[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20230323174129.GA2753619@hirez.programming.kicks-ass.net>
Date: Thu, 23 Mar 2023 18:41:29 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Valentin Schneider <vschneid@...hat.com>
Cc: linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-snps-arc@...ts.infradead.org,
linux-arm-kernel@...ts.infradead.org, linux-csky@...r.kernel.org,
linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
openrisc@...ts.librecores.org, linux-parisc@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
x86@...nel.org, "Paul E. McKenney" <paulmck@...nel.org>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Juri Lelli <juri.lelli@...hat.com>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Marcelo Tosatti <mtosatti@...hat.com>,
Frederic Weisbecker <frederic@...nel.org>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H. Peter Anvin" <hpa@...or.com>, Marc Zyngier <maz@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Russell King <linux@...linux.org.uk>,
Nicholas Piggin <npiggin@...il.com>,
Guo Ren <guoren@...nel.org>,
"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI
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
Powered by blists - more mailing lists