinse, and repeat.
>
> As a couple of examples of places we have this implemented just inside
> Facebook, although this isn't a Facebook-specific problem, we have this
> inside our netconsole processing (for alarm classification), and as part
> of our machine health checking. We use these messages to determine
> fairly important metrics around production health, and it's important
> that we get them right.
>
> While for some kinds of issues we have counters, tracepoints, or metrics
> with a stable interface which can reliably indicate the issue, in order
> to react to production issues quickly we need to work with the interface
> which most kernel developers naturally use when developing: printk.
>
> Most production issues come from unexpected phenomena, and as such
> usually the code in question doesn't have easily usable tracepoints or
> other counters available for the specific problem being mitigated. We
> have a number of lines of monitoring defence against problems in
> production (host metrics, process metrics, service metrics, etc), and
> where it's not feasible to reliably monitor at another level, this kind
> of pragmatic netconsole monitoring is essential.
>
> As one would expect, monitoring using printk is rather brittle for a
> number of reasons -- most notably that the message might disappear
> entirely in a new version of the kernel, or that the message may change
> in some way that the regex or other classification methods start to
> silently fail.
>
> One factor that makes this even harder is that, under normal operation,
> many of these messages are never expected to be hit. For example, there
> may be a rare hardware bug which one wants to detect if it was to ever
> happen again, but its recurrence is not likely or anticipated. This
> precludes using something like checking whether the printk in question
> was printed somewhere fleetwide recently to determine whether the
> message in question is still present or not, since we don't anticipate
> that it should be printed anywhere, but still need to monitor for its
> future presence in the long-term.
>
> This class of issue has happened on a number of occasions, causing
> unhealthy machines with hardware issues to remain in production for
> longer than ideal. As a recent example, some monitoring around
> blk_update_request fell out of date and caused semi-broken machines to
> remain in production for longer than would be desirable.
>
> Searching through the codebase to find the message is also extremely
> fragile, because many of the messages are further constructed beyond
> their callsite (eg. btrfs_printk and other module-specific wrappers,
> each with their own functionality). Even if they aren't, guessing the
> format and formulation of the underlying message based on the aesthetics
> of the message emitted is not a recipe for success at scale, and our
> previous issues with fleetwide machine health checking demonstrate as
> much.
>
> This provides a solution to the issue of silently changed or deleted
> printks: we record pointers to all printk format strings known at
> compile time into a new .printk_index section, both in vmlinux and
> modules. At runtime, this can then be iterated by looking at
> <debugfs>/printk/index/<module>, which emits the following format, both
> readable by humans and able to be parsed by machines:
>
> $ head -1 vmlinux; shuf -n 5 vmlinux
> # <level[,flags]> filename:line function "format"
> <5> block/blk-settings.c:661 disk_stack_limits "%s: Warning: Device %s is misaligned\n"
> <4> kernel/trace/trace.c:8296 trace_create_file "Could not create tracefs '%s' entry\n"
> <6> arch/x86/kernel/hpet.c:144 _hpet_print_config "hpet: %s(%d):\n"
> <6> init/do_mounts.c:605 prepare_namespace "Waiting for root device %s...\n"
> <6> drivers/acpi/osl.c:1410 acpi_no_auto_serialize_setup "ACPI: auto-serialization disabled\n"
>
> This mitigates the majority of cases where we have a highly-specific
> printk which we want to match on, as we can now enumerate and check
> whether the format changed or the printk callsite disappeared entirely
> in userspace. This allows us to catch changes to printks we monitor
> earlier and decide what to do about it before it becomes problematic.
>
> There is no additional runtime cost for printk callers or printk itself,
> and the assembly generated is exactly the same.
>
> Signed-off-by: Chris Down <chris@chrisdown.name>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Jessica Yu <jeyu@kernel.org>
> Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Cc: John Ogness <john.ogness@linutronix.de>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Kees Cook <keescook@chromium.org>
> Reviewed-by: Petr Mladek <pmladek@suse.com>
> Tested-by: Petr Mladek <pmladek@suse.com>
> Reported-by: kernel test robot <lkp@intel.com>
> Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
> Acked-by: Jessica Yu <jeyu@kernel.org> # for module.{c,h}
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> Link: https://lore.kernel.org/r/e42070983637ac5e384f17fbdbe86d19c7b212a5.1623775748.git.chris@chrisdown.name
>