Re: [PATCH v3 0/7] Statsfs: a new ram-based file system for Linux kernel statistics

[Paolo Bonzini]

On 27/05/20 23:27, Jakub Kicinski wrote:
> On Wed, 27 May 2020 23:07:53 +0200 Paolo Bonzini wrote:
>>> Again, I have little KVM knowledge, but BPF also uses a fd-based API,
>>> and carries stats over the same syscall interface.  
>>
>> Can BPF stats (for BPF scripts created by whatever process is running in
>> the system) be collected by an external daemon that does not have access
>> to the file descriptor?  For KVM it's of secondary importance to gather
>> stats in the program; it can be nice to have and we are thinking of a
>> way to export the stats over the fd-based API, but it's less useful than
>> system-wide monitoring.  Perhaps this is a difference between the two.
> 
> Yes, check out bpftool prog list (bpftool code is under tools/bpf/ in
> the kernel tree). BPF statistics are under a static key, so you may not
> see any on your system. My system shows e.g.:
> 
> 81: kprobe  name abc  tag cefaa9376bdaae75  gpl run_time_ns 80941 run_cnt 152
> 	loaded_at 2020-05-26T13:00:24-0700  uid 0
> 	xlated 512B  jited 307B  memlock 4096B  map_ids 66,64
> 	btf_id 16
> 
> In this example run_time_ns and run_cnt are stats.
> 
> The first number on the left is the program ID. BPF has an IDA, and
> each object gets an integer id. So admin (or CAP_BPF, I think) can
> iterate over the ids and open fds to objects of interest.

Got it, thanks.  But then "I'd hope that whatever daemon collects [BPF]
stats doesn't run as root". :)

>> Another case where stats and configuration are separate is CPUs, where
>> CPU enumeration is done in sysfs but statistics are exposed in various
>> procfs files such as /proc/interrupts and /proc/stats.
> 
> True, but I'm guessing everyone is just okay living with the legacy
> procfs format there. Otherwise I'd guess the stats would had been added
> to sysfs. I'd be curious to hear the full story there.

Yeah, it's a chicken-and-egg problem in that there's no good place in
sysfs to put statistics right now, which is part of what this filesystem
is trying to solve (the other part is the API).

You can read more about Google's usecase at
http://lkml.iu.edu/hypermail/linux/kernel/2005.0/08056.html, it does
include both network and interrupt stats and it's something that they've
been using in production for quite some time.  We'd like the statsfs API
to be the basis for including something akin to that in Linux.

To be honest, it's unlikely that Emanuele (who has just finished his
internship at Red Hat) and I will pursue the networking stats further
than the demo patch at the end of this series. However, we're trying to
make sure that the API is at least ready for that, and to probe whether
any developers from other subsystems would be interested in using
statsfs.  So thanks for bringing your point of view!

Thanks,

Paolo