Re: [RFC PATCH v2 8/8] sched, smp: Trace smp callback causing an IPI

Peter Zijlstra
2022-11-18T09:14:00Z

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..