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From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     Luigi Rizzo <lrizzo@google.com>, linux-kernel@vger.kernel.org,
        mhiramat@kernel.org, akpm@linux-foundation.org,
        gregkh@linuxfoundation.org, naveen.n.rao@linux.ibm.com,
        ardb@kernel.org, rizzo@iet.unipi.it, pabeni@redhat.com,
        giuseppe.lettieri@unipi.it, hawk@kernel.org, mingo@redhat.com,
        acme@kernel.org, rostedt@goodmis.org, peterz@infradead.org
Cc:     Luigi Rizzo <lrizzo@google.com>
Subject: Re: [PATCH v3 0/2] kstats: kernel metric collector
In-Reply-To: <20200226135027.34538-1-lrizzo@google.com>
References: <20200226135027.34538-1-lrizzo@google.com>
Date:   Wed, 26 Feb 2020 16:00:49 +0100
Message-ID: <87ftexz93y.fsf@toke.dk>
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Luigi Rizzo <lrizzo@google.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