[PATCH v2 0/5] Statsfs: a new ram-based file sytem for Linux kernel statistics

[Jonathan Adams]

On Fri, May 8, 2020 at 2:44 AM Paolo Bonzini <pbonzini at redhat.com> wrote:
>
> [Answering for Emanuele because he's not available until Monday]
>
> On 07/05/20 19:45, Jonathan Adams wrote:
> > This is good work.  As David Rientjes mentioned, I'm currently investigating
> > a similar project, based on a google-internal debugfs-based FS we call
> > "metricfs".  It's
> > designed in a slightly different fashion than statsfs here is, and the
> > statistics exported are
> > mostly fed into our OpenTelemetry-like system.  We're motivated by
> > wanting an upstreamed solution, so that we can upstream the metrics we
> > create that are of general interest, and lower the overall rebasing
> > burden for our tree.
>
> Cool.  We included a public reading API exactly so that there could be
> other "frontends".  I was mostly thinking of BPF as an in-tree user, but
> your metricfs could definitely use the reading API.
>
> >  - the 8/16/32/64 signed/unsigned integers seems like a wart, and the
> > built-in support to grab any offset from a structure doesn't seem like
> > much of an advantage. A simpler interface would be to just support an> "integer" (possibly signed/unsigned) type, which is always 64-bit, and
> > allow the caller to provide a function pointer to retrieve the value,
> > with one or two void *s cbargs.  Then the framework could provide an
> > offset-based callback (or callbacks) similar to the existing
> > functionality, and a similar one for per-CPU based statistics.  A
> > second "clear" callback could be optionally provided to allow for
> > statistics to be cleared, as in your current proposal.
>
> Ok, so basically splitting get_simple_value into many separate
> callbacks.  The callbacks would be in a struct like
>
> struct stats_fs_type {
>         uint64_t (*get)(struct stats_fs_value *, void *);
>         void (*clear)(struct stats_fs_value *, void *);
>         bool signed;
> }
...
> struct stats_fs_type stats_fs_type_u8 = {
>         stats_fs_get_u8,
>         stats_fs_clear_u8,
>         false
> };
>
> and custom types can be defined using "&(struct stats_fs_type) {...}".

That makes sense.

> >  - Beyond the statistic's type, one *very* useful piece of metadata
> > for telemetry tools is knowing whether a given statistic is
> > "cumulative" (an unsigned counter which is only ever increased), as
> > opposed to a floating value (like "amount of memory used").
>
> Good idea.  Also, clearing does not make sense for a floating value, so
> we can use cumulative/floating to get a default for the mode: KVM
> statistics for example are mostly cumulative and mode 644, except a few
> that are floating and those are all mode 444.  Therefore it makes sense
> to add cumulative/floating even before outputting it as metadata.
>
> > I'm more
> > concerned with getting the statistics model and capabilities right
> > from the beginning, because those are harder to adjust later.
>
> Agreed.
>
> > 1. Each metricfs metric can have one or two string or integer "keys".
> > If these exist, they expand the metric from a single value into a
> > multi-dimensional table. For example, we use this to report a hash
> > table we keep of functions calling "WARN()", in a 'warnings'
> > statistic:
> >
> > % cat .../warnings/values
> > x86_pmu_stop 1
> > %
> >
> > Indicates that the x86_pmu_stop() function has had a WARN() fire once
> > since the system was booted.  If multiple functions have fired
> > WARN()s, they are listed in this table with their own counts. [1]  We
> > also use these to report per-CPU counters on a CPU-by-CPU basis:
> >
> > % cat .../irq_x86/NMI/values
> > 0 42
> > 1 18
> > ... one line per cpu
> > % cat .../rx_bytes/values
> > lo 501360681
> > eth0 1457631256
>
> These seem like two different things.

I see your point; I agree that there are two different things here.

> The percpu and per-interface values are best represented as subordinate
> sources, one per CPU and one per interface.  For interfaces I would just
> use a separate directory, but it doesn't really make sense for CPUs.  So
> if we can cater for it in the model, it's better.  For example:
>
> - add a new argument to statsfs_create_source and statsfs_create_values
> that makes it not create directories and files respectively.
>
> - add a new "aggregate function" STATS_FS_LIST that directs the parent
> to build a table of all the simple values below it
>
> We can also add a helper statsfs_add_values_percpu that creates a new
> source for each CPU, I think.

