Re: [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI

Valentin Schneider
2023-03-23T16:26:33Z

On 22/03/23 15:04, Peter Zijlstra wrote:
> @@ -798,14 +794,20 @@ static void smp_call_function_many_cond(
> }
>
> /*
> + * Trace each smp_function_call_*() as an IPI, actual IPIs
> + * will be traced with func==generic_smp_call_function_single_ipi().
> + */
> + trace_ipi_send_cpumask(cfd->cpumask_ipi, _RET_IP_, func);

I just got a trace pointing out this can emit an event even though no IPI
is sent if e.g. the cond_func predicate filters all CPUs in the argument
mask:

ipi_send_cpumask: cpumask= callsite=on_each_cpu_cond_mask+0x3c callback=flush_tlb_func+0x0

Maybe something like so on top?

---
diff --git a/kernel/smp.c b/kernel/smp.c
index ba5478814e677..1dc452017d000 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -791,6 +791,8 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
}
}

+ if (!nr_cpus)
+ goto local;
/*
* Trace each smp_function_call_*() as an IPI, actual IPIs
* will be traced with func==generic_smp_call_function_single_ipi().
@@ -804,10 +806,10 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
*/
if (nr_cpus == 1)
send_call_function_single_ipi(last_cpu);
- else if (likely(nr_cpus > 1))
+ else
send_call_function_ipi_mask(cfd->cpumask_ipi);
}
-
+local:
if (run_local && (!cond_func || cond_func(this_cpu, info))) {
unsigned long flags;