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

[Valentin Schneider <vschneid@xxxxxxxxxx>]

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;