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Date: Thu, 23 Mar 2023 18:41:29 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Valentin Schneider <vschneid@redhat.com>
Subject: Re: [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI
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On Thu, Mar 23, 2023 at 04:25:25PM +0000, Valentin Schneider wrote:
> 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;

Hmm, this isn't right. You can get nr_cpus==0 even though it did add
some to various lists but never was first.

But urgh, even if we were to say count nr_queued we'd never get the mask
right, because we don't track which CPUs have the predicate matched,
only those we need to actually send an IPI to :/

Ooh, I think we can clear those bits from cfd->cpumask, arguably that's
a correctness fix too, because the 'run_remote && wait' case shouldn't
wait on things we didn't queue.

Hmm?


--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -728,9 +728,9 @@ static void smp_call_function_many_cond(
 	int cpu, last_cpu, this_cpu = smp_processor_id();
 	struct call_function_data *cfd;
 	bool wait = scf_flags & SCF_WAIT;
+	int nr_cpus = 0, nr_queued = 0;
 	bool run_remote = false;
 	bool run_local = false;
-	int nr_cpus = 0;
 
 	lockdep_assert_preemption_disabled();
 
@@ -772,8 +772,10 @@ static void smp_call_function_many_cond(
 		for_each_cpu(cpu, cfd->cpumask) {
 			call_single_data_t *csd = per_cpu_ptr(cfd->csd, cpu);
 
-			if (cond_func && !cond_func(cpu, info))
+			if (cond_func && !cond_func(cpu, info)) {
+				__cpumask_clear_cpu(cpu, cfd->cpumask);
 				continue;
+			}
 
 			csd_lock(csd);
 			if (wait)
@@ -789,13 +791,15 @@ static void smp_call_function_many_cond(
 				nr_cpus++;
 				last_cpu = cpu;
 			}
+			nr_queued++;
 		}
 
 		/*
 		 * 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);
+		if (nr_queued)
+			trace_ipi_send_cpumask(cfd->cpumask, _RET_IP_, func);
 
 		/*
 		 * Choose the most efficient way to send an IPI. Note that the