linux-kernel - Re: [RFC PATCH 1/1] smp: Add tracepoints for functions called with smp_call

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Date:   Wed, 19 Apr 2023 00:45:08 -0300
From:   Leonardo Brás <leobras@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yury Norov <yury.norov@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Nadav Amit <namit@...are.com>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Chen Zhongjin <chenzhongjin@...wei.com>,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [RFC PATCH 1/1] smp: Add tracepoints for functions called with
 smp_call_function*()

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 misses
> > are caused by remote CPU requests such as smp_call_function*().
> > 
> > For those cases, having the names of those functions running around the
> > deadline miss moment could help finding a target for the next improvements.
> > 
> > Add tracepoints for acquiring the funtion name & argument before entry and
> > after exitting the called function.
> > 
> > Signed-off-by: Leonardo Bras <leobras@...hat.com>
> 
> How are the patches queued there not sufficient?
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/log/?h=smp/core
> 

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, since it makes possible to filter what
all other cpus are scheduling on the requested cpu. OTOH it could be also be
misleading, as the requested cpu could be running something that was scheduled
way before.

The change I propose does exactly what my scenario need: track exactly which
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 pointed
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