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Message-ID: <f294a74cd17bf932621a6a746d592f49770d25ec.camel@redhat.com>
Subject: Re: [RFC PATCH 1/1] smp: Add tracepoints for functions called with
 smp_call_function*()
From:   Leonardo =?ISO-8859-1?Q?Br=E1s?= <leobras@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Yury Norov <yury.norov@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Nadav Amit <namit@vmware.com>,
        Zhen Lei <thunder.leizhen@huawei.com>,
        Chen Zhongjin <chenzhongjin@huawei.com>,
        linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org,
        Marcelo Tosatti <mtosatti@redhat.com>
Date:   Wed, 19 Apr 2023 00:45:08 -0300
In-Reply-To: <20230406095519.GG386572@hirez.programming.kicks-ass.net>
References: <20230406075718.68672-1-leobras@redhat.com>
         <20230406095519.GG386572@hirez.programming.kicks-ass.net>
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On Thu, 2023-04-06 at 11:55 +0200, Peter Zijlstra wrote:
> On Thu, Apr 06, 2023 at 04:57:18AM -0300, Leonardo Bras wrote:
> > When running RT workloads in isolated CPUs, many cases of deadline miss=
es
> > are caused by remote CPU requests such as smp_call_function*().
> >=20
> > For those cases, having the names of those functions running around the
> > deadline miss moment could help finding a target for the next improveme=
nts.
> >=20
> > Add tracepoints for acquiring the funtion name & argument before entry =
and
> > after exitting the called function.
> >=20
> > Signed-off-by: Leonardo Bras <leobras@redhat.com>
>=20
> How are the patches queued there not sufficient?
>=20
> https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/log/?h=3Dsmp/=
core
>=20

IIUC the last commits add tracepoints that are collected in the
requesting CPU, at the moment of scheduling the IPI, which are also useful =
in
some scenarios.

On my scenario, it could help a little,=C2=A0since it makes possible to fil=
ter what
all other cpus are scheduling on the requested cpu. OTOH it could be also b=
e
misleading, as the requested cpu could be running something that was schedu=
led
way before.

The change I propose does exactly what my scenario need: track exactly whic=
h
function was running at given time in the requested CPU. With this info, we=
 can
check which (if any) remotely requested function was running on given time
window.

(An unrelated thing I just thought: We could even use the commits you point=
ed
together with my proposed change in order to measure how long does it take =
for a
requested function to run / complete in the requested cpu)

Thanks for reviewing!

Leo