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

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Message-ID: <aef178472265e86b89f2af2632b95b8cd1d2d321.camel@redhat.com>
Date:   Wed, 03 May 2023 01:23:30 -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 Wed, 2023-04-19 at 00:45 -0300, Leonardo Brás wrote:
> 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


Hello Peter, 

The trace I proposed will run in the CPU which received the request, not in the
one which originated it. Meaning the trace will run in "cpu" from
smp_call_function*(cpu,...)

IIUC the trace introduced by the commits in tip/smp/core (which you pointed)
will run in the cpu which calls smp_call_function*(), which is different from
above.

Did I get it right?
Could you please help me understand how the commits in tip/smp/core are
sufficient for my scenario?

Thanks!
Leo



> > 
> 
> 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