linux-kernel - Re: [PATCH v2] trace/osnoise: Add trace events for samples

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Message-ID: <CAP4=nvRNvdULubjN1XPF8jNxY3tRdfy+9Vqv97EsKjzNGKyYig@mail.gmail.com>
Date: Tue, 4 Feb 2025 14:35:07 +0100
From: Tomas Glozar <tglozar@...hat.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	John Kacur <jkacur@...hat.com>, Luis Goncalves <lgoncalv@...hat.com>, 
	Gabriele Monaco <gmonaco@...hat.com>
Subject: Re: [PATCH v2] trace/osnoise: Add trace events for samples

po 3. 2. 2025 v 10:04 odesílatel Tomas Glozar <tglozar@...hat.com> napsal:
> A proof-of-concept bpftrace script using this feature:
> https://gitlab.com/-/snippets/4801190
>

I added another PoC using event histograms to the snippet. That one
captures data from all CPUs, and thus can be used for testing on
machines with a high number of CPUs where rtla cannot keep up with
timerlat samples (in our measurements, >100).

There seems to be an issue with division where most values are rounded up, e.g.:
max:        135  timer_latency:     134657

This also affects the main histogram and seems to be specific to the
event histogram PoC. The bpftrace one shows exactly the same results
as rtla when run concurrently with it. Another difference compared to
the bpftrace PoC is that you have to calculate averages manually from
the latency sum and the sample count.

Tomas