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Date:   Fri, 18 Nov 2022 10:12:43 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Valentin Schneider <vschneid@redhat.com>
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Subject: Re: [RFC PATCH v2 8/8] sched, smp: Trace smp callback causing an IPI
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On Thu, Nov 17, 2022 at 02:45:29PM +0000, Valentin Schneider wrote:

> > +	if (trace_ipi_send_cpumask_enabled()) {
> > +		call_single_data_t *csd;
> > +		smp_call_func_t func;
> > +
> > +		csd = container_of(node, call_single_data_t, node.llist);
> > +
> > +		func = sched_ttwu_pending;
> > +		if (CSD_TYPE(csd) != CSD_TYPE_TTWU)
> > +			func = csd->func;
> > +
> > +		if (raw_smp_call_single_queue(cpu, node))
> > +			trace_ipi_send_cpumask(cpumask_of(cpu), _RET_IP_, func);
> 
> So I went with the tracepoint being placed *before* the actual IPI gets
> sent to have a somewhat sane ordering between trace_ipi_send_cpumask() and
> e.g. trace_call_function_single_entry().
> 
> Packaging the call_single_queue logic makes the code less horrible, but it
> does mix up the event ordering...

Keeps em sharp ;-)

> > +		return;
> > +	}
> > +
> > +	raw_smp_call_single_queue(cpu, node);
> >  }
> >
> >  /*
> > @@ -983,10 +1017,13 @@ static void smp_call_function_many_cond(
> >                * number of CPUs might be zero due to concurrent changes to the
> >                * provided mask.
> >                */
> > -		if (nr_cpus == 1)
> > +		if (nr_cpus == 1) {
> > +			trace_ipi_send_cpumask(cpumask_of(last_cpu), _RET_IP_, func);
> >                       send_call_function_single_ipi(last_cpu);
> 
> This'll yield an IPI event even if no IPI is sent due to the idle task
> polling, no?

Oh, right..