I think I'd characterize this slightly differently; we have a set of
statistics which are essentially "in parallel":

  - a variety of statistics, N CPUs they're available for, or
  - a variety of statistics, N interfaces they're available for.
  - a variety of statistics, N kvm object they're available for.

Recreating a parallel hierarchy of statistics any time we add/subtract
a CPU or interface seems like a lot of overhead.  Perhaps a better
model would
be some sort of "parameter enumn" (naming is hard; parameter set?), so
when a CPU/network interface/etc is added you'd add its ID to the
"CPUs" we know about, and at removal time you'd take it out; it would
have an associated cbarg for the value getting callback.

Does that make sense as a design?

I'm working on characterizing all of our metricfs usage; I'll see if
this looks like it mostly covers our usecases.

> The warnings one instead is a real hash table.  It should be possible to
> implement it as some kind of customized aggregation, that is implemented
> in the client instead of coming from subordinate sources.  The
> presentation can then just use STATS_FS_LIST.  I don't see anything in
> the design that is a blocker.

Yes; though if it's low-enough overhead, you could imagine having a
dynamically-updated parameter enum based on the hash table.

> > 2.  We also export some metadata about each statistic.  For example,
> > the metadata for the NMI counter above looks like:
> >
> > % cat .../NMI/annotations
> > DESCRIPTION Non-maskable\ interrupts
> > CUMULATIVE
> > % cat .../NMI/fields
> > cpu value
> > int int
> > %
>
> Good idea.  I would prefer per-directory dot-named files for this.  For
> example a hypothetical statsfs version of /proc/interrupts could be like
> this:
>
> $ cat /sys/kernel/stats/interrupts/.schema
> 0                                          // Name
> CUMULATIVE                                 // Flags
> int:int                                    // Type(s)
> IR-IO-APIC    2-edge      timer            // Description
> ...
> LOC
> CUMULATIVE
> int:int
> Local timer interrupts
> ...
> $ cat /sys/kernel/stats/interrupts/LOC
> 0 4286815
> 1 4151572
> 2 4199361
> 3 4229248
>
> > 3. We have a (very few) statistics where the value itself is a string,
> > usually for device statuses.
>
> Maybe in addition to CUMULATIVE and FLOATING we can have ENUM
> properties, and a table to convert those enums to strings.  Aggregation
> could also be used to make a histogram out of enums in subordinate
> sources, e.g.
>
> $ cat /sys/kernel/stats/kvm/637-1/vcpu_state
> running 12
> uninitialized 0
> halted 4

That's along similar lines to the parameter enums, yeah.

> So in general I'd say the sources/values model holds up.  We certainly
> want to:
>
> - switch immediately to callbacks instead of the type constants (so that
> core statsfs code only does signed/unsigned)
>
> - add a field to distinguish cumulative and floating properties (and use
> it to determine the default file mode)

Yup, these make sense.

> - add a new argument to statsfs_create_source and statsfs_create_values
> that makes it not create directories and files respectively
>
> - add a new API to look for a statsfs_value recursively in all the
> subordinate sources, and pass the source/value pair to a callback
> function; and reimplement recursive aggregation and clear in terms of
> this function.

This is where I think a little iteration on the "parameter enums"
should happen before jumping into implementation.

> > For our use cases, we generally don't both output a statistic and it's
> > aggregation from the kernel; either we sum up things in the kernel
> > (e.g. over a bunch of per-cpu or per-memcg counters) and only have the
> > result statistic, or we expect user-space to sum up the data if it's
> > interested.  The tabular form makes it pretty easy to do so (i.e. you
> > can use awk(1) to sum all of the per-cpu NMI counters).
>
> Yep, the above "not create a dentry" flag would handle the case where
> you sum things up in the kernel because the more fine grained counters
> would be overwhelming.

nodnod; or the callback could handle the sum itself.

Thanks,
- jonathan