linux-kernel - Re: [PATCH v3 0/2] kstats: kernel metric collector

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Date:   Wed, 26 Feb 2020 16:00:49 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Luigi Rizzo <lrizzo@...gle.com>, linux-kernel@...r.kernel.org,
        mhiramat@...nel.org, akpm@...ux-foundation.org,
        gregkh@...uxfoundation.org, naveen.n.rao@...ux.ibm.com,
        ardb@...nel.org, rizzo@....unipi.it, pabeni@...hat.com,
        giuseppe.lettieri@...pi.it, hawk@...nel.org, mingo@...hat.com,
        acme@...nel.org, rostedt@...dmis.org, peterz@...radead.org
Cc:     Luigi Rizzo <lrizzo@...gle.com>
Subject: Re: [PATCH v3 0/2] kstats: kernel metric collector

Luigi Rizzo <lrizzo@...gle.com> writes:

> This patchset introduces a small library to collect per-cpu samples and
> accumulate distributions to be exported through debugfs.
>
> This v3 series addresses some initial comments (mostly style fixes in the
> code) and revises commit logs.

Could you please add a proper changelog spanning all versions of the
patch as you iterate?

As for the idea itself; picking up this argument you made on v1:

> The tracepoint/kprobe/kretprobe solution is much more expensive --
> from my measurements, the hooks that invoke the various handlers take
> ~250ns with hot cache, 1500+ns with cold cache, and tracing an empty
> function this way reports 90ns with hot cache, 500ns with cold cache.

I think it would be good if you could include an equivalent BPF-based
implementation of your instrumentation example so people can (a) see the
difference for themselves and get a better idea of how the approaches
differ in a concrete case and (b) quantify the difference in performance
between the two implementations.

-Toke