On Tue, 26 May 2020 13:03:10 +0200 Emanuele Giuseppe Esposito wrote:

There is currently no common way for Linux kernel subsystems to expose
statistics to userspace shared throughout the Linux kernel; subsystems have
to take care of gathering and displaying statistics by themselves, for
example in the form of files in debugfs. For example KVM has its own code
section that takes care of this in virt/kvm/kvm_main.c, where it sets up
debugfs handlers for displaying values and aggregating them from various
subfolders to obtain information about the system state (i.e. displaying
the total number of exits, calculated by summing all exits of all cpus of
all running virtual machines).

Allowing each section of the kernel to do so has two disadvantages. First,
it will introduce redundant code. Second, debugfs is anyway not the right
place for statistics (for example it is affected by lockdown)

In this patch series I introduce statsfs, a synthetic ram-based virtual
filesystem that takes care of gathering and displaying statistics for the
Linux kernel subsystems.

The file system is mounted on /sys/kernel/stats and would be already used
by kvm. Statsfs was initially introduced by Paolo Bonzini [1].

What's the direct motivation for this work? Moving KVM stats out of
debugfs?

In my experience stats belong in the API used for creating/enumerating
objects, statsfs sounds like going in the exact opposite direction -
creating a parallel structure / hierarchy for exposing stats. I know
nothing about KVM but are you sure all the info that has to be exposed
will be stats?

In case of networking we have the basic stats in sysfs, under the
netdevice's kobject. But since we're not using sysfs much any more 
for config, new stats are added in netlink APIs. Again - same APIs
used for enumeration and config.