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

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Message-ID: <af2ba926-73bc-26c3-7ce7-bd45f657fd85@redhat.com>
Date:   Wed, 27 May 2020 23:07:53 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Emanuele Giuseppe Esposito <eesposit@...hat.com>
Cc:     kvm@...r.kernel.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Jim Mattson <jmattson@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Emanuele Giuseppe Esposito <e.emanuelegiuseppe@...il.com>,
        David Rientjes <rientjes@...gle.com>,
        Jonathan Adams <jwadams@...gle.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mips@...r.kernel.org, kvm-ppc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, netdev@...r.kernel.org,
        Andrew Lunn <andrew@...n.ch>
Subject: Re: [PATCH v3 0/7] Statsfs: a new ram-based file system for Linux
 kernel statistics

On 27/05/20 22:23, Jakub Kicinski wrote:
> On Wed, 27 May 2020 15:14:41 +0200 Emanuele Giuseppe Esposito wrote:
>> Regarding the config, as I said the idea is to gather multiple 
>> subsystems' statistics, therefore there wouldn't be a single 
>> configuration method like in netlink.
>> For example in kvm there are file descriptors for configuration, and 
>> creating them requires no privilege, contrary to the network interfaces.
>
> Enumerating networking interfaces, addresses, and almost all of the
> configuration requires no extra privilege. In fact I'd hope that
> whatever daemon collects network stats doesn't run as root :)
> 
> I think enumerating objects is of primary importance, and statistics 
> of those objects are subordinate.

I see what you meant now.  statsfs can also be used to enumerate objects
if one is so inclined (with the prototype in patch 7, for example, each
network interface becomes a directory).

> 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.

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.

Thanks,

Paolo