linux-kernel - Re: [PATCH v2 0/4] perf: Fix perf_event

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Message-ID: <20201102035238.GI466880@tassilo.jf.intel.com>
Date:   Sun, 1 Nov 2020 19:52:38 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
        acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, namhyung@...nel.org,
        linux-kernel@...r.kernel.org, eranian@...gle.com
Subject: Re: [PATCH v2 0/4] perf: Fix perf_event_attr::exclusive rotation

> hm, it's too late for me to check ;-) but should I be able to do
> this with exclusive event.. running both command at the same time:

Yes. The exclusive part only applies during a given context,
but the two commands are different contexts.

You would only see a difference when in the same context,
and you have multiple groups (or events) that could in theory schedule
in parallel

e.g. something like

perf stat -e '{cycles,cycles},{cycles,cycles}'  ...

The main motivation is actually that the "multiple groups" algorithm
in perf doesn't work all that great: it has quite a few cases where it
starves groups or makes the wrong decisions. That is because it is very
difficult (likely NP complete) problem and the kernel takes a lot
of short cuts to avoid spending too much time on it.

With exclusive it will be possible for a tool to generate "perfect groups"
in user space and assume the kernel schedules it dumbly, but at least
without any starvation.

-Andi