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

Peter Zijlstra
2023-03-23T17:42:44Z

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