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Date:   Tue, 26 May 2020 17:45:17 +0200
From:   Emanuele Giuseppe Esposito <eesposit@...hat.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     kvm@...r.kernel.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Paolo Bonzini <pbonzini@...hat.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
Subject: Re: [PATCH v3 7/7] [not for merge] netstats: example use of stats_fs
 API


Hi Andrew

> How do you atomically get and display a group of statistics?
> 
> If you look at how the netlink socket works, you will see code like:
> 
>                  do {
>                          start = u64_stats_fetch_begin_irq(&cpu_stats->syncp);
>                          rx_packets = cpu_stats->rx_packets;
>                          rx_bytes = cpu_stats->rx_bytes;
> 			....
>                  } while (u64_stats_fetch_retry_irq(&cpu_stats->syncp, start));
> 
> It will ensure that rx_packets and rx_bytes are consistent with each
> other. If the value of the sequence counter changes while inside the
> loop, the loop so repeated until it does not change.
> 
> In general, hardware counters in NICs are the same.  You tell it to
> take a snapshot of the statistics counters, and then read them all
> back, to give a consistent view across all the statistics.
> 
> I've not looked at this new code in detail, but it looks like you have
> one file per statistic, and assume each statistic is independent of
> every other statistic. This independence can limit how you use the
> values, particularly when debugging. The netlink interface we use does
> not have this limitation.

You're right, statistics are treated independently so what you describe 
is currently not supported.

In KVM the utilization is more qualitative, so there isn't such problem.
But as long as the interface is based on file access, the possibility of 
snapshotting might not be useful; however, it could still be considered 
to be added later together with the binary access.

Jonathan, how is your metricfs handling this case?

Thank you,
Emanuele

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